Another thread to document this repair.
Yamaha AX-750, comes with no sound, no relays activate (it has 3). One channel is working, because heatsink gets warm. Not the other, it's cool. No power led on, neither the Pure Direct led, but the led on Volume knob is blinking. User manual says this indicates Muting (-20dB). Remote control doesn't do anything.
Inspection: [Q624] with broken solders and "-15V" are really -20V, not good. "+15V" are +15V, OK.
I resolder [Q624] and some other transistors near there. Now both channels are working (both heatsinks warming), but still the same problem, no relays, no sound.
I replace 16V zener diode [D617] that regulates -15V. Fixed this, now really -15V. But the problem doesn't go away.
I found some broken solders on the long bridge PCB (MAIN3 in schematics, [12-3] in exploded view), resolder everything but no change.
Days later I find that signal POFF is almost zero and should be -19V. It comes from the transformer (+-20V) after rectifying half wave. Then I notice that resistor [R720] is open (1Ω). Replace and voila!
The only problem remaining is inoperative remote control and persisting Mute. It stays in Mute mode (blinking led in volume know), so the only way to hear it is activating Pure Direct or raising the volume a lot. The remote control is OK, it emits signal, and the IR signal gets to pin 27 of microprocessor and it "looks" good at the Scope.
Well, the customer didn't ask that much, as he already knew that the remote wasn't working 10 years ago. The short solution: desolder pin 38 of microprocessor, disabling the user Audio Mute signal. There's another Mute signal for functional purposes, not user, and that is working fine.
Yamaha AX-750, comes with no sound, no relays activate (it has 3). One channel is working, because heatsink gets warm. Not the other, it's cool. No power led on, neither the Pure Direct led, but the led on Volume knob is blinking. User manual says this indicates Muting (-20dB). Remote control doesn't do anything.
Inspection: [Q624] with broken solders and "-15V" are really -20V, not good. "+15V" are +15V, OK.
I resolder [Q624] and some other transistors near there. Now both channels are working (both heatsinks warming), but still the same problem, no relays, no sound.
I replace 16V zener diode [D617] that regulates -15V. Fixed this, now really -15V. But the problem doesn't go away.
I found some broken solders on the long bridge PCB (MAIN3 in schematics, [12-3] in exploded view), resolder everything but no change.
Days later I find that signal POFF is almost zero and should be -19V. It comes from the transformer (+-20V) after rectifying half wave. Then I notice that resistor [R720] is open (1Ω). Replace and voila!
The only problem remaining is inoperative remote control and persisting Mute. It stays in Mute mode (blinking led in volume know), so the only way to hear it is activating Pure Direct or raising the volume a lot. The remote control is OK, it emits signal, and the IR signal gets to pin 27 of microprocessor and it "looks" good at the Scope.
Well, the customer didn't ask that much, as he already knew that the remote wasn't working 10 years ago. The short solution: desolder pin 38 of microprocessor, disabling the user Audio Mute signal. There's another Mute signal for functional purposes, not user, and that is working fine.
what did we say about hoarding too much stuff and depriving others of them?! *cough* socialism *cough* lol! 
