I bought a Korg MiniKorg 700FS from Thomann's couple of years ago.
I didn't get much chance to test it as my wife was in the hospital at the time.
But later on, I noticed that it would all of a sudden shut down.
Being owner of a pro audio and synth design company, I figured I would try to fix it, rather than sent it to Germany and then back to California.
Attempt 1, replace the power switch, funky looking solder joints...
No go.
Attempt 2, replace the power supply which was a PITA as Korg used an oversized barrel connector that no standard wall wart or lump would fit, so cut the cable and attached a new supply... No go
Attempt 3, complete dis-assembly and close inspection of the boards.
No go...
Attempt 4,
Another complete dis-assembly, and this time, I just happened to see a wire hanging out free from the Molex header.... These synths are built with wire, crimp terminals and plug housings instead of the horrible 3M ribbon cable crap.
This is good from the quality standpoint, but they are painstakingly hand assembled. This one wire was not "clicked" all the way into the housing and was making intermittent connections.
Anyway, I re-inserted the stray wire and terminal into the empty housing space and Viola! It has worked 100% ever since, and is the best built synth I own, and have owned dozens. Almost EVERY other maker uses that horrible IDC ribbon crap instead of wire harness. This may be OK in a computer or something like that but it has no place in musical instruments.
Anyway, maybe you have a similar failure mode?
Take care,
rf