YAMAHA P-45B Digital piano - standby switch fails

[raul320]

Hello,

Few days ago, my digital piano Yamaha P-45B got an issue with turning on. power adapter worked fine, so i decided to open the case and dig a little bit deeper. After unscrew about 40 screws (!!),  i've started analysing what is going on and found damaged transistor TR101 2N5551, so i've replaced it by  similar one MPSA42-D.


It seems to be repaired, but:
 - After connecting power adapter, piano is ON (as ussualy)
 - After pressing standby button for 2sec,  piano will OFF (as usualy)
 - Then, when i press standby buttoon again, it won't ON (as it should)

I don't have service manual for this model, but i found similar one - basically this part of schematic seems to be the the same:


Now, can anyone explain me:
 1. how this mosfet switch works?
 2. How it turns on and off?
 3. What can be the reason, that it can't turn on again? 

[Vicus]

The mosfet FT101 is the "switch". In order to turn on so the current can flow VGS has to be lower than -2, -4V. This is how I think it work: When pressing the on off button the signal PWSW goes to gnd and via D103 it lower the gate voltage so it tun FT101 on. After it turn on PSWD goes high so TR101 turn it on and this keep on FT101. At turning off PSWD goes away and at /PSWI a high voltage inhibite the PWSW signal so FT101 turn it off.
It would be interesting to know who generate the three signal PWSW, /PWSI and PSWD.
Now you have the multimeter in your have, have the right measerement in the right moment and guess what's wrong.