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Old 10th November 2000, 09:25 AM   #1
diyAudio Member
Join Date: Nov 2000
I'm repairing my 20+ year old McIntosh MC2125 poweramp.
I got the service manual already, so I have all the circuit details.
The problem with my amp is - occasionally, the left channel would gradually fade and starts to sound sniffy.
Many time, the sound will finally be gone. Lately, the right channel starts to have similar problem, but a lot less serious. The power guard light also turned red quite often.
The problem is I don't know where to start checking because it doesn't always happen. If I leave the amp off for a few days, it will normally come back to work properly for half and hour or more. Any suggestion ? I plan to have it overhauled anyway. But which parts of the circuits are most likely need to be replaced ? Thanks
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Old 10th November 2000, 09:13 PM   #2
diyAudio Member
Join Date: Oct 2000
Location: Austin, Texas, USA
Default Testing.

This sounds like a thermo-shutdown problem. So, one of the tubes or transistors is overheating (whichever this amp is?)

Try a can of Freon(sp?). Keep one component cold until the amplifier shuts down. Then you know it's Probably not that one. It's a slow process, but it will isolate the problem IF it is indeed a overheating problem.

Of course there's always the old standby... Ow! Yep it's that one!

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Old 2nd January 2001, 03:15 AM   #3
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Join Date: Jan 2001
Location: Lawrence, Kansas
This sounds like you have one or more leaky output transistors. Rebulid the amp with new quality outputs (Motorola is my favorite). Also the power supply filter caps are proably week and need replaceing.
Bob P
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Old 4th May 2005, 12:35 PM   #4
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Join Date: May 2003
Location: quebec
few questions?

Is that amp solid state ?

Is the power suppy regulated type ?

Is there current limiting in power supply ?

If yes to all find shunt resistor in current limiting circuit and the transistor used with it.

If the transistor is normally at cutoff with a high voltage ( emitter- collector ) across , it could produce your trouble.
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