Repairing an old McIntosh


2000-11-10 10:08 am
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

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! :)

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.