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That's usually a relay going bad. The way to test for sure
is next time a channel cuts, instead of cycling the power
off/on, turn off the speakers and turn the volume up about
3/4 for about 5 - 10 seconds. Turn the volume back down to
normal and turn the speakers back on. If both channels are
back it's definitely a relay. Sometimes cleaning can help
sometimes they're shot beyond hope. I'm sure Mac has the
replacements if necessary.
I would say it's in the POWER GUARD circuitry and it should pay a visit to Binghamton NY and see it's mother.
Chances are it's older design is just worn out. Also you maybe driving some hard to drive speakers i.e. varing resistance or possibly cross overs failing but I would lay a bet it's the PG curcuits though
You are correct in this sounds like a relay problem. If the amps are being overdriven or have poor ventilation they may thermally shut down. The MC2505 does not have power guard as it was not invented until the MC2205 by Ron Evans, the current head of McIntosh engineering.
All Mc amps have the sentry monitor circuit which will protect the amp if the load exceeds the safe operating area of the output transistors. A defective speaker or one with a shorting voice-coil will trigger this also.
Faulty relays is an extremely common problem with vintage
SS amps of every make and model. Paradoxically, I was told
by a tech I trust that many relays go bad because the amps
are never driven above a few milliwatts on average. The
current has a self-cleaning effect on the relay contacts so
relays actually benefit from a good workout once in a while.
3 out of 4 of my vinatge receivers have relay issues like
you describe. Interestingly, only my McIntosh MAC-4100
receiver (1979) still has reliable relays.