MC-754 intermittent dead channel issues

Mar 23, 2020
New here but not entirely to audio...I can't stop tinkering. At any rate, I've always dug the McIntosh gear and I just picked up a pair of MC-754's that are in GREAT physical shape but are giving me problems. The amps have no power switch so I use the switched preamp 120V to activate a power relay to supply AC. At turn on, with LOW volume input and gains set at about 1V, at least one channel will either cut in and out(making noise) or not play at all. If allowed to sit and "run" for about an hour, turning up the input(volume) or switching the impedance switch will cause the channel to start playing and it acts fine UNTIL the amp power is turned off then back on, then it reverts. I've had the amp in a repair shop in Tampa for about a month now and after 4 attempts all I'm being told is "I need a McIntosh pre-amp, the MiniDSP is no good...too low of output impedance/voltage." The repairman soldered a resistor in the input path(on the RCA's) to correct a 40KHz "high current oscillation" that he felt was the good...same problem. I connected not only an iPod but my Yamaha preamplifier DIRECTLY to the Mc(no miniDSP) to totally a different set of speakers...SAME results. Anyone out there had this? What was the fix? I've heard output relays but have NO idea where to start.
FYI, the system is: Turn table, DVD, TV>>Yamaha Rx-V750(pre-amp out RCA unbal)>>miniDSP 2x4 Balanced--OUTPUT 1(Bal)>>Crown XLS-1002>>subs, OUTPUT 2(Bal)>>Crown XLS-1002>>woofs, OUTPUT 3(unbal)>>McIntosh 754(L)>>mids, OUTPUT 4(unbal)>>McIntosh 754(R)>>highs. FYI, the minDSP unbalanced outputs are switchable between 2 and 4volts. I switched them to back and forth to no effect.
Help!!! :rolleyes:

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