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McIntosh - Hints and Kinks

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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?
 
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?
Click to expand...

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.
 
MC60 Bias update

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.
 
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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!
 
Speaker terminal screws

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.
 
preamp to amp Jumpers

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.
 
Tuner alignment MR65B

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.
 
dewickt said:
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.
Click to expand...


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.

r
 
MR65B, MR67 Increase Stereo Separation

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.
 
Fixing severe DC offset on MA-5100

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.
 
MC240 Bias update/mod

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.
 
Rebuilding Bias Supply on MC-40

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.
 
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.
 
MC-40 Rebias

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?

Kai
 
The 22 uF will stiffen the bias supply slightly, not critical but a little better with the 22uF
 
C712/C15 Buzz in output

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.
 
MR74, MX113 etc. Dark dial indicator

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.
 
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