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Old 28th April 2003, 11:21 AM   #1
Nui is offline Nui  Thailand
diyAudio Member
Join Date: Apr 2003
Location: Bangkok
Unhappy Not just repair but renovate

I have had McIntosh MC2125 stereo power amp (solid state with output transformer) and its service manual (complete schematics and PCB layouts). The amp is working but with occasional problems such as power drop in one channel (some time both), tik tik tik ... sound from tweeter even with input shorted to ground, and power meter(s) stop moving. Leaving the amp off for a few weeks and it usually comes back to work fine for a few days and then some of the problems creep in again. It has been like this for a few years now.

I asked Nelson Pass for his opinion on what could be the cause, and he suggested that for an approx 30 years old amp like mine, anything is possible and there probably are multiple causes.

Because of personal intimacy (of my 67-year old dad, in fact), I would like to attempt to renovate the amp. This is probably analogous to car restoration. It ain't really break, but I just wanna refresh it.

I have a few things in mind which I listed below, but hope to hear you folks' opinion on which actions would be worthwhile (or not worthwhile) for this kind of objective. My problem is - I have no idea how high quality the original parts are compared to current technological standard. The resistors, for example, are mostly 10% tolerant. Small caps are dark-red mylar. PS filter caps are big metal can type (4x10000 uF). Therefore, simply replacing with new parts may change it for worse (if that old part is of very high quality and in good condition).

- Replace electrolytic capacitors (with new audio-grade ones)
- Replace small caps with Wima polypropylyne type (stocked a few)
- Replace output transistors (Mr. Pass recommended Motorola MJ1502x serie)
- Rewire the signal path with some interconnect cable (like those from Kimber Kable, Monster, etc.)
- Bypass output transformer.
- Replace opamps (LM301) used as comparator in the McIntosh's power guard circuitry and a few more for driving power meters. Any stable opamp should work OK for those circuit.
- Replace input sockets and speaker terminals with new gold plated ones.
- Bypass 100k input attenuator.

Or if you have any other thing to suggest, you're welcome.

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Old 28th April 2003, 12:39 PM   #2
diyAudio Member
Join Date: Oct 2002
Location: Royal Oak, MI.
Thumbs up Good Start....

Hey Nui!,

From what I've seen in the past, Most of what You discribe sounds like the electrolitic caps have good bad. I'd go slow, and check everything!!

Keep us informed of Your progress.

Best of Luck!,
Tall Shadow
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Old 28th April 2003, 03:20 PM   #3
Nui is offline Nui  Thailand
diyAudio Member
Join Date: Apr 2003
Location: Bangkok
I just made a mistake, dropping a staple under the amp PCB. I heard its sound when I fliped the amp, but couldn't locate it. After turning on, the power meter shot maximum, then smoke came up!! I turned off the amp and finally found the staple. However, to my amazement, the amp still operated as if nothing had happened!!! I checked around the place where smoke came up, but couldn't locate any burnt spot!! Wonder what it is
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Old 2nd May 2003, 04:18 AM   #4
Nui is offline Nui  Thailand
diyAudio Member
Join Date: Apr 2003
Location: Bangkok
I changed most of on-board electrolytic caps (PCBs for voltage amplification, regulator, meter control) and also add bypassing 1.5uF caps for supply terminals of every audio signal PCB. Also replace the RCA input sockets (old ones have been broken). For all decoupling caps, I replaced those eletrolytic ones with WIMA polypropylenes.

With a little surprise, the amp seems to be in no-problem condition now. No power drop noticed after more than 2 hours of play, no strange meter movement (it bounces with the signal properly now, tracking either dB or Watt), and tik tik noise is gone. Hope it stays like this from now on. I didn't replace the main filter caps (10,000uF x 4) because the old parts look so high-grade and top-quality built. If the problem comes back, I may try replacing them. But for the past two days, things have been OK.
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