Yamaha PSR S770 Won't turn off after Powering On

Good morning my good friends, how are you all doing at this pandemic period. I wish you all safety and good health. Amen.

Please I have a PSR S770, the issue is it won’t power off each time i power it on, unless I remove the adaptor for it to turn off.

Please buddies, what do you think could be the problem and possibly solution to it?

Am very open to your professional advice now. Thanks, I’m most grateful.

Best regards.
Be safe.

Probably the power switch. You can try cleaning it with some contact cleaner, deoxit etc. But it is a component that gets a lot of use.

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Alright, I will try this. Thanks buddy.

Very much doubt this to be caused by the power switch


If you look at the 2nd pic and follow the yellow/red line - the switch just makes GND contact that is all.

1st pic - follow the red line and the switch GND just turns on the FT100 FET and enables +16V to go through.

Now it gets tricky - PWSO and PWSI go directly tp the CPU. As soon as power is on and the CPU is running it holds PWSO high - hence FT100 FET stays on (green line).

Next time the power switch is depressed follow the blue line … PWSI gets sent to the CPU and it then de-asserts PWSO and the unit powers off.

Therefore - my guess would be either the CPU is not doing what it should --> do a FACTORY RESET maybe - or, more likely, bad contact of sorts from D102 via PWSI to CPU (it not seeing the off command) …

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Thank you so much for the expository

Just as an update to this thread. (Thanks wernersaurus for the detailed explanation), I had this problem with a customer’s Yamaha and also doubted it was the switch as the switch turned it on ok. But it turned out the resistance of the conductive contact on the rubber switch was low enough to turn it on but too high to turn it off, you can verify by shorting /SPSI to ground to see if it turns off. It did in my case and a replacement power switch button solved the problem.

yeah, its easy to discard the switch as the problem since it turns on, but tolerances can be weird like that, id start with cleaning the pads and contacts, i assume its a rubber button since were talking about a PSR and go from there.