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McIntosh - Hints and Kinks

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blown fuse and red glowing tube on mc2102

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,

AY
 
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.
 
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.

Zekk
 
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?
 
replacing light bulbs on Mac1700

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.
Best,
Radu.
 
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 ;)
 
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.
 
HELP:?: Need Bias Set Procedure: MC2105/Service page and User manual

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.

Thanks,
John
 
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.
 
RE: 2105 Bias - yes NO adjustment

Terry,

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.

John.

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.

John.
 
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.

Thanks,
John
 
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.
 
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?
 
Terry and all, other poster,

Thanks.

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,
John
 
Bias Voltage MC240

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.
 
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.
 
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