Audio amp circuit only works when specific pin is touched

[magic]

Interesting. Are you sure there are no problems with the capacitor on this pin? Did anyone try replacing it instead of adding the resistor?

[keep_hope_alive]

I also have the Yamaha NP-31 keyboard. I tried changing virtually all the resistors / capacitors in a range of 6cm around the LA4625 and nothing changed.
BTW when you use the keyboard with 6 AA batteries, it's exactly the same, so I guess it's not some residual AC and wiring to earth is not necessary.
So my final solution, as it requires only a touch to pin 4 with any metallic object, has been to solder a wire to pin 4, and let is hang behind the keyboard (or you can place the other side of the wire anywhere). When I turn on the keyboard, I just need to touch the wire with some metallic object, and voila.
I don't even trust changing the LA4625, more risky than promising.

[update] SOLVED! Just solder a 1K resistor between the POP pin (7) and the +12V.

Thank you for updating.  I just acquired a NP-V80 with the same issue and you may have saved me from ordering a new IC. 

Looking at the datasheet for the LA4625, it seems we can snag a Vcc signal from Pin 14, correct?  That video appeared to suggest a random place on the board for Vcc.
https://www.mouser.com/datasheet/2/308/1/LA4625_D-2314925.pdf

Did you snag 12V from Pin 14 or elsewhere?  Thanks for sharing!

[keep_hope_alive]

Confirmed that soldering a 1k resistor between Pin 7 and Pin 14 works.  Slight pop on turn on and turn off but fully functioning sound again. 

[Doctorandus_P]

This reminds me of an old television (CRT) I once "repaired". It already was over 10 years old, and would not turn on anymore. After some poking around and reading datasheets, I discovered it did a "beam current measurement" during the boot process, and would not show a picture if that measurement was off. When fooling that measurement by momentarily adding a resistor, the CRT worked normally until powered down. My final repair was a resistor, a micro switch and a piece of string. So after turning the TV on, you had to pull the string to turn the TV on. It was good enough to get several extra years of use out of the thing.