Rotel RSX-972 no audio output, 60Hz hum instead

[Marc92FS]

Hi, people... new to this forum.

I have a 20 y.o. Rotel RSX-972 that worked perfect until the infamous memory supercap gave up (about 10 yr or so). Nonetheless, I still continued enjoying it, leaving it stand-by in order not to lose settings. From time to time, after unplugging it or after a blackout I had to re-configure it, but otherwise everything was fine.

Some time ago, one day I plugged it in, and got no sound from any source to any spkr output. No matter the configuration I tried, it wouldn't sound. Only a moderate 60Hz hum from the front spkrs, not altered by volume control.
Then I replace the broken 0.1 F 5.5V supercap... to no avail I assumed it would be the culprit, but apparently there's another issue. Supercap pins voltage is about 4.8V and radio station selected remains after 1 hr disconnected from the mains, so I believe memory backup is working OK.
Still, only a low buzz from the speakers...
Any ideas?
Thanks in advance.

Marc.

 

[avionic]

Welcome aboard.. My first suspect would be power supply filter caps.

 

[avionic]

Does it hum in all 5 channels ?

 

[Marc92FS]

Does it hum in all 5 channels ?

Yes. Hum is present in all channels no matter what input I choose.
And I must correct: volume control DOES alter hum intensity in the front channel only.

PS filter caps don't look swollen nor haven't leaked.

 

[avionic]

PS filter caps don't look swollen nor haven't leaked.

Doesn´t disprove anything..
I have pulled two 3900µf@50v filter caps out of a pre-amp last night. Both of the Elna Cerafine caps that look like new. No swelling, shrinkage or leakage. Both are dead " shorted " ..

 

[Marc92FS]

Hey, you've planted a doubt in my mind with that PS caps stuff...
Took it out again to inspect them thoroughly. And found two of them with some brown stains on the bottom (see pic). The other two look immaculate.
Measured voltage and curiously the stained pair displayed 10.15V each, but the clean pair only 1.45V. If bad, those caps would not retain voltage after being de-energized, would them?

 

[avionic]

Brown stain could be old dried glue. Remove the caps from the board and inspect the solder pins. That glue has a tendency to cause corrosion. Ideally you would need and ESR meter and a good LCR meter.