locating ground buzz in Boss ODB-3

[MrStab]

hi,

i know this isn't strictly a "build your own stompbox"-type question, but i've seen a few of these and this is just a one-off so hopefully i'll be forgiven!

my bassist asked me to fix his ODB-3, which has the clear symptoms of a ground fault (ie. buzz that goes away when touched), but i can't seem to find the source.

i've hooked up the unconnected (but grounded via. enclosure) sleeve lug of the output jack to a test wire, but i can't seem to find the source in all the obvious places. i've tried touching the wire to: the input jack (both the sleeve and ring for kicks), the board ground, the grounded lug on the  Tone pot(s)... nothing. i have the non-bass OD-3 schematic handy, i'm guessing it's not too different.

seems to work fine otherwise. only apparent cause was some kind of non-obvious physical trauma. no components touching on the board, it seems.

cheers!