YAMAHA PSS-680 STRANGE PROBLEM SOLVING ROUTINE

[samcameron]

just got a pss-680 for £15 in newish condition but i was fairly convinced it would have a fault.
Someone has clearly had it open thinking the problem was physical.
Baiscally it actually works fine but on power up it splats out a horrible percussion sequence
that you can not stop. Not just that but you can not use most functions either.
Thinking just sending sysex of the factory presets would cure it i connected it to a pc.
What i found was that it works 100% fine so long as:
YOU HAVE MIDI IN TO IT FROM A PC OR PC RELATED INTERFACE (such as emu or m-audio)
THAT IS PLUGGED IN TO MAINS POWER - IT DOES NOT NEED TO BE SWITCHED ON
JUST PLUGGED IN
this seemed a bit weird so i reasoned it must be to do with Midi pins. It seems like it is
caused by midi pin 2. This is different (or should be ) on pc interfaces in terms of
shield/grounding. I have no idea why this happens though and can find no discussions of this
anywhere which is why i am posting it here even though it may not be 100% a tip.

[Circuitbenders]

hmmm, it could be that the grounding via the power supply is disconnected somehow, like the solder joint has broken in the power jack socket, and its using the ground of the midi cable somehow instead. Although i'm not sure how much sense that makes as if recall correctly midi connections are only grounded at one end, either the out of the PC or the in of the Yamaha.