Ghost in the machine (Pioneer SX-790)

[jeeem]

I picked up a very handsome Pioneer SX-790 (the dark-dial equivalent of the SX-780) and had to replace bulbs and clean it up a bit. (As you can see, I'm experimenting with some ice blue LEDs.). Odd thing is: the meters sometimes dance even when there's no input attached (PHONO or AUX -- not even using the radio). There's an occasional audible click, which seems to be coming from the box (relay?) you see below, that seems to turn the meters on and off at random. (If that's the problem, how do I replace it?) I hooked up some speakers and there is indeed a signal passing through (a low hum and random static) that corresponds with the movement of the meters. Sometimes only the meter on the right jiggles all by itself; other times both do.

I suppose I could just disconnect the meters and see what happens, but that might do more damage. And they're cool looking whether they're moving or not.

Any ideas for how I might be able to vanquish this poltergeist?

 

[larryderouin]

Protection circuit is cycling. The relay is doing what commanded, so it's ok for now. 1st thing is to do the center voltage adjustment in the manual (IIRC pg 18 or 19 of the SX-780 manual). BIAS is set by the STK module and is not adjustable. Make sure the voltage regulators (3 transistors with heatsinks next to relay) have not desoldered themselves. Refresh solder points as needed on board. It's 43-44 years old. It may need an overhaul. Lot's of rebuild threads on here for the 780.

 

[zebulon1]

It wouldn't be a bad idea to clean and lube the controls. Start with the Volume and Balance controls see where that takes you.

 

[Watthour]

...Then measure the DC and AC voltage on the main filter capacitors.

 

[merlynski]

+1 to Larry's post!
The noise may be coming from old/failing transistors, and causing enough dc offset to trigger the protection circuit.
Here is a list of known problematic transistors, compare to your schematic and what is in there: Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements
It could also be bad (failing) solder connections on Q25, Q19, and Q20 (the three with the little U channel aluminum heatsinks). Reflow the solder on the three. Q25 is the worst, and causes the protection relay to cycle or drop completely. Prime candidates for replacement and heatsink upgrade!