200A sound randomly cuts out (onboard speakers)

[Eleven]

To the best of my knowledge it was manufactured in 82’. I bought it NOS in 2008. After playing it for an hour or so the sound cut out. I opened it up and gently pressed down on the amp board and the sound was back, temporarily. I examined the PCB for bad solder joints or breaks anywhere in the trace but everything is solid. I got fed up and put the piano in storage back in 2008 until just a couple of weeks ago. It played good for an hour then cut out. I used contact spray on the volume, vibrato, and pcb trim pot working them good while doing so. It worked until (lid back on) I started to press the volume knob back on. Removing the lid, this time I gently pushed the trim pot and the sound was back. I sprayed more cleaner and worked the trim pot. All pots seem solid and there is no evidence of component damage. It continues with the random outages. There seems to be ample insulation under the pcb. I doubt the isolated preamp is involved because the outages occur only when the amp rail is disturbed, perhaps by flexing?  The sound continues to go out from either putting the lid on, installing the knobs, or randomly. Speaker wiring is solid. Any ideas what would cause this?

Thanks…Rich

[pianotuner steveo]

I really think it's cold solder joints. They are not always obvious from visual inspection.
Careful resolder the amp board. Don't do this if you aren't sure what to do. In that case, find a local tech.

[gacki]

I'd probably check if the Aux out drops out at the same time as well. This would give a hint whether the problem is located just in the power amp section or earlier. Could also be that the power supply is generating the drop outs.

[Eleven]

Thanks for your responses guys. I respect and value your insight. I’ll try the aux out approach and if no luck reflow the solder joints, which I was putting off. I wasn’t aware the joints could look good (shiny/tight) and still be bad. I have a variable heat pencil soldering set I can use.

I removed everything from the piano to inspect, clean and lube. The only snag is I can’t figure where a ground wire leads to. It connects to the front center (to the front right of the preamp on a 200A). I can’t get my picture file small enough to attach.

Thanks…Rich

[Eleven]

I found the problem, one that I’ve read about so many times. It IS the large top shield. Looks like it is directly screwed to the harps so I’m not sure where or what is causing the short. I’ll loosen the shield mounting screws and position it higher. I don’t know why pressing the knobs back on would cause a short involving the top rail. At least I can rule out bad solder

Other than that, since my serving the action  there is a noticeable improvement regarding consistent sound (no more hot spots) and a major improvement with the action.

Could the shorting episodes have caused damage somewhere or is a a case where it would work or it wouldn’t?

Thanks again…Rich