Denon AVR-789 Receiver Repair Advice?

[AmmoJammo]

Got this amp for free.
It powers up, starts its self tests, then powers down, and flashes the power light red.

If I disconnect the main AC power to the amplifier board, so the rest of the amplifier still powers up, it passes its self test, and doesn't shut down.

I expected to find a shorted output stage, but now I've completely torn it down (as half the parts aren't accessible) they all appear to be fine.

I prefer devices that set the faulty part on fire when they fail... 

Thanks!

[kripton2035]

when it's not the output stage that fails on the denons, it's some power supply that is not in the tolerance.
check every power supply output, one must be missing or out of specs.
the schematic of the amplifier is mandatory to fix them.

[AmmoJammo]

when it's not the output stage that fails on the denons, it's some power supply that is not in the tolerance.
check every power supply output, one must be missing or out of specs.
the schematic of the amplifier is mandatory to fix them.

Can't test voltages on something that doesn't turn on

I found the issue though.
The 78/7915 regulators that are known to fail, had been replaced before.

However, the rectifier and current limiting resistors that feed the regulators had failed.
Two diodes were short circuit, and half the resistors had gone open circuit.

I had tested the regulators themselves out of circuit.
I've replaced them again now anyway

[kripton2035]

Can't test voltages on something that doesn't turn on

yes you can. only the amplifier doesn't turn on, the power supplies output their voltages anyway.
and yes often some 78/79xx fail. 7x15 are good candidates.

[AmmoJammo]

Can't test voltages on something that doesn't turn on

yes you can. only the amplifier doesn't turn on, the power supplies output their voltages anyway.
and yes often some 78/79xx fail. 7x15 are good candidates.

No, the amplifier was going into "protect" mode, and shutting everything down, apart from its standby power supply.

[jogri]

Do you know if your 789 has some hand-soldered resistors on the underside of the bottom PCB? I have a 1909 (same board) that won't start up after i replaced a few opamps and caps on the input board, and it looks like my AVR blew a few caps before i got it (it had some new Nichicons on the power supply when i received it). Now i don't know if those resistors (a few 1/2 W resistors with foam between them and the pcb) are supposed to be there (design fail, forgot to add them) or if someone added them for whatever reason.

[AmmoJammo]

Do you know if your 789 has some hand-soldered resistors on the underside of the bottom PCB? I have a 1909 (same board) that won't start up after i replaced a few opamps and caps on the input board, and it looks like my AVR blew a few caps before i got it (it had some new Nichicons on the power supply when i received it). Now i don't know if those resistors (a few 1/2 W resistors with foam between them and the pcb) are supposed to be there (design fail, forgot to add them) or if someone added them for whatever reason.

Yes, my board has them. Sorry I didn't take any pics, and its all back together, but I can confirm, they're a factory fitted bodge.

[tooki]

Can't test voltages on something that doesn't turn on

yes you can. only the amplifier doesn't turn on, the power supplies output their voltages anyway.
and yes often some 78/79xx fail. 7x15 are good candidates.

No, the amplifier was going into "protect" mode, and shutting everything down, apart from its standby power supply.

No, kripton2035 is absolutely right.

FYI, here's how you would have measured that, since you certainly can do it in this kind of situation: hook up a meter to a rail, and then turn it on, and see what voltage it reaches before protection shuts it down again.