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07-01-2004, 04:48 PM
Since I get to work on McIntosh equipment most every day I think a Helpfull Hints topic can benafit all of us in the McIntosh fourum. Feel free to post your hint or kink, each one of us may have a unique solution for a problem they have run into and others may need a solution for.

07-01-2004, 04:55 PM
Here is a good starter, MC30 bias upgrade for safety and reliability.
Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is exactly what it should be with these 3 changes, and the long life of your MC30 is one step further.

07-02-2004, 03:23 PM
MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2, 12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin 7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias voltage resulting in the phase inverter outputs being unbalanced.

Rich Andrews
07-02-2004, 09:50 PM
The internal cable dressing is VERY important on the MC2500. Had one with a hum/buzz in the left channel. Turns out the someone had moved the leads coming from the thermoswitch on the heatsink to the other side of the wiring harness. A distance of about 1/2 inch. That was enough to cause a hum in the left channel. Moving it so that it was on the inside of the wire bundle solved the problem.

Number 9
07-02-2004, 11:11 PM
I learned about this not that long ago.

I had a bit of a problem with my MR67 Telefunken 6U8 MPX tube. Had it tested out on two different tube testing machines by knowledgeable technicians, and it checked out fine, but continued to run into problems.

I replaced the tube with a new NOS, and everything is just fine.

By my understanding, tube testers, test the tube cold, and it may perform differently when hot.

Rich Andrews
07-03-2004, 02:22 AM
A 6U8 in a MR67? I tried a few and they don't work as well as a 6EA8 in my MR67. I have some tech notes on a MR71 and the tech recommened a 6GH8 instead of a 6U8 or 6EA8 in the MPX circuit. I personally have not tried this as the 6EA8 works great.

I discovered that my MR67 does not match the schematic. There is an additional pot on the chassis that adjusts the MPX light. Mine was set wrong resulting in a modulation of the lamp in direct accordance to the program material.


I can't tell you how many times I have had to test a tube hot which required me to leave it on the tester for 5-10 minutes. The most recent was when I tested tubes a few months ago that were from my MR67. I had one tube that after about 10 minutes would go so leaky that it would peg the meter and start to show short. I had another that after about 5 minutes would go quite leaky.

My rule when dealing with a tube tester is like this. I have seen plenty of bad tubes test good, but rarely (once?) have I seen a good tube test bad.

What was with the good tube that tested bad? It was low on emission and I think it just needed to be used. IIRC, it was NOS that had been sitting around since 1955 or earlier.

07-03-2004, 07:30 AM
I had Rich do his Modification on my MR-67 and he suggested using the 6GH8 for the MPX section. I have not had any problems. I also tried a couple of different 6U8 tubes.

Number 9
07-03-2004, 10:45 AM
It was a BAD tube that tested GOOD. It was the original diamond-bottom Telefunken that came with the MR67. And it is as you say, because it was being tested cold rather than hot.

Now, I had asked about the 6GH8 at the time PRM, because I saw your comment once before. The fellow who sells me my tubes (and I trust him, he's been at it for nearly 40 years and has no alterior motives) told me that they are very unreliable. He had some of those in stock too, so no skin off his back if I had bought one. He said they were designed for TV use, and said they used to fail very frequently. Miserable in fact. He would buy them buy the case once a year because they were the most common part to fail on old RCA televisions.

Ampex (Made in France) NOS 6U8s are supposedly the best but are extremely expensive.

07-03-2004, 12:04 PM
The brand of 6GH8 is an Amperex (France) 6GH8 which Rich Modafferi had available. I have heard from two veterans of McIntosh that 6U8 tube was notorious for failing. I myself have not had a problem with them. I did use a 6EA8 which I did have problems with. If it works for you that is good. If I had to go back with the 6U8 I would for I have a couple on hand. I am glad it works for you. Man, I love my MR-67. Great tuner. We are lucky. Enjoy.

07-06-2004, 11:04 AM
This series of amplifiers have 4 lamps on the panel, 2 normal (#7382) and 2 power guard (#7373), on power up the power guard lamps blink on for about a second and then the normal lamps come on. If they never come on (all 4) and you know that they are good, check the power supply board behind the front panel. This board has 2 capacitors 2200uF at 16V (C307 and C308) in a voltage doubler to drive the lamps - these capacitors tend to short - thus no lamps. While you are at it check the 2 diodes between them, if the amp has been used for an extended time with a shorted cap the diodes will be bad, 1N4004 is a good replacement.

Rich Andrews
07-06-2004, 01:32 PM
The 6GH8A was notorious for failing in tube TV sets. It was used as a horizontal driver and they would get soft. Now whether it was due to the type of use or just crappy lifetime I don't know.

Speaking of MR67, does everyone know about the factory tech bulletin regarding the addition of a cap and resistor to improve separation?

look at MR71S13.JPG in the zip file for more info.

Yes it applies to the MR67 as well.

Originally posted by Number 9
It was a BAD tube that tested GOOD. It was the original diamond-bottom Telefunken that came with the MR67. And it is as you say, because it was being tested cold rather than hot.

Now, I had asked about the 6GH8 at the time PRM, because I saw your comment once before. The fellow who sells me my tubes (and I trust him, he's been at it for nearly 40 years and has no alterior motives) told me that they are very unreliable. He had some of those in stock too, so no skin off his back if I had bought one. He said they were designed for TV use, and said they used to fail very frequently. Miserable in fact. He would buy them buy the case once a year because they were the most common part to fail on old RCA televisions.

Ampex (Made in France) NOS 6U8s are supposedly the best but are extremely expensive.

Rich Andrews
07-14-2004, 02:32 AM
Originally posted by Number 9

Ampex (Made in France) NOS 6U8s are supposedly the best but are extremely expensive.

Don't you mean Amperex?

Rich Andrews
07-22-2004, 10:42 PM

I cleaned up the RCA jacks on the back of my MA5100 with Brasso and they look like fine jewelry now! Thanks!

Number 9
07-23-2004, 05:06 PM
Hi Rich ... yes, I meant Amperex. BTW thanks for the MR71 mod tip. I'll git it a try when I have some time.

07-23-2004, 06:23 PM
Does your MA5100 sound dull, lack bass, behave tired - replace the 100uf (4 each) capacitors on the preamp board. If yours has white Mallory caps in it, replace them as they will fail with age.

Number 9
07-23-2004, 08:28 PM
Terry ... I have an MA6200 ... would the same apply?

07-23-2004, 09:04 PM
MA6200 is a full generation newer, the MAC4100 uses the same output section, very trouble free, have not seen any repeating problems in this design.

07-24-2004, 12:38 PM
Originally posted by dewickt
Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is exactly what it should be with these 3 changes.
I am new to DIY. I have replaced the bad filter caps with 80+40+20+10uF/525V from AES in my MC60. Can I use one of the unused 10uF for this Bias filter cap or use both 10uF in parallel (or just get one 22uF/450V cap for this purpose). If I replace the diode with 1N4007, what series resistor is need to replace the 1.8K resistor in MC60?

Rich Andrews
07-24-2004, 08:09 PM
Originally posted by KTT
I am new to DIY. I have replaced the bad filter caps with 80+40+20+10uF/525V from AES in my MC60. Can I use one of the unused 10uF for this Bias filter cap or use both 10uF in parallel (or just get one 22uF/450V cap for this purpose). If I replace the diode with 1N4007, what series resistor is need to replace the 1.8K resistor in MC60?

You cannot use the unused section as the polarity is wrong. This is a negative voltage that is being generated and attempting to use the unused section would likely result in the total failure of all the sections.

Don't cheap out- get the right parts.


07-24-2004, 08:13 PM
First - the bias filter has it's + end to ground, opposite of the main filter capacitor, you will have to use a seperate capacitor for this filter. Tie the unused section of the filter capacitor to the 80uf section, more is better at this point.
Second - you may have to put a series resistor at the transformer lead to the bias rectifier, try without one first and measure the bias voltage at the KT88 with a high impedence meter, most DVM are OK, it should be -45 volts. If the voltage is -50 or more you will need to add the resistor to bring it back down to -45.
I do this with no input signal, and the AC plugged into a variac to set the plate voltage on the KT88 (pin 3) to 435 volts when I measure the grid (pin 5).

07-24-2004, 11:52 PM
Thanks Rich Andrews and dewickt.
I found that I have several spare parts: a NOS CD multisections can 12+12+12uF/450V, a NOS Sprague 10+10+10uF/450V, a Solen FAST METALIZED POLYPROP 22uF/630V and a Sprague Atom 16/475V caps.
Which one is better for this filter cap? Tie all 10uF or 12uF, or just use one solen or Sprague Atom?

Rich Andrews
07-25-2004, 12:34 AM
Originally posted by KTT
Thanks Rich Andrews and dewickt.
I found that I have several spare parts: a NOS CD multisections can 12+12+12uF/450V, a NOS Sprague 10+10+10uF/450V, a Solen FAST METALIZED POLYPROP 22uF/630V and a Sprague Atom 16/475V caps.
Which one is better for this filter cap? Tie all 10uF or 12uF, or just use one solen or Sprague Atom?

If it were me, I would drop the sprague atom in and call it a day. Don't forget the + side goes to ground! The cans are worthless in this application as the can case is negative. Insulating the case so that it can be tied to the bias supply is unnecessary grief no one needs.

08-02-2004, 06:20 PM
When changing out old bias diode to a silicon (1N4007) change the 1.8K series resistor to 5.1K, check bias for -45 or slightly higher (-46) when B+ is at 435V on the KT88. If you have a fully meterd variac it should draw 1.2A at 120V.

Note: some very eary MC60 did not use a series resistor, you may have to experiment to come up with a good value for the resistor.

Rich Andrews
08-02-2004, 07:49 PM
The series resistors in the fan circuit on the MC2600 are 7 watt with a part number of 139179.

Number 9
08-07-2004, 01:07 AM
If anyone plans to work on some of the older preamps, there are some nice internal pics of the C28, 29, C32 on this Japanese website.

MC60s MC275 and C22 here

Rich Andrews
08-07-2004, 09:03 AM
There was one website that I came across some time ago that showed all of the internals for a MC275. The fellow even labeled the resistors with the values, but he got the values wrong!

08-13-2004, 03:12 PM
If your screws on the speaker barrier strips are chewed up and you want to replace them watch out - if the new screws are too long they will short to the chassis.

08-26-2004, 02:45 PM

This chart should help determine the values of Sprague bumblebee capacitors since they used coloured stripes to display their value.


09-09-2004, 03:48 PM
Got a MAC1900 in for repair the other day and it had a wadded up 2 conductor cable at the Pre Amplifier Out to Amplifier In connectors on the rear panel, seems that someone had lost the original solid loops that were used from the factory.
Most all McIntosh Integrateds use these jumpers, and a lot of other brands, need a set ?? Home Depot carries a solid #8 copper ground wire sold by the foot in the electrical department. Buy a foot or so, using some pliers a vice and a little ingenuity you can make your own that will fit and work as well as the original.

10-01-2004, 02:56 PM
Here is a useable complete alignment procedure for the MR65B, can also be use for a MR71 and MR67 with some minor part number changes.
Alignment for MR65B

IF Alignment :
Remove V11 osc.
Inject 10.70 mHz into pin 7 V2B mixer
measure at TP1 and adjust
T2, T3, T4, T5 for maximum
T6 bottom for maximum voltage at D2
(may have to misadjust T6 top to get voltage near .2 V)
T6 top set for 0 V at D2
T7 bottom for maximum voltage at D3
(may have to misadjust T7 to to get voltage near .2 V)
T7 top set for 0 V at D3

The top adjustments of T6 and T7 set the center tune points of the discriminators for audio(T7) and AFC(T6) they can have more than one 0 point, choose the ome that gives the most change if the 10.7 frequency is shifted (most sensitive).

RF Alignment:
install V11 osc
FM Signal to antenna terminals
keep signal low for good reading at TP1
at 106 mHz peak C2, C5, and calibration trim C53
at 92 mHz peak L1, L2, L7, and L11 for calibration
the calibration at 92 and 106 may have to be repeated several times to make the dial correct.

Demultiplex Alignment:
FM stereo signal to antenna center tuned
T501 for maximum voltage at V14B pin 9 (19 kHz peak)
The following should be done with the stereo signal set for L or R modulated with the other channel not modulated (L only) read output voltage at the non modulated channel output and adjust for minimum output, L501 and T502 (38 kHz Osc).
L504 is the SCA filter and is peaked for maximum seperation and SCA rejection.

If setting of T502 is in question pull V14 and set frequency at V15 pin 6 for 38 kHz by adjusting top of T502.

All adjustments in the stereo demodulator an be slightly re adjusted to maximise the seperation to a minimum of 30 dB.

A 65B in good condition can get as good as 33 to 35 dB seperation and RF sensitivity of 2 uV for 3% THD (30 dB SNR).

For improved distortion you may want to replace C75 & C77 (10 uF) at the volume control, and C74 & C76 (1 uF) at V9 and V10 with new quality components.

Rich Andrews
10-24-2004, 04:53 PM
Does your MA5100 sound dull, lack bass, behave tired - replace the 100uf (4 each) capacitors on the preamp board. If yours has white Mallory caps in it, replace them as they will fail with age.

Also replace the 1.5 uf output coupling caps on the filter board. The filter board is the smaller board on the top side of the chassis.


11-18-2004, 03:29 PM
While giving a MR65B the work over on the bench I found that I could not get better than 28dB of separation, I remembered a modification used in the MR71. Since all three models use the same basic demultiplex and output stages I figured that the modification would help my separation problem.

Connect a 27K 5% 1/2W resistor and a .0015mF 10% capacitor in parallel. Connect this network between pin #8 on one 6BL8 audio tube to the pin #8 on the other audio tube.

Amazing --- the MR65B now has over 40 dB of separation just like the late production MR71, and a definite increase in sound quality…..

This modification makes up for a slight loss in the L-R portion of the composite signal caused by the SCA filter by increasing the difference signal gain in the left and right audio amplifiers while leaving the common signal gain unchanged.

M Jarve
01-12-2005, 02:50 PM
If you have a MA-5100 outputting severe DC at the speaker terminals, and it seems intermittent (or not) look at replacing C207 and C208 on the power amp boards. The cap may be passing voltage, causing devastating DC offset.

02-11-2005, 12:02 AM
When replacing the selenium diode in the bias supply with a silicone diode (1N4007) use a 470 ohm resistor in series with the transformer lead going to the diode, this should put the bias voltage on the button !! While you are at it replace the bias filter, a 22uF 450V cap will stiffen it up and last forever.

02-14-2005, 08:28 AM
I'm currently restoring a pair of MC-40's and would like to replace the selenium bias rectifier with a silicone diode (1N4007) as you have suggested here. My question is which resistor should I change to get the proper voltage and what value should the resistor be?

There is a -150V coming from the selenium going to the cathods of a 12AX7. Then a 100K resistor drops it down to -52V going to the grid of the same tube. The 100K is followed by a 56K to ground.

I've changed the bias filter from a 12uF to a new 10uF.

Any advise would be helful.

02-14-2005, 11:37 AM
I have 2 MC40's to do this to waiting on the shelf, the resistor will be in series with the diode and the lead from the transformer. The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5 of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470 before I measure pin 5.

02-14-2005, 12:52 PM
Thanks for the advise.

I saw on your post on the MC-240 that you suggest a 22uF bias filter at 450V. I currently have a 10uF in there, should I change it to 22mF?


02-14-2005, 01:20 PM
The 22 uF will stiffen the bias supply slightly, not critical but a little better with the 22uF

03-17-2005, 10:15 AM
Interesting problem, sudden occurrence of Buzz (60HZ) in output of a C712, at the same time I noticed all of the lamps were burned out, put 2 and 2 together and realized that if all the lamps are out the lamp leads will radiate 60HZ into the circuit board at the front panel. Replace one bulb and Buzz vanished, five #7373 and one #7382 lamps later the front panel looks like new and the Buzz is gone. Keep the lamp wiring as far as possible from the circuit board, keeps residual noise down.

06-15-2005, 09:12 AM
Many of the McIntosh tuners use a illuminated pointer, if yours is no longer the red pointer it used to be it probably is not a burned out bulb. This pointer uses a red filter and sometimes a white diffuser between the lamp and the slot on the pointer, pull out the filter and check to see if it is burned, if so turn it end for end and use the unburned end as the new filter and you will have your red pointer back. You will have to remove the dust cover and front panel to gain easy access to the dial pointer; a small pair of needle nose pliers will be needed to extract the filter.

06-21-2005, 02:56 PM has some good advice on the MAC-1700, which shoul apply to other units as well. Check out
It reads:
McIntosh Mac-1700 Receiver
This is a receiver of the early '70's which has the distinction of being a hybrid. The tuner is tube while the preamp and power amp are solid state. It is not large, weighing in at 45 watts rms/channel but will power relatively efficient speakers very well. These will require all of the usual cleanups of controls, tube sockets, tuning capacitor, and switches. The tuner will often require tubes and alignment. They have rather large capacitors for positive and negative power supply and these often go bad, causing hum or distortion in both channels. There are other supply caps which are prone to failure. MAC1700's have been known to occasionally pop their supply fuses, (located in the fuse block underneath) and they will sometimes damage speakers due to an intermittent dc offset problem (dc appears at the speaker terminals). This is caused by the differential amp transistors on the small driver boards underneath. These must be replaced on every machine, regardless of what other repairs are needed. After all this is done, do the cosmetic to make it beautiful and enjoy.

06-29-2005, 09:20 AM
The MR80 is the best of the analog tuners, immune to overload, selective, lots of separation, and good sound. It does have some quirks, especially with the capacitive touch controls for the tuning. If your MR80 behaves strange when you use the manual tune - will not go into stereo - hangs the preset tunes - or just acts strange when you touch a control on the panel, it may have a very simple fix. Unplug the tuner, rotate the plug 180 degrees, plug it back in, good bet your problems are solved !!

Masonic Man
08-13-2005, 05:10 PM
Greetings Everyone:

Iam looking to put external meters on my MC2200 amp. Does anyone know if the blue bulb is a McIntosh proprietary (sp) color or locally available at Radio Shack? And has anyone made/purchased a meter kit that will accomplish this goal?


08-13-2005, 05:33 PM
The McIntosh blue meters are made blue by using blue filter material between the lamp and the meter and to keep the blue look when turned off it is also between the meter and the glass front.
I use blue vinyl window tint material to achieve the effect.

09-04-2005, 01:40 PM
I have 2 MC40's to do this to waiting on the shelf, the resistor will be in series with the diode and the lead from the transformer. The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5 of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470 before I measure pin 5.
Posting here mainly because I have not found this answer elsewhere - even after extensive searching.

On the MC-30s, what is the bias voltage value and where is it best measured?

I believe it should be -52 volts at the grids of the 12AX7 used to drive the 6L6s. Correct, Terry?

Reason I ask is that there are minor differences in the bias circuit topology:

* One version of the A-116/MC-30's power transformer uses a separate winding to drive the bias circuit.
- The power transformer's label indicates that its a M-152-D

* Another version of the A-116/MC-30 derives its bias voltage from the high-voltage winding of the power transfomer
- power transformer on mine is labeled a M-152-B
- I note that the schematic I have includes drawings for an M-152-C which also shows bias being derived from the high-voltage windings.

Figures I have one of each to restore doesn't it?

So, Terry, any help you can provide on this detail is greatly appreciated.



09-04-2005, 02:25 PM
All three of my MC-30's have the 152B transformer so no help on the other one. Pin 2 of the second 12AX7 should be -42v. This from the "Instruction Manual" for MC-30, Type A-116B.

My MC-30's, well, 2 of them, show about -40V here, IIRC, with 117v input.


09-04-2005, 10:17 PM
440V on the plate and -42V on the grid of the 6L6, power transformer type is of no consiquence, replace the selenium and change the value of the resistor in series with it to set the bias, about 9.1K should get you close, a value either way may be needed to get it right on. I find anywhere from -42 to -43 is good when plate is at 440V..

10-21-2005, 09:59 PM
MC2300 was produced in 4 basic production runs that can be identified by the first 2 parts of the serial number.
1Y--- , first run, no themostats on fans (noisy), red power switch.
2Y--- , Had thermostat controlled fans, red power switch.
3Y---, and 4Y---, had Had thermostat controlled fans, white power switch (some 3Y had red).

There were also several possible variations within a run that were special ordered, like the MC2300C that has a 64 ohm output and no .5 ohm it uses a standard MC2300 top cover and front panel with a new rear panel showing the changes.

01-17-2006, 06:24 PM

Please let me know if you can help me on my MC30 problem.
I read that some months ago you received a question about high red glowing (and therefore high sound distortion) in a 6L6 tube, is there a solution out of the shelf since it is a repetitive problem? I would apreciate your advice.

Regards. Maquico.

01-17-2006, 08:35 PM
Need to check the bias on the tube, possibly replace the bias rectifier and filter, check the answers above about the bias update, if bias is OK you may have a transformer problem.
No off the shelf parts kits that I know of to do the update.

01-18-2006, 04:17 PM
Thanks a lot for your answer.
In fact while waiting for help I changed the whole tube set for the MC30 for a new one and the result was that one of the 6L6 (metal) went well over heated, (the other one not) and the sound distortion become a terrible noice just for blowing up the speaker, is your diagnosis the same with these sintoms?


01-18-2006, 05:11 PM
Same problem different tubes, start checking voltages or get it to a tube man for a check up.

01-21-2006, 11:01 PM
McIntosh Electronics Production History (1949 to 1999)
T=Tube, SS=Solid State, W=Watts, AF=Autoformer, PG=Power Guard, DC=Direct Coupled, SL=Slim Line

Model Product Type Description Years Price
15W-1 Power Amplifier T 15W 1949 (1 year) $149.50
50W-1 Power Amplifier T 50W 1949-1950 $249.50
20W-2 Power Amplifier T 20W 1951-1954 $149.50
50W-2 Power Amplifier T 50W 1951-1955 $249.50
A-116 Power Amplifier T 30W 1953-1954 $143.50
MC-30 Power Amplifier T 30W 1954-1962 $153.50
MC-30A Power Amplifier T 30W-with DK-30-60 damping kit 1954-1962
MC-60 Power Amplifier T 60W 1955-1961 $219.00
MC-60A Power Amplifier T 60W- aith DK-30-60 damping kit 1955-1961
MC240 Power Amplifier T 40-40W 1960-1969 $288.00
MK30 Power Amplifier T 30W Kit 1960-1961 $120.00
MC225 Power Amplifier T 25-25W 1961-1967 $198.00
MC75 Power Amplifier T 75W 1961-1970 $229.00
MC275 Power Amplifier T 75-75W 1961-1970 $444.00
MC40 Power Amplifier T 40W 1962-1979 $159.00
MC250 Power Amplifier SS 50-50W-AF 1961-1971 $429.00
MC2505 Power Amplifier SS 50-50W-AF-Meters 1967-1977 $549.00
MC2105 Power Amplifier SS 105-105W-AF-Meters 1967-1977 $799.00
MC3500 Power Amplifier T 350W-Meter 1968-1971 $1,099.00
MC2100 Power Amplifier SS 105-105W-AF 1969-1977 $599.00
MC50 Power Amplifier SS 50W-AF 1969-1970 $199.00
MC100 Power Amplifier SS 105W-AF 1970-1973 $299.00
MC2300 Power Amplifier SS 300-300W-AF-Meters 1971-1980 $1,799.00
MC2205 Power Amplifier SS 200-200W-PG-AF-Meters 1975-1979 $1,649.00
MC2120 Power Amplifier SS 120-120W-PG-AF 1976-1982 $999.00
MC2125 Power Amplifier SS 120-120W-PF-AF-Meters 1976-1981 $1,199.00
MC2200 Power Amplifier SS 200-200W-PG-AF 1976-1982 $1,199.00
MC502 Power Amplifier SS 50-50W-PG-DC-Slimline 1978-1989 $1,200.00
MC2500 Power Amplifier SS 500-500W-PG-AF-Meters 1980-1990 $3,800.00
MC2255 Power Amplifier SS 250-250W-PG-AF-Meters 1981-1986 $2,995.00
MC2155 Power Amplifier SS 150-150W-PG-AF-Meters 1981-1986 $2,295.00
MC2250 Power Amplifier SS 250-250W-PG-AF 1982-1987 $2,495.00
MC2150 Power Amplifier SS 150-150W-PG-AF 1982-1987 $1,925.00
MC2002 Power Amplifier SS 200-200W-PG-DC-Meters 1984-1988 $1,850.00
MC752 Power Amplifier SS 75-75W-PG-DC 1985-1988 $895.00
MC7270 Power Amplifier SS 270-270W-PG-AF--Meters 1985-1991 $2,499.00
MC754 Power Amplifier SS 100-100W-PG-DC 1988-1992 $895.00
MC7200 Power Amplifier SS 200-200W-PG-DC-Meters 1989-1993 $2,779.00
MC2600 Power Amplifier SS 600-600W-PG-AF-Meters 1990-1995 $6,000.00
MC7150 Power Amplifier SS 150-150W-PG-AF-Meters 1991-1994 $2,400.00
MC7300 Power Amplifier SS 300-300W-PG-AF-Meters 1991-1995 $3,400.00
MC7100 Power Amplifier SS 100-100W-PG-DC-Slimline 1992-1997 $1,400.00
MC1000 Power Amplifier SS 1000W-Mono-PG-AF-Meter 1992- $6,500.00
MC7106 Power Amplifier SS 160W x 6 @ 4 Ohms-PG-DC 1992- $3,500.00
MC275 Power Amplifier T 75-75W-Commemoritive 1993-1996 $4,000.00
MC500 Power Amplifier SS 500-500W-PG-AF-Meters 1994- $7,000.00
MC7108 Power Amplifier SS 50W x 8 @ 4 Ohms-PG-DC 1994- $2,500.00
MC7104 Power Amplifier SS 100W x 4 @ 4 Ohms-PG-DC 1995-1997 $2,750.00
MC150 Power Amplifier SS 150-150W-PG-AF-Meters 1995-1997 $3,000.00
MC300 Power Amplifier SS 300-300W-PG-AF-Meters 1995-1998 $3,750.00
MC352 Power Amplifier SS 350-350W-PG-AF-Meters 1998- $4,500.00
MC7205 Power Amplifier SS 200W x 5 @ 4 Ohms-PG-DC 1998-
MC2000 Power Amplifier T 130-130W-Meters-50th Anniversary 1999- $15,000.00

McIntosh Electronics Production History (1949 to 1999)
T=Tube, SS=Solid State, W=Watts, AF=Autoformer, PG=Power Guard, DC=Direct Coupled, SL=Slim Line

Model Product Type Description Years Price
AE-2 Preamplifier T Mono 1950-1952 $79.50
C-104 Preamplifier T Mono 1950-1954 $59.50
C-108 Preamplifier T Mono 1954-1955 $86.50
C-4 Preamplifier T Mono 1954-1959 $64.50
C-8 Preamplifier T Mono 1955-1959 $88.50
C-8S Preamplifier T Mono-Stereo Controls 1958-1960 $99.00
C20 Preamplifier T Stereo 1959-1963 $269.00
C11 Preamplifier T Stereo 1961-1963 $209.00
C22 Preamplifier T Stereo 1963-1968 $279.00
C24 Preamplifier SS Stereo 1964-1968 $249.00
C26 Preamplifier SS Stereo 1968-1978 $449.00
C28 Preamplifier SS Stereo 1970-1978 $649.00
C27 Preamplifier SS Stereo 1977-1983 $749.00
C32 Preamplifier SS Stereo 1977-1981 $1,649.00
C29 Preamplifier SS Stereo 1978-1985 $1,399.00
C504 Preamplifier SS Stereo-SL 1980-1988 $1,090.00
C33 Preamplifier SS Stereo 1981-1985 $2,450.00
C30 Preamplifier SS Stereo 1985-1987 $1,649.00
C34V Preamplifier SS Stereo 1985-1992 $2,499.00
C31V Preamplifier SS Stereo-Remote Control 1987-1989 $1,895.00
C35 Preamplifier SS Stereo-Remote Control 1989-1992 $1,995.00
C37 Preamplifier SS Stereo-Remote Control 1991-1992 $2,399.00
C100 Preamplifier SS Stereo-Remote Control-2 Chassis 1997- $6,500.00
C15 Preamplifier SS Stereo-Remote Control 1998- $1,500.00
C36 Audio Control Center SS Stereo 1992-1995 $1,700.00
C38 System Control Center SS Stereo-Remote Control 1992- $2,500.00
C712 System Control Center SS Stereo-Remote Control-SL 1993-1997 $1,500.00
C39 A/V Control Center SS 6 CH-Remote-Dolby-PL 1993-1998 $4,000.00
C40 Audio Control Center SS Stereo 1992-1998 $3,500.00
MX130 A/V Tuner Control Center SS 6 CH-Remote-Dolby-PL 1993- $4,800.00
MX132 A/V Tuner Control Center SS 6 CH-Remote-Dolby-AC3-PL-DTS 1998-
THX-M THX Add-On Module SS THX PL Enhancement 1993- $500.00
MX118 A/V Tuner Control Center SS 6 CH-Remote-Dolby-PL 1994-1997 $3,000.00

McIntosh Electronics Production History (1949 to 1999)
T=Tube, SS=Solid State, W=Watts, AF=Autoformer, PG=Power Guard, DC=Direct Coupled, SL=Slim Line

Model Product Type Description Years Price
MA230 Integrated Power Amplifier T-SS 30-30W-Tube Pwr-SS Preamp 1963-1966 $349.00
MA5100 Integrated Power Amplifier SS 45-45W-DC 1966-1972 $449.00
MA6100 Integrated Power Amplifier SS 70-70W-DC 1972-1979 $699.00
MA6200 Integrated Power Amplifier SS 75-75W-DC-PG 1978-1991 $1,899.00
MA6400 Integrated Power Amplifier SS 100-100W-DC-PG-Meters 1995- $3,000.00
MA6800 Integrated Power Amplifier SS 150-150W-PG-AF-Meters 1995- $5,000.00
MA6450 Integrated Power Amplifier SS 100-100W-DC-PG-Meters 1998-
MA6850 Integrated Power Amplifier SS 150-150W-PG-AF-Meters 1998-

MAC1500 Receiver T/SS 30-30W-Tube Pwr-Tuner-SS Preamp 1965-1967 $499.00
MAC1700 Receiver T/SS 40-40W-SS Pwr- Tube Preamp-Tuner 1967-1973 $599.00
MAC1900 Receiver SS 55-55W-DC 1973-1978 $949.00
MAC4100 Receiver SS 75-75W-DC-PG-Pwr LED's 1978-1985 $1,999.00
MAC4200 Receiver SS 75-75W-DC-PG 1985-1987 $2,890.00
MAC4275 Receiver SS 75-75W-DC-PG 1988-1990 $2,099.00
MAC4280 Receiver SS 75-75W-DC-PGp\-Remote Control 1988-1991 $2,099.00
MAC4300V Receiver SS 100-100W-DC-PG-Pwr LED's-Remote 1988-1993 $2,995.00

McIntosh Electronics Production History (1949 to 1999)
T=Tube, SS=Solid State, W=Watts, AF=Autoformer, PG=Power Guard, DC=Direct Coupled, SL=Slim Line

Model Product Type Description Years Price
MR55 Tuner T AM-FM-Mono 1957-1959 $249.00
MR55A Tuner T AM-FM-Mono 1959-1962 $276.00
MR66 Tuner T AM/FM Simulcast, Stereo Add-On 1960-1961 $325.00
MR65 Tuner T FM-Mono 1960-1962 $225.00
MR65A Tuner T FM-Mono-Stereo Add-On 1962-1966 $284.00
MR65B Tuner T FM-Stereo 1962-1964 $329.00
MR67 Tuner T FM-Stereo 1963-1968 $299.00
MR71 Tuner T FM-Stereo 1963-1969 $399.00
MR73 Tuner SS AM-FM-Stereo 1969-1971 $549.00
MR74 Tuner SS AM-FM-Stereo 1972-1979 $849.00
MR78 Tuner SS FM-Stereo 1972-1979 $1,699.00
MR75 Tuner SS AM-FM-Stereo 1980-1985 $1,349.00
MR80 Tuner SS FM-Stereo-Digital 1980-1985 $2,499.00
MR500 Tuner SS FM-Stereo-Digital-SL 1983-1985 $1,499.00
MR510 Tuner SS FM-Stereo-Digital-SL 1986-1990 $1,699.00
MR7082 Tuner SS AM-FM-Stereo-Digital 1986-1990 $1,579.00
MR7083 Tuner SS AM-FM-Stereo-Digital 1990-1995 $1,750.00
MR7084 Tuner SS AM-FM-Stereo-Digital-SL 1996- $1,500.00

MX110 Tuner-Preamplifier T FM-Stereo 1962-1969 $399.00
MX112 Tuner-Preamplifier SS AM-FM-Stereo 1968-1971 $599.00
MX114 Tuner-Preamplifier SS FM-Stereo 1969-1971 $549.00
MX113 Tuner-Preamplifier SS AM-FM-Stereo 1971-1980 $1,099.00
MX115 Tuner-Preamplifier SS FM-Stereo 1972-1974 $599.00
MX117 Tuner-Preamplifier SS AM-FM-Stereo 1980-1984 $1,649.00

McIntosh Electronics Production History (1949 to 1999)
T=Tube, SS=Solid State, W=Watts, AF=Autoformer, PG=Power Guard, DC=Direct Coupled, SL=Slim Line

Model Product Type Description Years Price
MCD7000 CD Player SS Single Disk 1985-1987 $1,399.00
MCD7005 CD Player SS Single Disk 1987-1988 $1,599.00
MCD7007 CD Player SS Single Disk 1988-1994 $2,000.00
MCD7008 CD Player SS 7 Disk Changer 1992-1996 $2,200.00
MCD7009 CD Player SS Single Disk 1995- $2,750.00
MCD7010 CD Player SS Single Disk 1998-
MLD7020 Laser Video Disk Player SS 2 Side Auto Play- AC3 RF Out 1995- $3,200.00

MI-2 Performance Scope T Multipath Indicator 1963-1964 $219.00
MI-3 Performance Scope T Multipath Indicator 1964-1971 $249.00
MPI-4 Performance Scope SS Multipath Indicator- Audio Scope 1973-1977 $599.00
MQ101 Equalizer SS 3 Band EQ for ML Speakers 1970-1978 $250.00
MQ102 Equalizer SS Bass EQ for ML Speakers 1970-1978 $74.50
MQ104 Equalizer SS 4 Band Variable EQ 1977-1991 $500.00
MQ107 Equalizer SS 7 Band Variable EQ 1981-1993 $650.00
MQ108 Equalizer SS 7 Band Variable EQ-Balanced I/O 1994-1996 $500.00
SCR-1 Speaker Relay Switch 2 Pairs Speakers 1970-1971 $49.50
SCR-2 Speaker Relay Switch 2 Pairs Speakers-AC Switch 1971-1991 $200.00
SCR-3 Speaker Relay Switch 2 Pairs Speakers-AC Switch 1991- $219.95
MVS-1 Video Switcher Composite 1986-1992 $299.00
MVS-2 Video Switcher Composite-S Video 1991-1996 $375.00
MVS-3 Video Switcher Composite-S Video-Audio 1992- $700.00
CR10 Audio Multizone System SS 4 Zone Audio 1992-1995 $2,000.00
CR12 A/V Multizone System SS 4 Zone Audio/Video 1995- $3,300.00
MAC-3 Dolby Digital Decoader SS AC-3-Pro Logic-Remote Control 1997- $1,800.00
MSD-4 AC-3 RF Demodulator SS Accessory for MX132 1998-

01-30-2006, 03:52 PM
Same problem different tubes, start checking voltages or get it to a tube man for a check up.
Now at last and with great help from Terry I have compiled a great deal of information to repair my MC30 (heat on 616 and bussy noice) but before initiate de ordeal to find a tube man I found that there are several modifications (at least 30 years ago) made in the amp, the power resistors, bias cap, diode, the three horizontal caps on resistors board and most visible the can cap and amp resistors, some of these modifications recommended by Terry, but not the can cap which in this case is substituted by three huge capacitors 500vd 60pf Dubilier drilitie. Now I beg for your recommendation, should I persue only on repairing the actual status or reinstalling a new can cap and amp resistors or any other more desirable update?

Thanks in advance

03-20-2006, 03:08 AM
You guys are insane, well at least I know where to send my MX110 preamp when it needs work. I would be afraid to change a light bullb in the thing, let alone the stuff y'all are talking about.

My question is more towards the limits of my technical know-how.

No. 1.: What do you reccomend for cleaning/preserving the chrome chassis on an MX110 that will not harm the lettering.

No. 2: What brand/equiv. tubes do you reccomend for V16 and V17 (6U8). My are the original telefunken and one of them has developed pops and noise confirmed by switching them. I have replaced the marginal tube with a NOS USA tube but it is a bit noisy. Have had a very difficult time locating 6U8 NOS Telefunkens.

Thanks in advance, and sorry these questions are so basic compared to bias voltage, bumble bee cap. color codes, etc.


03-20-2006, 08:27 AM
McIntosh chrome responds nicely to a good car wax, I use Turtle wax Carnuba paste, just don't rub real hard on the lettering, and also gives the black paint a nice shine. If your chrome is getting tired with pits or hazing you can use Wenol metal polish (, it can help revive the chrome to be as good as it can be. Whatever you do use a clean soft rag to do it, keep using a clean section as the dirt it picks up can scratch the chrome leaving swirls and hazing.
No real help for the 6U8 except that a 6EA8 or 6GH8 will also work, check for a previous thread on this.

Audible Nectar
03-20-2006, 08:38 AM
The 6U8 is a bit of a "buggy" sort of tube.....I go through a few sometimes in certain brands before I find a linestage quality pair that will work well without shorting or microphonics.

I have had very good luck with the Seimens 6U8, as well as the RCA (but the RCA's seem better in the earlier vintages). I buy 6U8 by the sleeve, since I know that 2-3 out of 5 may not be linestage suitable.

Best advice I can give on 6U8 linestage tubes in the MX110 is that once you find a good pair that you like and work well - leave 'em. Not a spot for "roll happy" folk, based on my experiences.

03-21-2006, 08:22 PM
How do I clean the laser on a Mc 7005? I posted this in another thread, but this one seems to attract the real expert (Terry D.) Thanks, I need allthe help I can get.

03-21-2006, 09:11 PM
I clean the optics with a cotton swab dampened with household ammonia, be careful as the optics mount is delicate.

04-28-2006, 06:27 PM
Hi all,

I'm a newbie so please forgive me if I'm dropping this in the wrong
section. I have a C32 preamp I bought about a year ago and as of
late it's doing something very strange.

After about 15 or 20 minutes the volume will ramp up to full output!
(moving the volume knob has no effect when this happens). Is there
something I could check before I send this unit out for service?
Could it be a simple ground problem from a solder joint? And have
any of you heard of this happening with a C32?


04-29-2006, 12:18 AM
C32 had cable problems, it uses a variety of ribbon cable that gives problems, both delaminating and corrosion under some humidity and temperature conditions. Check all the cables especially the ones at the volume control, if you find a bad one the best fix is to use ribbon cable (computer style) and hard solder to the board - will never have a problem again.

04-29-2006, 09:51 PM
I clean the optics with a cotton swab dampened with household ammonia, be careful as the optics mount is delicate.
Have you ever heard of the 'lint-free' swabs used by the semiconductor industry? They utilize what looks like foam/sponge-wrapped plastic sticks for the purpose of avoiding any cotton fiber shedding, etc. However, having worn plastic lens glasses for the last 31 years, I'm also aware that wet cotton doesn't scratch plastic like most synthetic fibre can. (Besides, lint-free products are probably prohibitively expensive, so I usually get free sample packs from the vendors at trade shows.

Also, was wondering why ammonia rather than alcohol? More streak/residue-free? Would this apply to tape heads as well (cassette & vcr)?

04-29-2006, 11:47 PM
Most dirt I find on the transport optics does not require alcohol type of solvents, smoke, dust and such are best cleaned with a gentile cleaner, Reel to Reel is a different matter as the tapes leave behind nasties that require stronger solvents.

05-29-2006, 09:48 PM
I bought an MR-74 and it was in excellent condition except that the stereo light bulb was out. It was a 1835 bulb, 55 V. The wiring diagram indicates 40 V on the circuit. Apparently there are two versions of the stereo light-one with an 1828 bulb (37.5 V) with a 270 ohm 1 watt resister and one like mine which uses an 1835 bulb and the resistor was replaced by a jumper. The previous owner said all bulbs were working and, considering the condition and function, I am inclined to believe him. Even though it was very well packed with double boxes, I thought it might have suffered vibration during shipment. I replaced the 1835 bulb with a new 1835 bulb. The circuit worked as expected and I cycled it by tuning different stations numerous times. Although the light appeared dim to me, I have never owned an MR-74, so I assumed that was the way it is supposed to be. The light appeared to be steady without flickering and I had the tuner on for several hours. Later, I noticed that the bulb was out again. I am not sure if the bulb went out in that first session or if I had turned the tuner off and back on. A local tech friend of mine, with no experience with older McIntosh equipment, suggested that it was nothing more than a bad bulb.
Before I try another bulb, I thought I would ask the group if there is an issue with this stereo light, if anyone has any thoughts about why this bulb burned out so quickly, and if there is a solution other than just replacing the bulb again.
Thanks, Ed

05-29-2006, 10:08 PM
Hopefully, I can get this posted before Terry shows up with the right answer. :D

I don't have the sheet for a 74 but the mpx lamp is driven by Q306 and you should have 40v at the collector when a stereo program is detected (I'm looking at the sheet for a 77). Then, going backwards there is Q305, a diode and then T301.

It should be fun doing a little detective work.


05-30-2006, 12:17 AM
OK, here we go, the 73-74-77 are basically the same demux, if it has the 270 ohm resistor use a #1828 (37V), if no resistor use a #1835 (55V). Either lamp has a slim long filament that hates vibration, and looks for an excuse to break.
Have gotten several tuners with a #47 in them, is bright for a while till the 270 ohm burns open after the transistor switch shorts.

05-30-2006, 07:09 AM
Thanks Murray and Terry for your prompt replies. I am relieved to learn that the bulb is the problem. I had checked the other components but was worried I had missed something. ..........Ed :yes:

09-14-2006, 04:04 PM
If one channel of your MAC1900 is running hotter than the other it might be a good idea to check R415/R416 on the driver board, it must not be over 22 ohms. Had a 1900 that the 22 ohm R415 measured 27 ohms, the channel ran hot and would go into shut down after an hour. Replaced with a measured 21.8 ohm resistor, and heatsink barley get warm after 2 hour idle time...

10-05-2006, 01:14 PM
Assuming someone else might make the same mistake I did, I thought I would post it here: If you replace the voltage doubler caps in the MC240 (or similar) with non-twistlock caps, don't forget to ground the positive cap to the chassis...

Also in my MC240, I used the IXYS DSEI 8-06A HexFRED as my replacement bias diode and that got the voltages close but still a little high at -55V (instead of -48) and -155 (instead of -150). Any idea what resistance value I need in series with this diode to get them in check? Also, is 1/2W resistor sufficient? I couldn't figure out the current draw from the schematic and I don't really want to do trial-and-error or use an underrated pot. Great thread, thanks for all the help!


10-05-2006, 04:59 PM
Well I just did some tube datasheet reading and some rechecking of my bias voltages. Maybe this info will help others using 5751s in place of 12AX7s in a MC240.

It appears that the 5751 can handle more grid voltage than the 12AX7s - specifically, -55V max, -50V design, whereas most of the 12AX7s I checked are -50V max. I measured my pin 2 voltages on my GE 5751WAs and have -50V on the dot so I don't think I need a bias resistor afterall since last time I tested the tubes, they were as new. The IXYS diode is close enough for these tubes. All the other tube voltages were at or slightly lower than spec.

If someone else is using this IXYS diode with 12AX7s, a "correcting" bias resistor value would still be helpful to post here. Maybe I will go back to 12AX7s down the road myself. Thanks


10-31-2006, 11:51 AM
Any advice on cleaning older 60's and 70's Mac gear would be appreciated. Specifically the cleaner to use for cleaning the brown transformer covers and such like so that I don't remove the silk screened lettering. Any advice on what cleaner to use for cleaning light rust and oxidation from the chrome.

10-31-2006, 01:41 PM
For chrome I use Wenol (, it gets the rust and oxidation. For the glass I use Turtle Wax Carnuba, can use it on painted surfaces just don't rub it a lot or you will end up with a beautiful gloss finish. For general dirt I use clear Ammonia, is very gentile on lettering, both the Wenol and Turtle Wax can eat at painted lettering if you rub to hard, the metal ID/information plates are safe.. Do not use Windex, or 409 type cleaners as they attack lettering and painted surfaces.

10-31-2006, 04:29 PM
I've followed Terry's suggestion about using auto wax on the chrome, glass and painted surfaces. It looks great and a side benefit is that it seems to repel fingerprints a little better and makes the surfaces easier to clean.

Wenol is indeed great for getting the corrosion off of chrome. It'll (as will most cleaners) get your stamped ink serial number off too. :tears: Does a nice job on zinc plated hardware.


10-31-2006, 11:08 PM
A rag and some 3/8 hose pushed over an RCA with some Wenol, twist till it shines like chrome....

11-18-2006, 03:26 PM
MAC 4300 question for you Terry: The unit did not come with jumpers for pre-amp to amp when I bought it new back in 1988. Should I get some anyway? Why would some units need jumpers and others not?


11-18-2006, 06:12 PM
The MAC4100 and up do not need external jumpers for their preamp to amplifier connection, they have internal contacts in the RCA connectors that if an RCA is plugged into them the connection between the amp and preamp is automatically opened. This is a nice feature that helps keep the clutter on the rear panel a bit less, but if a unit has been in a damp area and has some corrosion problems these switching contacts have been known to fail necessitating the old style external jumpers.

11-25-2006, 11:34 AM
Are the phono RCA shorting plugs on the back of a preamp/MX really helpful? Should I bother with replacing missing ones?

12-08-2006, 04:10 PM
Are the phono RCA shorting plugs on the back of a preamp/MX really helpful? Should I bother with replacing missing ones?

No :thumbsdn:

12-15-2006, 12:26 AM
After having all the bad bulbs replaced, including the stereo indicator, the stereo indicator now ALWAYS glows red, but it does get brighter when tuning in the station.

Why doesn't it stay dark and glow only when tuning?

12-18-2006, 08:03 AM
I just picked up a MC2205 this weekend at a garage sale (along with a C28 and a MR71 that is in rough shape) and noticed a couple of things that weren't working:

1. The 2 right channel lights in the Output Mode box do not light (Limit, Normal).

2. The right channel big blue analog meter doesn't appear to be in synch with the left channel when the Meter Range knob is set to Watts mode. When set to any of the Decibel modes it funtions normal.

One last question on what you use to treat the Walnut cases with. I have used Formsby Lemon Oil on them and they have definitely improved, but they are somewhat soaking up the oil in a hurry.


12-18-2006, 09:24 AM
I recently purchased a C2200 preamp and the left meter sometimes sticks while playing music. The right side meter registers output normally, but the left will sometimes register a certain output and kind of just stay there. Other times it works normally. Is this something that has to be repaired by Mcintosh or is there something I could do to adjust the meter?

12-23-2006, 08:45 AM
McIntosh will sell you a new shipping box for most all models, better to be safe with a new box to protect vintage gear when shipping. If your old shipping box is in good condition but starting to look tired with bunches of old labels and stickers remove them and the old tape with a heat gun or hair dryer, heat will release the glue and let you peel them off without damaging the box.

12-27-2006, 09:06 AM
HI Terry, Hope you got my email. The mac 1900 receiver has gone dead! The line fuse is ok! I borrowed the good pwr output transistor from the good channel and placed it in the place of the shorted one the 5 a amps fuse opened again! The strange thing is that the pwr pc boards are ok because when i swapped the board from the one from the channel that the fuses were opening in the good channel to pwr pc board worked very well. My limited testing with the meter the has capcitance check on it found that one of those large can power supply GE 9300uf/50VDC cap was not showing the right cap and would not register any reading most of the time. Also what problem would there to be no sound coming from the fm section , the signal strength meter moves when the tunning dial is moved across the fm dial , the tunning meter dont moves and no sound as mentioned from the fm section. This was before the unit when dead! Do you think because one those large can power supply shows bad reading it might have to do anything with the unit to go dead?! Also should I check the diode in the 5a fuse circuit by desoldering them?!! YOUR KIND! HELP IS GREATLY APPRECIATED! Thanks! Ali

01-29-2007, 12:29 AM

02-16-2007, 07:39 AM
On the MCD7800, should I use fixed or var? :scratch2:

02-24-2007, 12:25 PM
I have a Mac 1500 that was a gift and one of the knobs is missing. It is a Base control with double knobs on the same contorl. If any one has a source for these knods I would appricate any feed back.

02-25-2007, 11:48 PM
Knobs are hard to find, McIntosh has very few older model ones, sometimes Audio Classics has some so give them a call to find out. I hate to say it but the last resort for odd parts can be eBay, one never knows what will show up.

03-09-2007, 03:01 PM
Hi everyone. Since I rebuilt my MC240, I have been wanting to make sure that I wasn't overdriving any tubes. Replacing the old diodes with new HEXFREDs changed the voltages a little bit so I have been verifying in several ways that the changes are ok. I thought a DIY rebuilder would be interested in this information and/or would like to add to it.

I have 3 different schematics for MC240, two of which have different voltages listed. I used the IXYS DSEI 8-06A HexFRED instead of the 4007 diode and the IXYS is very close to the original one except for a bump in the negative grid voltage of the 12ax7 drivers. This was the main thing I was worried about so I set off to find out if it was a problem.

One schematic shows the following plate voltages:
split 12ax7: 103
12au7: 205
12BH7: 350
driver 12ax7: 430
6L6: 430

The other one shows
split 12ax7: 108
12au7: 275
12BH7: 355
driver 12ax7: 430
6L6: 430

Note the large difference on the 12au7s. Both schematics show the negative grid voltages for the 12ax7 and 6L6 to be -46V. Mine measure at -51V with no load and the new diodes. Data sheets show that some 12ax7s are limited to -50. However, EH 12ax7as and 5751s are good to -55. Also, the MC40 amps run the 12ax7s even harder than the 240: the negative grid voltage is set at -52. So it looks like the IXYS is an excellent choice since it doesn't need any voltage adjustment and you get the benefits of latest technology.

03-10-2007, 02:44 PM
Burned R19 and R20 12K's, fried R17 1.2 cathode resistor and half the 12bh7 gone- Terry warned about the 12k's going bad. So have rebuilt the power supply filter. Put in the IN4007 diode and series 9.1K resistor. Decided C13 10uF on the serial number 15329 and above schematic is the bias filter capacitor he refers to in recommending replacement with a 22uF. Replacing the burned and low resistors seems to have gone well. Bought Hovland Musicaps and between the red and green leads on them, the banding and orientaion of the original bumlebees ,(most of which fail testing with a Heath IT-11) and the schematic there seems to be places where there are conflicts in how to orientate the red high end of the Hovlands. So I figure the designers orientated the caps right so which end of the Hovlands red or green correlates with the banding on the Spragues? Advice appreciated-really miss listening to this thing. Did not want to mess with this either. Was hoping to have more time to work my way up to it.

03-11-2007, 01:07 PM
Problem resolved. The old Mc-30 is singing away as I type. Correlated the schematic to the capacitor orientation-tied myself up in knots trying to do otherwise plus a good nights sleep doesn't hurt.. Checked the original can 15/35/80uf and they were above 90%. The 15 didn't show any leakage but the 35 and 80uF sections did. Seems to like the new Spague atoms in the power filter.

03-11-2007, 02:24 PM
Don't worry about the C12c, the 15uF section, it is only used if you plug in a C8 into the preamp socket.

04-25-2007, 08:41 AM
HI Terry! My McIntosh MX114 Stereo light does not stay on it light bright breifly and then goes off! I have the service manual. There is a part called LDR on the MPX board with a resistor attached to one of its four legs do you think this part is causing the stereo light and stereo sound to not function properly?!! The MX 114 is without question one of the finest! FM tuner I have ever heard even with its mono sound at present is sound fantastic! the sound is absolutely stunning! and I have at one point also owned a MR 78 and couple of Marantz 10B both in mint condition but for some reason! the sound from the MX 114 Fmtuner/pre-amp to my ears sound much better. Any kind help to fix the stereo and the stereo light problem in the beauty will be much appreciated! Thanks Terry! With Best Regards! Ali

05-22-2007, 08:46 AM
Here is a tip on resistors used in McIntosh tube amplifiers.
If a pair is marked 1% in the schematic this means to each other not the listed value.
When purchasing resistors here is something to remember -
When resistors are made the 1% ones are removed from the batch first, then the 5%, then the 10%, and what is remaining is called 20%. Think about this process,a batch of 10% resistors will have NO values closer than 5% as the 1% and 5% values have been removed already. This doesn't say you will not find resistors of the same value within 1% or less, just that none of them will be closer than 5% of the nominal value.

05-22-2007, 04:53 PM
Speaking of resistors... The MC225 schematic shows 10 or so resistors that are to be 5% tolerance. What are all of the other, unmarked resistors supposed to be?

05-22-2007, 06:01 PM
For all resistors in McIntosh, unless noted otherwise they are 10% tolerence.

07-25-2007, 03:26 PM
If your MC225 has cooked the two 22K, 1W resistors and the 4.7K, 1/2W resistor, replace the pair of .047uF bumble bee capacitors feeding the grids of the 12BH7 at the same time you replace the resistors. When the .047uF caps get leakage they will raise the grid voltage of the 12BH7, I have seen 96V on the grids, replaced the caps and had it go down to 33V, close enough to get the tube running in spec. The second thing this will do is raise your output power from a sad 22W to a happy 30W+ as it did for me on a customers 225 that was running all original McIntosh tubes.

09-14-2007, 05:54 AM
Hi everybody,
I have a 6100 in 110V, bought in the US.
I'm wandering whether the transfo can be modified to provide 220V? I did it on my 7270 and it works perfectly, the transfo was designed to work with 220V with a small modification.
Is anyone knows if it's the same on the 6100?
Thanks for your answer :-)

09-15-2007, 07:06 AM
Adding a quick note about a the 1% matched resistors - check them even if they look fine. I did that to my 240 last week and found the ones powering the power tubes were off 20%! I don't know how it happened (voltages were ok before and after) but replacing them improved focus tremendously. All the other pairs were still matched but I replaced them while I was at it. The amp now sounds more "hifi" than I ever thought possible.

09-22-2007, 09:47 AM
Dear Sir,

Can you advise the process to bias and balance the MC2002?

I've searched the net for a manual, service. I am an EE and can do this if provided the process. May end up buying a service manual from Mc. I have all tools, scope, freq. generator, variac, etc.

Any kinks with this one, the amp.? The amp sounds fine, no issues, says on back stickers that it went to clinic and serviced fine 1999 - 8 years ago.

In addition, no feet for the amp, Mc wants $2 per foot and recommend 1 inch from bottom without mounting in the walnut panloc cabinet I have. I'm currently using in shelf with clearance above at 8 inches, sides 4, bottom less than .5 inches. - using rubber bump on 3M feet on the panloc rails. It gets hotter on the front meter top of chassis than near heat sinks.

It hums quite a bit as a unit, not specific to the speakers, dead quite...chassis humm...probably just solid state and the power supply's in pretty tight acoustic space, maple ent. system...please advise any airflow issues, what should the sound of the chassis make - hummm....

Please advise as a courtesy.

Thank you for a prompt and kind reply with guidance or exact answer(s) to my ?'s. I appreciate it.


09-22-2007, 12:29 PM
Here is a scan from the manual, have Bias set and meter only, no offset.

09-23-2007, 01:02 PM
Please leave the file here so I can print it on Monday.

I appreciate it.

Any advise on the heat/ventilation issue?

The unit ran for 5-7 hours straight this weekend with no heat issues, room temp: 82 degrees and it was warm but not hot, the AC was 120-122VAC. The PT hums a bit but it's probably normal the result of the cabinet enclosure space magnifying it.

Again thank you. I think I will get the maint./serv. manual along with the feet from Mc.


09-25-2007, 01:54 PM

Your scan was useful.

I appreciate the help.

The bias at 120-122VAC was about 22.6mV. I lowered it a bit to 20.4-.6 and both sides came out about right at:

L - 20.7
R - 20.7-21

The meters were very slightly sticky at ZERO...just above it and a tap in the unit showed it dropped. I realigned the meters via the adj. no issues.

Unit is less hot now, believe it or not at idle and at run.

Ran it hot and got both the power guard function to work properly and the temp never came on.

Gave it a good dusting on the meter bulbs and it's just fine.

Could not have done it without your help.

The owner got a quick electronics lesson and a free tune-up.

Thanks again,

10-03-2007, 02:59 PM
I bought a MC-2505 from ebay a year ago. It arrived but one of
the channels was not working properly. Finally got off my butt and did some debugging last evening. Basically checked for shorts between transistor
junctions and found a bad power transistor (Q22). 2 of it's pins shows a short.
The surprising thing was Q22 was replaced some time ago ie. Q21,23,24 was
a Motorola 32-070 7452 and Q22 was a newer SanKen 2SC1116A.
So, I was wondering if the 2SC1116A stop working because it is an
incorrect replacement part or some other thing is causing the part to blow.
I'm going to order a replacement part from McIntosh but just want to be
make sure the new power transistor I put in will not blow again.

And since I'm at it, are there any parts I should replace since this beast is
about 30 years old ie. maybe some caps that needs to be replaced?

Thanks in advance for any advice.

10-03-2007, 03:44 PM
The correct replacement for the 132-070 is a MJ15003 by ON/Motorola semiconductor, odd transistor was probably blown due to mismatch.

10-03-2007, 05:28 PM
Terry, I have an ms-300 that I bought in April, and have had no issues until recently. I cannot seem to get it to record a cd. I have made sure that the cd is for music recording, but there really is no trouble shooting in the manual for a machine that does not record. Thoughts?

10-03-2007, 09:11 PM
The MS300 is listed as 'Factory Service Only', looks like this one is a call to Chuck Hinton at the factory 800-538-6576. To be truthfull I have looked at one in a display but never had any hands on time.

10-03-2007, 10:04 PM
Terry, I have an ms-300 that I bought in April, and have had no issues until recently. I cannot seem to get it to record a cd. I have made sure that the cd is for music recording, but there really is no trouble shooting in the manual for a machine that does not record. Thoughts?
Are you having trouble recording to the MS300 hard drive or making a CD. If you are having trouble making a CD do you get an error message?

For MS300 service and tech support there is a dedicated phone line. 866-458-6910 (I think 8am-6pm). The guys at this number are great, they have helped me out many times.

Good luck,


10-04-2007, 02:23 PM
Terry, Thks for the replacement part # info. I've placed the order and will update. Rgds, CH

10-15-2007, 03:30 AM
Hi Terry,
I'm wondering if you could clear something up for me. Your bias mod shows replacing SR-1 with a 1N-4007 and bumping up C-13's capacitance to 22UF@450V. I can see that however, the schematic I have only shows a 3.3K at R5 which is at the other end of the circuit on V1. Also, I'm running 5881s in my amps and they ask for a slightly beefier bias voltage. Do you think i'll be okay without throwing in the series resistor behind the diode? Also, the schematic shows -45V for a bias voltage, not -42 as you mention.

The second issue is that one of my amps seems to have a whopper of a surge at power up on the filaments. R27 and 28 measure within 1% of rated values so I'm thinking that maybe the can needs to be replaced.. any thoughts?

Thanks much,

10-15-2007, 01:30 PM
I found this thread while looking for a fix for a turn-on popping issue, lots of great ideas - thanks! Here's the problem I've been working on.

MC30 type A-116-B amplifier.

Problem: Makes popping noises for a few seconds after you plug it in. It sounds much like a relay closing. It would pop 5 to 10 times, then settle down.

Solution: The insulation of the yellow wires going from the P/S xformer to pins 2 & 8 of the 5U4 socket was cracked, the wires were exposed & were arcing to the chassis. The wire was stiff, brittle & discolored around those areas. I cut the yellow leads 1/4" back into fresh wire, attached new lengths of wire, covered the joints in heatshrink & connected the new leads to pins 2 & 8.

To confound things, I replaced the quad P/S capacitor at the same time as I fixed the wiring... but the popping noises are gone now. I'm guessing that the wiring was the culprit.

10-15-2007, 01:59 PM
Turn on surge is common, this is due to cold filiments getting their initial hit of voltage, if this bothers you add an NTC resistor like the MC240 uses, thruthfully I would not care since it hasent caused the transformer to fail for the past 40+ years.
My schematic is for the later MC30 drawing # MC-30-4 (#15329 and above, 6-15-56), R30 is the bias droping resistor with C13 the - bias filter, the bias is called out at -42V with B+ at 440V, this is the most common version of the MC30.
Watch out on the early version that uses the 2 extra .47uf capacitors and has the square bias rectifier on the capacitor side of the board, it does not have a droping resistor and if you replace the selenium you will have to add a resistor in the transformer feed side of about 8.2K.
Remember to much bias makes the amp run cooler but also decreases output power and raises distortion.

10-15-2007, 03:31 PM
okay... thanks very the way... since I don't have a variac to get the B+ right at 440, what if I used a 1/2W 10K trim pot instead of the drop resistor? how do ya interpret the serial numbers on these things? I'm sure mine aren't from the same run, or same year for that matter but i am curious.:yes:

Thanks again,

11-01-2007, 03:27 PM

I'm running a couple of mc502's in mono, the other day I plugged my headphones into them and noticed that on one of them only the righthand side has any signal. the other one plays as expected a mono signal through both sides of the headphones. I switched the amp back to stereo and plugged in the left input and then both channels had the stereo signal through the headphones. Swtiched back to mono and only the right channel again. Would this be a fault with the mono/stereo switch or something further down the line ?



11-02-2007, 03:52 PM
At low volumes on my MX115, I hear a constant static noise. Once I go past 1/3rd, it goes away. I used some Rat Shack contact cleaner and it helped a little, but it's still there. Is the static noise a problem with the switch itself, do I need to use Deoxit, or is something else the problem? A bottle of Deoxit arrived today, but I don't want to take the unit apart again unless I have to.

11-30-2007, 01:16 PM
Factory-Recon'd '501 came last week and even though the knob on the right would power up the amp, using the remote start would not. The red LED for Remote would light up, but no matter what combination of remote cables and sockets tries would not fire her up.

Even though the shipping container is in perfect shape, sending the '501 for a trip from Tampa to upstate NY is out of the question. Two different carriers already cracked glass on a C-28 and an MC-352, all within the last month!

So the amp came out of the box and on to the bench. Lotsa terrycloth towels is the way to go. As the amp is rolling over on its back to get the bottom plate yanked there is the unmistakeable sound of bits and pieces tumbling inside the amp chassis.

Disassembly starts with the remote board and a magnifier inspection looking for cold solder joints, then moves on to yanking the entire front panel, then the amp modules and moving on to the autoformers.

Removing the transformers revealed that one of the rivets holding the threaded transformer case securing screws was snapped. The nut just turned and turned inside the chassis. Best idea was to remove the outer part of the rivet and use a 4-40 stainless allen screw into the hole of the securement stud.

The debris was found in the undersides of the output autoformer: Little pieces of transformer potting compound had cracked off the edges of the dried compound right near the wires as they emerge from the tar-pit and were flopping around. Whew! It coulda been much worse. Putting the chassis back together gave rise to a warnig for everyone who wants to work on their stuff: The edges of the stainless chassis metal are like little razor blades. Especially when you need to jam your hands into tight spots to start hardware. First blood-Hmmmm.

Using a continuity setting on a DMM, the tip of a remote cable was used as the signal injector and the circuit was traced through the J-5 connector on the input board, throught he connectors on the right channel amp module and into the power supply board. There the problem ended. The mddle connectors on the power supply board are J-14 and J-15. The plugs are not lettered and look exactly the same. You got it: The plugs were jacked in backwards, meaning P-14 went to J-15 and vice versa. The amp would still play in manual mode but the +5 VDC start signal would never reach the optoisolator's LED pin 1 as the P-15/J-15 is not redundant.

A bit of Windex to wipe off the blood, a bit of Wax'n'Feed to nourish the walnut and she's back in shape, operation normal.


12-09-2007, 03:05 PM
I've just moved up from an Eico 2080 (which I have enjoyed for years) to an MC240/MX113 which is blowing my socks off! (Playing through E-V Aristocrats). I've had the amp for about a week: an untouched find from a used furniture store. I haven't done anything to it yet. A concern: one of the output transformers gets warm to the touch; the other stays totally cool. I've switched the 7027A's from channel to channel with no change in the effect. I'm not sure that there is any component beyond the power tubes and speaker impedance that could affect this. Any hints or suggestions?


Giles D.

12-10-2007, 07:51 AM
The power transformer is the one on the right. The two on the left are the output transformers. The middle one will get much warmer than the left one because it is cooling the hot power transformer. The heat from the power transformer is conducting into the middle transformer through the chassis. So if you have the far left one cool, the middle one warm, and the right one hot, then it is all normal. You are simply feeling conducted heat on the middle one - and not heat induced by the circuit. Hope that is clear.

You will want to have the capacitors, power supply diodes and main bias resistor pairs replaced. This will guarantee another 40 years of trouble-free life and have it sounding better than ever. I have done this to 2 MC240s now and it makes a world of difference. Enjoy your find! I wish I could be that lucky.


12-10-2007, 01:02 PM

I made the erroneous assumption that the center transformer was the power transformer! What a relief!

Do you advocate replacing the signal caps, or are you referring to only the power supply caps? I replaced all in my Eico to very good effect, but there appears to be two schools of thought on doing the same for the McIntosh instruments.


12-10-2007, 01:59 PM
Replace the power supply capacitors, 2 doubler and 2 filters, possibly the bumblebes (striped oned), and leave the Black beauty's alone, the 160P black beauty is a mylar-foil capacitor and it's not likley you can beat it's sound and quality at any price you can afford.

12-10-2007, 04:43 PM
Huh. I thought that Black Beauties were oil-impregnated paper dielectric, unless more than one variety of cap got that nickname. With old Collins ham radio gear, first thing anyone tells you to do with it is replace every Black Beauty cap in the radio.

12-11-2007, 08:27 PM
HI folks,

I need some advice. I am new to McIntosh gear and I am having some problem with my second-hand mc2102. After running problem free for 6 months, when I crank up the volume, the right speaker would crackle and hiss. One of the tubes (vr3, methinks) would glow very very red, then the fuse would blow. I replaced the fuse and swap the tube in vr3 with the one in vr4, the same tube, now in vr4 position, would glow really red and the fuse would blow. I bought a new tube from Upscale Audio, of the same SED Winged-C KT88 to replace the bad one. Now a different tube is glowing really red. The new one is also glowing red but not as badly.

Any advice or suggestions? Is it possible that I have two tubes going bad catastrophically one after the other? Should I have replaced all the tubes at the same time to guarantee matching? I think the tubes are pretty new but I am not an expert in these matters. The McIntosh logo on the tubes looks pretty unfaded.

Thanks for any and all advice/suggestions,


12-11-2007, 08:57 PM
I think you should move or re-ask this question down below.

12-11-2007, 10:19 PM
There are several varieties of Black Beauty, the oil impregnated are 1600V and higher models, you also have the DiFilm Mylar and paper Black Beauty, this is why I used the type number 160P as it is the Mylar foil version.

12-19-2007, 05:50 PM
moved to general area

12-27-2007, 07:46 AM
Hi Folks !

Took the plunge to stripped my MC-30 down to bare metal and have the chassis nickel plated. Still have the other unit intack for reference.

Got a 80/40/30/20 can cap, but don't know how to wire it up.
40uf to rectifier tube cathode ...?
80+30 after 150 ohm resistor.....?
20uf for invertor tube supply ....?
Can someone simplify that for me, eg: 40uf to 5U4 pin #?

Do I needs to add anything for the increased in capacitance ?

What's the two resistors that connected up pin#3 to pin#4 of the 1614 ?

Which is the diode ? Is it the horizontal one between R19 & R20 ?

I'm a retired refinery engineer and all this is new to me. So, please help.

Thanking All in Advance.


03-17-2008, 11:01 PM
anyone know if an older amp (mc150) can have the aluminum ends modified to the newer style (mc162 and mc202 seem to have the exact same size face, etc....).

Can it be done?

03-21-2008, 08:47 AM
Greetings all,

I was thinking that the result of a few hours of head scratching and sweat should get in "hints and kinks" so others would not have to go through the same procedure as I did, but cut through to just getting this done.

It regards replacing the dial glass light bulbs (4 of them) on a Mac1700 receiver. It turns out all four CAN go - against all odds and probability calculations - at the same time.

Just for the record, per service manual, the receiver uses bulbs #1866 (dial glass), #1850 (FM/MPX), #1847 (meter), #51 (selector lights), and #1828 (tape monitor). Most are available from Mouser (cents per bulb), some from Radioshack (per most testimony, Radioshack #47 can replace #1847, I have not tried). I hope this is not taken as advertising, there's no affiliation, it's just that other places charge dollars per bulb.

In order to replace the dial lights, the front fascia has to go off. It is held in place by two screws on top (visible after taking the back sheet metal dial mechanism cover off, the one that says McIntosh on top of), and also two washers that are visible after taking the balance and volume knobs off (and only if). All knobs need to be taken off anyway, so the front panel can be detached.

Dusting might be in order as this whole work has been done at that point.

Hope this helps someone.

03-29-2008, 02:05 PM
I had some serious buzzing when using unbalanced interconnects with my Rotel preamp and MC501s (I am waiting for the AP1000 to replace the Rotel with balanced interconnects).

I fixed the problem by disconnecting all analog inputs into the preamp. My adcom 5 disk changer was replaced with a MS750. This leaves only the MS750 and a PS3 both connected with optical inputs...

I am still using unbalanced interconnects (obviously the AP1000 isn't out yet) and the buzz is absolutely gone!

Hopefully this helps some others ;)

03-29-2008, 02:11 PM
Is anyone knows where to find some Bumbblebee .047/400V besides Ebay? It would be a great help for me!
Thanks guys.

03-29-2008, 02:55 PM
DO NOT use Bumble Bee capacitors, 40% or more of them will or have failed due to a manufacturing process problem. All bumble bee capacitors are liable to leakage and 40% or more will crack the molded case and slowly become close to shorted, why take a chance on one killing your amplifier when you can NOT predict if it will fail or not.

04-07-2008, 03:45 PM
Terry and or anyone,

I need to tune up a Mc 2105.

The amp is pristine but the bias needs checking and offset if applicable.

Can anyone post the service page and/or process to set the bias/offset?

Also, anyone have the user manual.


04-07-2008, 07:30 PM
Most first generation McIntosh autoformer amplifiers have no adjustments, to check for a problem let it idle for an hour then check the heat sinks, they should be only warm - never hot. I one or both get hot a problem may exist and it is time for a repair. Some of the 50 watt amps had an adjustment, how to do it is in the manual, watch out for the serial number to match the manual range.

04-08-2008, 10:50 AM

Thank you so kindly.

I opened it up with the schematic last night in hand.

No adjustment. It's just warm to the touch after 1 hour or so and in the cabinet only slightly warm at idle.

Thanks again.


By the way the interior is a blessing to work on...very minimalist and simple circuit tracing.

No burned or charred resistors and all voltages check out.


04-09-2008, 10:39 AM
Help ?:

MC2105, what is the Normal/Direct switch on the back for?

I checked the service and owner's manual and nothing.

Moving it does not seem to initiate any difference in performance.


04-09-2008, 01:38 PM
Normal has a capacitor in series with the input for preamps that have a DC component in their output, direct bypasses the capacitor and allows even DC to get to the preamp section in the amplifier. Using the direct with a tube preamp can zap the input stage since many tube preamps can put out several volts during warm up.

04-09-2008, 03:51 PM
Should it always be left on normal? I have a MC2105. Actually you have jazzed mine up.

04-09-2008, 10:00 PM
So, your warnings came true. The upper bass on my MC240 started getting flabby and distorted. Terry, a phone call to you confirmed a power supply rebuild was in order. I've replaced the caps and I'm working on the bias supply diode now. But: the transformer bias supply winding is at about +250V (no load); wiring the diode as in the schematic (anode towards the transformer), it passes no voltage. Shouldn't it be neg. with respect to ground? Any hints for this kink?

04-10-2008, 09:45 AM
Terry and all, other poster,


The manual did not say and I was just about to check the schematic.

I use a Tube Preamp; not sure it outputs DC or not.

I will leave it in normal mode.

Thanks again; the MC2105 is a very nice unit.

Flawless presentation.

Have a good day,

04-10-2008, 11:26 AM
The banded end of the new diode should be tied to the transformer, this will give you a - voltage, it may need to be tweaked with a resistor in series at this point to get the proper -46V (loaded) when the B+ is 430V at the output tubes. When all is working as it should with the AC line at 120V the AC current should run 1.2 to 1.3 amps when you have a set of good tubes installed.

04-10-2008, 11:04 PM
Brought my oscilloscope home from work today, and wonder of wonders, the new diode was bad.....passes distorted AC in both directions. I spent three hours last night with my Simpson 260 trying to figure out what was wrong with the bias circuit, and 5 minutes with a scope showed me that I'd wasted all that time. Sometimes a little knowledge is a dangerous thing. Doesn't stop me from trying, though.

05-01-2008, 01:14 PM
I'm loosing my hair trying to restore MPX operation on this 1700. The 6EA8 tube was very weak on both sections and was replaced with a NOS. Also swapped the 12AU7. IF and RF tubes check fine. IF alignment in accordance with manual went fine, as did RF. The problem persists: there's no response from the 19khz transformer or phase coil- the manual refers to "...max negative DC at pin 7 of V8", but there is no DC present at this point; regardless. The 19khz transformer checks fine for continuity, as do voltages in the circuit. I pulled V7 and adjusted 38khz. at pin 6 of V8. I can see 19khz pilot at TP1, but not on T6-- should it not be present on pin 6 of V7?? I suspect an undetected bad ceramic cap but have not been able to get any closer- the caps check OK as do diodes and resistors.
Any help appreciated.
Thanks, Cornell

05-01-2008, 02:12 PM
Common failure in the MAC1700 is C62 a 100uF at 16V capacitor, this capacitor will become leaky or short thus killing the 36K oscillator.

05-01-2008, 02:20 PM
Thanks Terry; I'll swap it out... Cornell

05-01-2008, 03:56 PM
Terry, changed the cap; no difference. T6 and L5 still have no effect. With the generator in stereo, the l and R output is a modulated envelope, with one channel peaking as the other is approaching minimum. Mono looks OK.
Thanks, Cornell

05-05-2008, 10:48 AM

With the weather change, the room the MC2105 is in is tiled and temp in the cabinet on top of the cage above the right Heatsink if you are facing the face plate is 98-100 degrees F in a 80 degree room.

The left side far left is 96 degrees F in a 80 degree room.

The heat could be from at idle mind you after 1-2 hour, the POWER Transformer being on the right front, correct?

It obviously gets hotter than the autoformers would on the left.

The whole unit seems warm, but not hot, when I put my hand on cage above the heatsinks seems like chimney is doing what it should.

Any comments or is this ok, given your last advise re: heat and bias not adjustable.

The PT is a bit hummy, if you will but probably due to the tile floors and acoustics it can be heard in a very quite room.

Thanks for any advise.


PS...I did notice a slight channel imbalance on the meters but then it could be the source or preamp - tube adjustment.

right seems a bit low

05-05-2008, 12:59 PM
Temperatures seem to be OK, the right side with the PT will be a bit warmer then the left, the HS and PT tend to aid each other in warmth.

05-05-2008, 01:47 PM
Any comments on the HUMM.?

I know it's not the preamp, but it could be a cheapo RCA Cable I have, I prefer cheap...will replace with some 75 Ohm HOSA Quad shielded RCAs tonight...see if it makes a diff.

Not sure.

I do hear some humm in speakers so I "think" it could be one of two things...ground loop or connection not firm.

Thanks, again, John.

Not sure about the meter issue and gain balance, seems like it's always FM and always the same channel...I am going to try a direct connect from my IPOD tonight to the amp and see if that makes a diff.

I don't want to mess with getting a 150W dummy load and trying to calibrate the meters as per service manual..the largest load I have is about 50W.

I guess I could get some large(ish) wirewound non-inductive resistors...but 20W is all I have in this area.

What is the best way to test, in your opinion, the gain controls and the meter accuracy...last night I put about a few volt 250Hz signal through it and the left meter was about 25% higher than the right.

Of course it's possible since I am running them through a phase inverting Van Alstine Bridge/Inverter and summing the inputs inverting at the amp and then correcting for phase at the speaker it's the problem...that and the humm/.

05-05-2008, 06:15 PM
Loose ground.

replaced cable No trouble found.

Need to adjust meter calibration.

Have to take to 105W Dummy Load


05-09-2008, 12:25 AM
A question from a newcomer to AK McIntosh audio : I bought a 2nd-hand MC7150 in 2003, and I would like to check if everything is OK after 15 yrs.
Can anyone advice about the bias setting : which idle current, where to check and adjust ?
Is there any offset adjustment necessary with autoformers ?
When close to my speakers, I can hear a faint buzz, which does not increase when cranking the level pots ; any idea what it is, and if it can be cured ? (I don't hear it with my vintage Marantz 1200B)
Any help is welcome.

05-09-2008, 08:08 AM
The service manual has the information you need, call the parts department to order one 800-538-6576. Here is the procedure from the manual.

05-19-2008, 01:54 PM

As b4 the offset I am seeing is as in title.

Is this something to be concerned about or is it alright, within spec.

This is the MC2105 I asked you about when discussing bias/temp on heat sinks, etc.

No output problems or other issues, just wanted to make sure this is not a problem on the outputs - idle no input about 30-40min on or at first start up, same as above.

No offset adjustment.


05-19-2008, 05:56 PM
No problem, the heatsinks are cool this says the bias is OK, if the bias is OK the offset you are seeing is nothing to worry about, more than likely some noise that is showing up as offset to your meter.

05-19-2008, 06:15 PM

Thanks, could be. All voltages spec out inside the unit per the schematic +/- 10%.

Hot or cold no issues.

Meter is a Simpson VTVM

Other meter is a Fluke.

Thanks again.

05-20-2008, 09:37 PM
Been running a MC240 for burn in after rebuilding the complete power supply, 1/2" feet, on the workbench.
After 5 hours here are the readings.
Room = 75F
Front = output transformer = 88F
Middle = output transformer = 92F
Rear = power transformer = 110F
Can still hold a hand on the hottest transformer but yes it is a bit warm to touch though entirely in the safe zone...

05-21-2008, 01:24 PM
I am guessing as always that the close proximity of the Output and Power trans, in my case to the heat sinks on the rt side, looking from the front are driving the slight elevation in Rt v left temp with a room temp as stated.

Unit of course was in the cabinet with cage, a few inches from back wall and clear on both sides, elevated about 3 inches on a wood block.

Seems normal...

Room at about 80 degrees F, PT runs the hot part it seems varies when incoming AC is 115 or over 120VAC.

This is the MC2015 I spoke to you in email about, within this forum.

I've seen hotter tube PT's...but generally not "smokin': hot.

Good to know your thoughts though.

I have had one Cap can on a MX110 that is always hot due to proximity to rectifiers underneath and PT...even in open space but the one next to it is less hot, further away by inches....these never fail, run fine and leak not one bit.

i rebuilt the PS some years back and it runs and runs.

Not a problem anywhere except a 6CS tube failure.

For what it's worth. MC stuff runs and runs./


05-21-2008, 01:36 PM
Been running a MC240 for burn in after rebuilding the complete power supply, 1/2" feet, on the workbench.
After 5 hours here are the readings.
Room = 75F
Front = output transformer = 88F
Middle = output transformer = 92F
Rear = power transformer = 110F
Can still hold a hand on the hottest transformer but yes it is a bit warm to touch though entirely in the safe zone...
Adding to the database; with roughly the same room temperature, I get maximum 117F transformer temp from my MR-71 in a McIntosh slant-foot case. I took readings several times over roughly a week.


07-19-2008, 07:27 PM
Another question regarding the amp mentioned above. The outside barrier block has four dividers broken off. I will have to call Mcintosh shortly to see if replacements are available (I'm crossing my fingers. They already said no regarding the the front panel.) What is the procedure to replace them? Do I unscrew the old one and pull it out to expose the solder terminals or do I have to work from the inside? The inside is very tight between the back wall and the headsink. Does the heatsink have to be unscrewed and raised out of the way? This amp must have been used by roadies. The accessory outlet is also cracked and the two volume controls were rewired so they are bypassed. The output level has to be controlled by the source.


07-19-2008, 07:29 PM
I purchased an MC2120 that was rack mounted with no feet attached. I have a set of four feet with threaded screws that I got a few years ago, probably for a C34V. I cannot figure out how to attach them to the bottom plate of the amplifier. There are four holes that look to be in about the right position but they are relatively small and not threaded. Which holes do I use and what are the correct fasteners?

07-19-2008, 11:36 PM
Not sure if this should be a separate post (I'm new here but I already love it). Recently got a 2505, but one channel does not work. The meter does not move either. Any suggestions?

07-20-2008, 12:02 AM
Hi onlysleeping and welcome aboard! :)
Read this post for the typical DIY preliminary things and should that not do it a new thread for your specifics would be best.


08-31-2008, 09:43 AM
Most all the tubed McIntosh tuners MX110, MR65, MR67, and, MR71, use a 12AT7 in the front end of the tuner. This tube is used as an RF amplifier and mixer stage and can have a big effect on the overall performance.
Here is the problem, overall sensitivity is only 5uV and has a tendency to oscillate on the high end of the band, is very tweaky when trying to align. You have put in a new tube and expect more than this !! this is a NEW stock tube... Well take it out and use an old stock or used tube of the old design without the internal shield, all the problems will go away. It seems that the new tubes with the internal shield works fine in preamps at audio frequencies, but at RF they will not work!! The internal shield has plate to plate capacitance that will cause the tube to oscillate messing up the front end of your tuner.
This is one case where new is not necessarily better.

09-04-2008, 05:14 PM
Hello all,

I am restoring and repairing a MA-5100 with a bad right channel. I know one of the output transistors (Q220) has blown, so I want to replace both of them in this channel as a pair.

The problem is, a previous owner cleaned the chassis and wiped off the serial number!! I have a spare parts MA-5100 that is S/N 99H62, but its output transistors have different part numbers than the unknown serial number unit I am working on. I have seen some information around here on part number 032-070 transistors, which are the ones installed in my spare parts chassis. However, the transistors in my new unit are stamped "025 6-44", and I can't find out what they are. I don't know where to find out what the manufacturer part number of the transistor is so that I can order them. Help!!


09-04-2008, 08:44 PM
The replacement part for the 132-025 is the 132-070, the generic replacement for the 132-070 is the MJ15003 by ON Semiconductor (Motorola).

09-07-2008, 04:43 PM
I have an MC250 that has a hard to diagnose problem. After about 2-5 minutes of playing, crackling static noise will suddenly appear. The static will get louder rather rapidly where upon, after 30 seconds to a minute, there will be a loud snap/pop in the right speaker and noise will go away. The right channel then becomes extremely weak but not distorted. The left channel seems to emit some static but there is no popping sound and it maintains normal volume levels. The right channel level will come back after I unplug the amp and wait awhile. This all happens rather quickly and randomly enough that I cannot do much troubleshooting. If I pull the plug while it is crackling, the noise will just fade away until all the caps discharge and the amp goes silent. I wish I had a scope because I really don’t know between the RCA inputs and speaker screws this noise is created. Originally, the problem with the amp was just low output on the right channel. I changed most of the circuit board electrolytic caps (except for the two larger filter caps on one end of the input board) and the low output went away. I thought it was a bad NP cap at the right channel output of the driver board. This noise is new. I have not touched any of the four chassis mounted can caps. Their measured voltages seem to match the service manual. Thanks

09-22-2008, 09:08 AM

If I may ask. Having a problem at 89.7 FM...MX-110 starts to fade, signal goes about the same time in the evening each night...called the station engineer...he said could be a pirate radio station...mentioned getting in car and trying it there when it happens on the MX-110...the station jumps from 89.7 FM to about 90.1 or so...the tuning eye opens and the station vanishes, then after a bit it fades back, then back and forth.

Coudl this be an alignment issue? It only happens on this one station and mostly 10pm to 12pm or so at night?

ANy advise?


09-22-2008, 10:34 AM
I doubt if it is the 110, a problem in the 110 would be continuous or slowly getting worse over time, especially an alignment issue.
This problem is either station related, signal path related, or caused by some type of predictable interference. The capture ratio in the 110 is being exceeded by the strong signal at 90.1, possibly a pirate station or maybe they are changing their antenna pattern at night. Remember the 110 was designed long before the crowding of the stations on todays FM band. As a side note this could be an artifact from an adjacent HD FM signal, known about but ignored by the FCC.

09-22-2008, 04:20 PM
Talked to the station engineer and he said go out to the car and use that unit asa the problem occurs again.

He said yes they are broadcasting in both digital HD FM and Analog BOTH but the issue is probably some sort of interference OR my MC. he asked if I has a Phase locked loop Digital set, which I do; a Grundig...fully digital and that get static but no drop out.

I have a Pioneer SA-9xxx-II analog tuner and a HK Citation 15...all of them have the therefore all a bad?

I don't think so...

he did say while the Mc is a tuner and a oldy, it could be the problem.

We'll see...oddly enough it ONLY occurs at about 10pm to midnight.

I think I agree with you and believe he is finding any reason to get me off the phone; as I am the "only" person to complain.

We'll try tonight by getting in the auto with it's brand new Honda Radio...woo hoo.

Thanks again for the diagnostic. By the way all the tuners work perfectly.


09-22-2008, 05:05 PM
I don't know if I should post this here but since johndoe1 asked a question about the tuning eye I thought I'd ask. My MX112 tuning eye never totally closes and usually stays open about a quarter inch or so, is this normal? If not what type of antenna should be used and where can I buy one? Thanks

09-22-2008, 10:19 PM
MX112 uses the AGC voltage to drive the eye, a signal strength indicator. Reason the eye will not close fully my be alignment, or a bad 6HU6 indicator tube, I don't remember ever seeing one close all of the way so I wouldn't worry.

09-23-2008, 09:57 AM
Dear readers,

First thanks Terry. I spoke to the station engineer again, I only listen to 1 station ever. NPR.

He said he thinks it's the tuner(s), then he said the stuff about going to the car when it happens...guess what, last night i turned it on; all 3 tuners for grins; turned the tuning knob both directions slightly left and right of 89.7 from 8pm through midnight and no fade, no problems no nothing, nothing but muting ON and OFF switch and no extra stations.

My guess is either they fixed something OR it's interference from somewhere. Is it possible that someone could be broadcasting an iPOD via one of those little tiny FM Transmitters? I have one Griffin and it's at 88.6 as a std. I wonder?

Anyway, MT restored tuner and the other tuners worked flawlessly for 4+ hours not a change rock steady.

Now as for the tuning EYE, mine uses the 6 tube Terry indicated v. what you have...mine originally did not close right nor did the MPX Light fade and function properly. I had it aligned per the Service Manual and then restored and aligned it again by myself...yes it can be done.

My version has every knob and POT one could ask for with the exception of the T4 being a tunable transformer, mine has the old style TOP - rounded no adjustment.

I did find that without a NOS 12AT7 and then without a very good 6GH8 in the back left position for the MPX...the thing just would not adj. properly. Once I went with all NOS tubes, Telefunkens, Valvo and Amperex Bugle Boys...yes used were very cheap where I live...the tuner came alive...this was some 8 years ago and I have had not one problem.

At 120V or higher AC line coming in, the transformer is HOT but not as Terry says hot enough to burn.

I did put HEXFREDS in the diode section for the PT and added nice solen caps, the big non-polarized ones for the old might mite paper covered jobs.

I replaced all the coupling caps, the RED ones on the inside support rail to ASC custom made 1% Mil spec teflon and polyprop...600VDC...I got some samples from my work.

I've lost only one tube in 10 years due to age...

I DO NOT use the PHONO Section so I removed the tubes. I use a diff. Left Right amplifier tube, can't recall the euro version but it's for a higher DC filament voltage to offset what comes out or the wall...get back to you on's in a cabinet.

Bottom line on the Tuning eye, once aligned, properly mine closes and then I find the widest OVERLAP after the beams come together and that is the strongest signal and best MPX Light.

As I said it's flawless...but this pesky non-sense on NPR was driving me crazy.

I do use a cheap - Radio Shack TERK like square coax connected antenna - in house on a 8 foot tether...converted from 75 Ohms to 300 ohms via balun n the antenna IN on MX-110.

I have tried my rooftop antenna, old ARCHER job...on a diff.

For what it's worth.


09-24-2008, 08:17 AM

It could be started again at 5pm last night and I jumped in my car and nothing, works just fine.

Maybe the old non digital tuners are the problem.

It took fiddling again at the 89.7 dial to more like almost 90 to get a muted station in.

I think it's a neighbor maybe...not sure.

Oh well.


09-28-2008, 04:16 AM
Hi Terry,
Sad day for me. My mc 275 started making a loud farting noise.
I quickly switched it off and let it cool down and put it back on after about 30 min. It played fine for about 5 min then the sound came back.
the noise is coming from the right speaker. it is very loud and sounds like it is in motion
all the tubes light up and noting seems amiss other than the loud noise
the amp is an original 275 and is still in its original state.

09-28-2008, 10:05 AM
First place to look is at the tubes, swap good channel tubes into the bad one and see if the problem moves with the tubes or stays with the channel. If the problem stays with the same channel it is time to trouble shoot, if it moves wit the tubes swap them back one at a time to find the bad one that carries the problem with it.
A MC275 in original condition is due for updates, bumble bee capacitors need replacement, power supply needs a rebuild, and all resistors need to be checked foe correct value.

09-28-2008, 10:48 AM
Hi Terry thanks for getting back so soon.

i tried swapping the tubes. On the first attempt i noticed the sound started on the left side and then when i switched off the amp the sound stopped on the left and came out of the right while the amp was powering down.

i swapped the tubes back to their original position and the noises did not change. started on the left and then moved to the right while powering down after i switched off the amp.

on another attempt without further tube rotation the initial sound came out of both speakers and on power down came out on the left speaker.

thanks for the help. i cant concentrate on anything else. and i am hoping it is not the transformer. would like to have an idea before i have to send it off for repair and full service.

09-29-2008, 08:44 PM
hi i just purchase a c-39 without the 6 channel upgrade my question is (i'm a bit confused) is will this unit still function as a 6 channel surround without the upgrade

09-30-2008, 08:46 PM
help with my 2205 when i turn the volume to about 6 oclock the lights on the meters start to blink is this something to be concerned about? also the transformers get quite warm not hot but quite warm

10-02-2008, 08:49 PM

Does your MAC 1900 emit a horrible squeal? Is it most noticeable at the top of the dial? Check the soldering of the 2 capacitors near the tuning capacitor and if the solder joint is cold or debatable, reflow the joint. Cured this problem on my late version with impedance switch.

10-03-2008, 07:00 AM
Terry good day,
can you help me with a question?
my name is Lev and i live in Israel i have purchased mcintosh amplifier mc 352 and mcintosh preamplifier c2200 new. I have a problem with 352: it doesnt output bass and it feels like it doesnt work at all its possible power.
in Israel i couldnt find anyone who can fix mcintosh (technician ).
This is mine equipment :
1.amplifier mc 352
2.preamplifier c2200 DCD 700AE
4.cables by "monster cable"
5. speakers KEF aq9
could you help me to diagnose the problem? why its doesnt work properly?
thank you
my email :

10-03-2008, 09:15 AM
You didn't mention your speakers, possibly they do not like an autoformer amplifier like the 352, assuming they do, try changing the speaker output tap to another value, some speakers that are rated 8 ohms like the 4 ohm tap better.
A lack of bass can also be caused by having the speakers out of phase, reverse the leads to one of the speakers and listen again...

10-03-2008, 10:16 AM
Terry, thank you for your quick answer.
my speakers are KEF aq9, i tried all options 8 ohm, 4 ohm and 2 ohm.
i tried to replace speakers for TANNOY mercury F3 and for another kind but the same problem remaind.
the same about the bass, no change.
In generally there is no drive at the sound of music.
thank you

10-03-2008, 06:37 PM
I have just finished putting a new faceplate and bulbs in the tuner and she looks and sounds wonderful.....I have noticed the multipath light cut out at times though....I have turned all knobs and nothing happened accept when I turned the mono-auto-stereo knob to the far left, then it came off and on, could this be a simple deoxit or is it in need of some "Terry TLC"???

10-03-2008, 06:46 PM
Deoxit D5 to the Mode switch...

10-03-2008, 07:25 PM
Deoxit D5 to the Mode switch...

Thanks Terry, will do tomorrow, thinkin I will send her yer way in the new year for a thorough once over!!:thmbsp:

10-04-2008, 06:45 AM
Terry good day,
I dont know what to do with MC352 problem, i tried all option that you recomended with no luck, the same problem remained, i changed -/+ on speakers it did not influenced on a sound.
I look forward for your advice, what more can be done?
please help me.
thank you

10-08-2008, 06:47 PM
What would be the logical place to look if the following symptoms occur. One channel begins to get loud static and popping sounds followed by a loud snap/pop. The channel then goes dead. It is not completely dead but you have to turn the volume all the way up to hear faint music. I have replaced every internal electrolytic capacitor as well as all four output transistors. I have a DVM but no scope or signal generator. I do not see any resistors that obviously look burnt or open. Any suggestions? I'm tempted to try swapping internal signal cables to at least see if the problem is in the first preamp section or the driver/output section. Thanks.

jean lafite
10-12-2008, 11:56 AM
please forgive me if this has been addressed already but i am having problems with my stuff. i recently bought a great pair of speakers and am hearing my gear essentially for the first time so naturally it has to start acting up. i have an mc275, c22, and mr71 (mi-3 too, but it isn't hooked up). i have long wrestled with reception issues, but that is not my biggest problem. lately there has been a hum that i cannot seem to isolate. i am not at all adept at this, but usually i can figure out if it is the amp or the preamp by flicking switches around on the pre to see if anything illicits a change and if it does i assume it's the pre (of course i could be dead wrong about that). in this case nothing seems to change it which indicates the amp, but again i am not sure about this. so what happens is once i turn everything on the humm slowly starts to build as it warms up maybe even gradually getting worse over a long period and then, every so often, there is a loud pop and the hum disappears completely and everything sounds perfect for a variable period of time and then the hum comes back. there is also a similar loud pop when i shut the power down at the pre if the hum is present although shutting down and powering back up does not fix the problem for any length of time the way the random pop does.
can anybody help me with this? i sure as hell don't want to haul that thing back to the shop. thanks,

10-14-2008, 01:48 AM
I've searched through the forum for anyone noting humidity issues. I've got a MHT200 that's shutting itself completely off and then returning to standby at random intervals. Once it starts, it won't stay on. As soon as I take it out of standby, it will shut off and return to standby a few seconds later. I believe it's somehow related to the relative humidity in my house as it only occurs during periods of greater than 60% relative humidity. A couple of times I've noticed the glass appeared "foggy" from the inside of the blue LED readout. I'm wondering if this is some sort of safety function or if I need to have a professional service done on the unit. Has anyone else has seen something like this. I know there are a ton of folks from Florida on the boards so maybe this has happened to them as well.

10-25-2008, 11:54 AM

10-31-2008, 11:03 AM



11-12-2008, 01:52 AM

I'm about to start the restoration of a pair of MC-50's and a MC-250. Just a few quick questions concerning repairs. Are there any current semiconductor replacements(faster,more stable transistor/diode replacements) that should be upgraded?

How about cross ref's for:

145A136 red dot

F 021 718 yellow dot

Any ideas in play on extending the HF signature slightly(cap bypass, film coupling, blocking etc)?


11-12-2008, 08:56 AM
I use Nichicon Muse KZ and ES for the preamp and driver sections, not sure about replacement parts except the outputs are MJ15003. I normally use the recommended part for diodes, buy them from the factory, I can cross reference most from a model and schematic reference number from the service literature, the numbers on the actual part are mostly date codes and variations of the house part number that is assigned by McIntosh. The house number is listed in the service literature and is the starting point for me. Watch out for the sand block 5 watt resistors, the type used by McIntosh are also used a fuses, they are designed to open when hit with an over current condition, this is a safety feature designed into the part.

11-13-2008, 01:46 PM
Hey Terry,

Any sources where I can purchase the correct replacement 5w sand emitter resistors? Are they a special design?


11-15-2008, 12:32 AM




11-20-2008, 03:39 PM
Ok, it is an MC 2100 that I have been helped with by Terry already as to cap replacement. Thanks Terry, the Nichikon caps worked out great.

I am ready to begin looking for the source of a right channel hiss.

So to understand, I need to disconnect my RCA preamp inputs and see if the hiss goes up and down with the gain controls to isolate C 32preamp/other inputs from amp issues? I just read the C 32 ribbon cable post a bit ago.

With everything connected I already note the hiss does goes up and down with the right gain control. It's just enough "there" to note at no preamp volume, and warn me it is time to find the issue.

I think tomorrow I will disconnect the turntable, tuner, and CD from the preamp and see if it is an input.

If I am directed to disconnect the preamp from the amp, I think I will connect the garage speakers rather than the Cornwalls for that test.

Presuming I am only concerning myself with the amp, after a no preamp no input gain knob turn and listen gets me the hiss, where to start?

The amp runs cool. I never did put the screws back in for the cage after the cap replacement. I have slid the amp out, removed the cage, and placed my hand on the sinks after 30 -45 minutes of play time. The sinks are quite comfortable to the touch.

The amp hisses more noticeably at start up than after playing a while, I think (maybe).
There was no hiss after the cap replacement, this hiss just sort of "came on" a bit later.


Should I disconnect power and begin checking across resistors with my ohm meter for rated resistance versus actual?

Is the "freeze" spray I have heard about valid? Play, spray, ( pray :yikes: )and listen for increase in noise? Didn't see one suggestion to freeze spray in this entire thread, so the lack of that suggestion makes me suspicious.

Well, I wanted to jump in so I will get notification of replies, and begin this. I am on vacation for the next ten days, so I wanted to get this done, among other things.
Thanks Jeff R

Ah yes, and if I do find out of tolerance resistors, source from the McIntosh factory, or from whom should ? purchase new resistors?

11-28-2008, 05:10 PM
I am sure this is not a McIntosh problem; but perhaps someone can give me some good advice. I have a pair of new McIntosh 1.2KWs. They work great. Don't laugh but for now, I have them hooked up into the Preamp Ins of a very old Sansui QR 6500 Quadraphonic Reciever. I have them hooked into the front two channels of this reciever. When I play my Cd player or IPOD Classic through this system; the sound is fine. If I hook in my laptop listening to an internet radio program such as those on Live Radio 365 or Hearts of Space; I get a very low hum/static sound through my speakers. Can anyone tell me what this is about and what I can do to remedy this? Thanks Scottsmrnyc

11-28-2008, 05:30 PM
I would wonder about the volume setting on the laptop headphone output, if that is what you are using. I bought a set of those Bose powered laptop speakers. The Bose manual states to be 60% or lower on the headset volume setting.
I blew the first set of Bose up forgetting and having it too high. Or at least they went dead. The second set has no had an issue since I keep the laptop volume below 50%.
When I have hooked the laptop up to my C 32, I can't recall hearing any noise. When I connected it to my KA 7100, I think had to turn the laptop volume down or I heard noise, best I recall.
My best guesses. Lower the laptop volume. I presume you are running a mini RCA split to two RCA jacks in all this reply.

12-03-2008, 08:50 PM
Yes, sorry for the delay in responding. I am running a splitter. I will try your suggestions. I have put the laptop volume to the max. Maybe I will lower it to half and then increase the volume on the Sansui QR 6500 which I use as a pre amp. By the way, of course I am thinking about getting a new McIntosh pre amp. Perhaps the2300. Do you think a new Mcintosh preamp will make a substantial difference in the sound? Thanks Scottsmrnyc

12-03-2008, 10:18 PM
I am not at all qualified to advise on which Mac preamp to buy. I would email Mr DeWick about that, or other members.
Did lowering the volume output help the noise?

12-05-2008, 05:02 PM

My 2205 just arrived from ebay! It is a heavy behemoth, hooked up and sounds wonderful and looks even better. I noticed the only lights on the front are the meters, WATT/DECIBEL and the L & R green lights. The McIntosh logo and all lights I presume on the bottom row are not lit. Am I corrrect in assuming those DO light up?

12-05-2008, 10:08 PM
Has 4 each #1866 (sub #1847 or #47), they are in line with the controls. You have 2 options to replace them, pull the panel and do it from the front or I think the easy way by pulling the bottom and popping the holders out with a squeeze and replace them.

12-07-2008, 12:16 PM
I had an idea this am as the wife would like to have Christmas music on during a family gathering this evening.
I looked up the input voltage on my C32 inputs, and thought to use my multimeter to read what I was putting out at what setting on my PC volume control.
Unfortunately I could read voltage with my postive on the RCA male parts of ajack, but when I grounded the negative to the shell the voltage just dropped down.

So maybe I would have to do this while the output is actually running something, I don't know.

Maybe someone will chime in on what I was doing wrong, and how to do it.
If possible.

12-12-2008, 08:50 PM
Hello All:

I noticed last night on my MC 2205 amp that when you are facing the unit the rear right "heat sink" is warm to the touch and the rear left is cool, no heat. The amp is working in stereo and both meters work independently and I can turn the gain down seperatly so I don't understand why only one "sink" would be warm any thoughts

12-13-2008, 09:18 AM
MC2205 has a bias adjustment on the driver board, correctly set the heatsinks will get slightly warm at idle (no output). The bias setting procedure requires an AC amp meter in the AC line, procedure is described in the service manual.

12-23-2008, 08:14 AM
I've got an MC-30 which I'm completely rebuilding since it was in pretty sad shape. For the resistors: what type is best? Stick with the carbon composition or go to a newer type? Capacitors; new orange drop OK? How do you get the transformers out of the potting compound in the can? It seems to melt with a heat gun (but slowly) so I was thinking of sticking the whole can upside down in some boiling water or an oil bath. Reasonable?


12-23-2008, 08:32 AM
The tar melting point is 600dF, neither hot water or oil can safely melt it, I have used a hot plate, question is why. If you must remove the potting use the heat gun from the inside to pull the tar from around the transformer, then pry the core out.
The resistor type is up to you, I personally use carbon or metal film for temperature stability. Orange drops are OK but may give the 30 to modern a sound, I mix types to mellow the sound, paper foil or PIO in at least one stage.

12-23-2008, 08:41 AM
I need to remove the transformer from the can since the wire insulation is badly cracked (power transformer). I'd like to replace it as close to the winding as possible and have the cans powder coated while they're empty.


12-24-2008, 10:54 AM
Have seen the covers removed by hanging the transformer by it's leads and using a torch to heat the cover, do it fast and don't catch the tar on fire are my thoughts.

01-07-2009, 01:42 PM
Dear experts,

ok, the mc2002 is now a problem...on occasion at any volume the rt channel goes quiet, if I apply a solid smack to the wood cabinet, presto it's back...

Any guesses, it could be an old RT channel interconnect gone bad or the output of the MC 4xxxx eq where I use it OFF and just the Bass controls.

Maybe I should swap the EQ for the other backup unit?

Maybe a new interconnect for the rt channel?



01-08-2009, 04:26 AM
Terry, I have a MQ101 which sounds distorted when put on the tape loop on my C2200 pre-amp. I tried inserting it between my tape deck and the pre- and I still get the same results. Does this unit has a history of caps drying up? I don't have the SM so I can only guess at this point. Any help is greatly appreciated.

01-08-2009, 07:44 AM
Terry, I have a MQ101 which sounds distorted when put on the tape loop on my C2200 pre-amp. I tried inserting it between my tape deck and the pre- and I still get the same results. Does this unit has a history of caps drying up? I don't have the SM so I can only guess at this point. Any help is greatly appreciated.

Here's a PDF of the MQ-101's service manual:

01-08-2009, 10:32 AM
No real history on the EQ101, a transistor or two and a power supply filter, only had 2 on the bench in 3 years.

01-08-2009, 10:52 AM
I've searched high and low and can't find it, but I swear I recall seeing a bit of discussion on how to take apart and troubleshoot those pesky IF cans. The tech who sat on the 113 for a year before returning it "unable to fix- part not available" said that the 19KHz transformer (Mc part 162-055) was bad. How do I get it out of the can, and which pins are which?

01-08-2009, 10:25 PM
mrz80, thanks and a very Happy New Year! You are right Terry. This unit has been very reliable and I am the second owner. It is the first time it has broken down in its history. Let me go through and post my findings on it. Thanks once again gentlemen.

01-08-2009, 10:30 PM
I've searched high and low and can't find it, but I swear I recall seeing a bit of discussion on how to take apart and troubleshoot those pesky IF cans. The tech who sat on the 113 for a year before returning it "unable to fix- part not available" said that the 19KHz transformer (Mc part 162-055) was bad. How do I get it out of the can, and which pins are which?

There was a discussion some time back regarding MX-110s and I/F cans with where they were rebuilt to eliminate a small silver-coated cap in the can's bottom. Might search with those elements.....I've got an MX-110 with the symptoms that will need attention at some point.



01-08-2009, 11:50 PM
Well, as it turned out, it was dead simple to get the can off the transformer; it was just crimped to the sides of the base. When I got it apart, this is what I found - all the wires leading from the coils to the base were snapped off. (see the picture)

I just got a quote of $400.00 and 2 months' time to have it rewound. That seems a trifle steep; I could nearly buy another 113 for that! :)

I'm wondering what to do now. I've browsed until my eyeballs dried and can find no source online for even a near match of this little guy. I'm guessing that someone a lot more adept than I at resonant circuits could cobble up something in the way of a design for a substitute.

01-11-2009, 10:10 PM
Terry it seems that the dial pointer on my MX-110 is not lit up. Is there a bulb I need to replace?

01-12-2009, 09:21 AM
MX110 never had a lit dial pointer, if the glass does not glow from the edge lighting it could be a bad festoon lamp, bad filter between the festoon and the glass, or tired glass that the lettering is getting loose and does not pick up the light from the glass. If the letteing is getting loose do not attempt to clean the inner dial glass as the lettering can fall off with any touch.

01-12-2009, 11:12 AM
Thanks Terry. The glass glows, dimmer setting works lettering is intact no bubbles. I just did not know if the pointer was suppose to light up.

01-12-2009, 03:25 PM
So Terry, what *is* your approach to dealing with one of these units (like my 113) where that 19kc xfrmr is damaged like that (all the coil wires broken)?

01-12-2009, 08:07 PM
Either figure out the wiring, ohm the leads, and wire them back, or order a new one from the factory. You would be surprised what they still have in stock, I just ordered a 162-031 19K coil for a MR73, retail at $6, better than telling the customer his tuner is scrap.

01-12-2009, 09:42 PM
I've already contacted Mc. They haven't any more of the 19kc transformers. And, just to show that Murphy still gets to be in charge, one of the 6 leads broke off *at* the coil, up inside the ferrite, and is 100% inaccessible.

01-15-2009, 08:02 AM
No real history on the EQ101, a transistor or two and a power supply filter, only had 2 on the bench in 3 years.

The second transistor, a BC137 had blown, shorting the power supply. I was amazed to see the row of vintage Siemens transistors inside. The replacement for the BC137 is a BC237 which comes in a standard packaging. Some bending of the leads are needed to get it to seat nicely.

Thanks Terry and MRZ80!

02-01-2009, 12:31 AM
Either figure out the wiring, ohm the leads, and wire them back, or order a new one from the factory. You would be surprised what they still have in stock, I just ordered a 162-031 19K coil for a MR73, retail at $6, better than telling the customer his tuner is scrap.

Ok, I should really be in bed, but your comment about the MR73 got me digging out the schematics. Aside from the collector and base voltages on the amp transistor ahead of the 19kc transformer, the circuit topology looks identical on the MR73, the MX112 and 113, and the MR77 and 78... so I'm wondering if maybe a 162-031 would be a workable substitution for the 162-055. Heck, at $6 I might as well give it a shot, huh?

02-01-2009, 07:56 AM
Ahha yes, the sign of a learning technician, make do with what is available, check the pin out of the new can, the numbering and layout is a bit confusing.

02-01-2009, 01:02 PM
Ahha yes, the sign of a learning technician, make do with what is available, check the pin out of the new can, the numbering and layout is a bit confusing.

Hot soldering iron and a can do make do attitude. That's me! :)

And doggone it, Mom Fantastick'ed ALL the lettering off the 275 :( . At least I don't have to worry about destroying it cleaning it up.

02-02-2009, 06:01 PM
Have you ever repaired a MC7205? I just picked one up cheap that has the entire left side out. I'm assuming that one channel has shorted and the power guard protects the entire heatsink (2 channels). This was purchased from an ex-McIntosh rep. He indicated that heard a pop, then smoke, which sounds like a resistor/transistor shorted. This happened with less than one watt output while listening to a TV source. My question is, does McIntosh stamp their part numbers on the transistors or will these be easy to source elsewhere? I would tend to believe McIntosh sources these transistors from motorola. Or do you suggest using McIntosh replacement parts? I do have the equipment to match parts but may not be worth the effort if its just a couple transistors. Your thoughts?

02-03-2009, 08:20 AM
My first suggestion is to get the service manual, the one for the 7205 gives a fairly complete parts list with the generic part numbers included. I buy most of my parts from the McIntosh parts department, saves time on finding a source then coming up with a minimum order, one stop shopping.

02-03-2009, 10:04 AM
There was a discussion some time back regarding MX-110s and I/F cans with where they were rebuilt to eliminate a small silver-coated cap in the can's bottom. Might search with those elements.....I've got an MX-110 with the symptoms that will need attention at some point.




Here's one

And another


02-06-2009, 10:19 AM
This is my first newer McIntosh amp I've repaired and I couldn't believe the number of parts damaged by a shorted output. The circuit board was also burnt to a crisp underneath R8 resistor (flameproof, yeah, right). I have probably repaired a half a dozen McIntosh amps over the years but have never seen such a catastrophic failure as this in an MC series amp. I have a poor quality schematic found on the berners site so troubleshooting is difficult except for the obvious. I requested a schematic from McIntosh but not sure if these are free or not.
These are the known parts shorted or out of tolerence due to the short.
I am replacing all the outputs on the one channel as preventive maintenance even though one side tests OK. What concerns me is the board is double sided and some of the output transistor connections are made by the solder only since the pins were not turned thru the circuit board. This is probably why the output shorted as some of the solder did not fully penetrate the holes and appear to be a "cold" joint. Will probably order all of these from McIntosh unless they charge me $10 a piece for outputs.

1 6644600 47MF 35V C1
2 7004700 DIODE D4,D5
2 13222300 MPS4124 Q1
1 13222400 MPS4126 Q2
1 13226100 A1145 Q7
1 13226200 C2705 Q6
1 13704200 47 OHM 5% 1/4W R12
1 13706500 430 OHM 5% 1/4W R7
1 13707000 680 OHM 5% 1/4W R2
2 13708000 1.8K OHM 5% 1/4W R6,11
1 14103300 220 OHM 5% 1/4W R8
3 NOT LISTED 2SA1302 Q8,Q26,Q10
3 NOT LISTED 2SC3201 Q9,Q11,19
2 13222300 MPS4124 Q6,Q44
2 13222400 MPS4126 Q1,Q43
1 13225000 3068 Q7
1 13225100 1238 Q3
1 13225400 C2302 Q5
1 13225500 A1016 Q2
1 13226100 A1145 Q18
1 13226200 C2705 Q17


MCM Electronics is a very good source and at 30% of the cost.

02-14-2009, 04:15 PM
Hello Terry and everyone,

About the MA 230:

If I were to tube roll some EH7591s in a matched quad, instead of the currrent Westinghouse/RCAs, I understand that the MA 230 would have to be rebiased to reflect the new EHs.

Is this something I can do myself, and if so, how and with what? Thanks in advance.

By the way this thread is remarkable. You guys are engaged in something really valuable here. Thanks!


02-14-2009, 07:54 PM
Owners manual gives procedure, each tube has a test point and associated pot for adjustment. The manual calls for each tube to be set for .7 Volt at the test point, it will take several passes to get them close, I usually set them for .6 a bit on the light side. A small screwdriver from the top will access the adjustment, start with the pots CCW for minimum voltage for safety, a 10 minute warmup volume set to 0.

02-14-2009, 11:20 PM
Many of us are reusing our shipping cartons many times, some even modified to hold different models, I have been known to get out my 1" wood drill (blade type) and drill new holes for the shipping feet.
Another thought is what about cleaning up your box with 10 different shipping labels and assorted layers of tape - you can remove old labels and tape from a box with a heat gun or hair dryer, heat makes the adhesive soft and release it’s grip, the box may be well used, but now it is clean and ready to go another trip.

02-15-2009, 11:39 AM
Terry thank you for your directions and advice. imho this thread makes many valuable contributions to member not just now but into the future as an archive of knowledge and experience not easily passed down or accessible otherwise.

02-18-2009, 09:50 AM
Are there any LED lights available for this model?

02-18-2009, 11:55 AM
Hello Terry:

I installed new glass on my 2255 amp and mr 77 tuner several years ago. The new glass from McIntosh doesn't diffuse the light evenly like the old glass did. To see if I didn't do it right I put the old glass back on and in spite of the all bubbles in the old glass the light was perfectly even. Is this an art or a science? I have noticed new Mac gear in the showrooms that don't have even lighting across the board. The back of the new glass doesn't appear to be the same as the old glass.


02-18-2009, 12:02 PM
Might try some games with the light diffuser, look at how the pattern of white paint is on the backside to make it glow behind the lettering areas, possibly more white. A material you can experiment with that is easily removable is White Out.

02-18-2009, 07:00 PM

I replaced all the smoked parts. But, after firing back up with the variac, the outputs on both sides of the left channel go into a thermal runaway. Its the input board but I dont understand why it happens on both channels. McIntosh forgot to send the service manual so its frustrating trying to read a blurred copy I found on The bias is not high because the variac reads less than 1/2 amp. I'm afraid to leave this on too long for fear of smoking the outputs again.
Do you have any experience with this amp?

02-18-2009, 07:31 PM
Minor experience, turn bias completely off, the last part of the turn should do nothing, this will let you know if the bias circuits are working correctly. Last one I worked on had DC offset, was one of the 6 lead pack transistors, check them all with a Simpson.

02-18-2009, 08:10 PM
I have suspected some of those but don't have a data sheet, do you know the part number, 1237 does not show anything.

02-25-2009, 01:57 PM

You were right, it was the 6 lead pack transistor. I found out the real part number from McIntosh and was able to test the device. This 6 pack lead device is a 2SC3064 differential amp and is no longer available from McIntosh. I'm about to match it up with some standard transistor devices but am concerned since these matched pairs. Don't know why McIntosh cant come up with a solution.


DO NOT ASSUME the ribbon cables can be swapped from Front to Surround on the preamp board of a MC7205. The pos 59v pin is swapped with the ground and could smoke a channel, had I not used a variac, this channel would of been smoked. THE SCHEMATIC IS IMPORTANT on this amp, follow the cable connections carefully!!!

03-03-2009, 12:13 PM
I am recapping my MC2125. I cannot find or decide I guess what caps to put in the power supply. The main caps are the silver Mepco/Electra 10000uf, 50 VDC.
This is my first recap, and I wanted to change these since the amp is 31 yrs old.
Would appreciate any suggestions / sources for these big power supply caps.

added note:
I did find caps that are exactly the same size, including the spacing for the screw mounts at Mouser. However it is not available in the 10,000 uf. The new one is a Vishay/Sprague 15,000 at 50 V. I was worried about getting caried away with inrush or charging peaks since it is going to up the capacity about 50% at the output of the bridge.


03-03-2009, 06:03 PM
You are safe up to 20,000uF or more, the 2125 has inrush limiters to protect the bridge and power transformer.

03-03-2009, 07:28 PM
Hi I just got a C35 and MC2255. They seem to be running pretty good for their age. What kind of maintence should be done at this point? The 2255 seems to have a meter thats a little low and slow. Is it in need of cleaning or is there a deeper problem?

03-03-2009, 08:24 PM
Meters calibrated, controls cleaned, a good coat of polish, and a full test to specs are what is needed, from this the next step would be decided.
The next step would be a freshening up with some capacitors in the audio path, might smooth things out some, at this age it's a tossup as to how much to do and how much it will help.

03-14-2009, 10:04 AM
Hi Terry,
Im alittle embarrassed I didnt question this sooner,
As Ive had this MC 2505 amp for over 4 months now ....

Should the "Red" light above the Mcintosh logo stay "ON"
during entire operation ???
(I hope so~)
Thanks - Mark

03-14-2009, 11:16 AM
Same lights that do the lettering for the controls do the red indicator indicating power ON, yes it stays on if power is on..

03-14-2009, 12:27 PM
Thanks Terry,
Just wanted to make sure,I feel better!

03-29-2009, 07:14 PM

I finished recapping my MC2125, everything looks ok at first check. Bias adjusts OK, etc. I thought I had read on here that all 4 lights (two normal and two red limits) come on briefly when you power up. Actually the normals stay on if I think a second.
Should the limits blink? The pwr circuit was toast at first but new small caps and diodes in the front board fixed that.

Thanks for any info.

03-29-2009, 07:41 PM
This is a hard to answer question, the circuit as not specifically designed to blink the limit lamps then go to normal on, but most PG amplifiers do this when turned on from a cold start, good way to check the limit lamps.

04-01-2009, 01:45 PM
Just letting you know what I found with my limit lamps......I shouldn't even tell this one....I did a little troubleshooting, found that pwr guard was working well on the output if I pushed it too hard with a sig generator. When it was limiting the clipping, nothing seemed to be driving the lamps. I had put a piece of scotch tape on a couple of pins on the single sided edge on the pc card while isolating circuits at early troubleshooting . After letting the project sit several months, guess I forgot I had done that, so the tape was doing its job- blocking the limit drive inputs on the pwr supply board!
Unit puts out 180 w/pc before noticing clipping on the output. The pwr guard when switched back on, lights up at almost the same pwr out. So I guess it is working OK.

Just for the record, my red limits do not come on at power up on this particular unit, but they work fine now that I found my goof up.