theoretical repair of a Boss TU-2 ?....

[Johan]

my pedal isnt here, so I cant check anything, but please help me determening the likelihood of a therefor theoretical repair...( I'll only have a few hours between daytime work and gig tomorow, so any ideas to check are good ideas)
http://www.synfo.nl/servicemanuals/Boss/TU-2_SERVICE_NOTES.pdf
schematic on page 5

here is the problem.
My tuner has started acting weird. when operating on battery, it is dead silent(as far as hum and hiss is concerned), but often goes out of tuner-mode into bypass(play-mode) for no obvious reason.
when using a well filtered regulated powersupply, it hums but work as it should in all other respects.

I figure this has to be a power/ground issiue and the most likely culprit seem to me, should be the "blk" (cable?) going from input jack ground to board connection "4" pcb-ground


..I will off course bring a spare tuner to the gig, but I hate unplugging to tune between songs, and I hate to tune by ear in front of an audience ( ruins the drama/show..) so I really want to get it working again

any ideas to check in the short time I have tomorow would be much appreciated...
thank's
Johan