Second time I've come across this prea ground connected to casing and
separate pa ground connected to casing via standoffs to metalwork. Underside
of pcb large solder pad, upperside 2 pieces of tinned copper wire. The
underside solder seems to compress , maybe pcb as well, and then exceed the
give in the star washer and looses good grounding. This time prea end ,
previous time was pa end problem.
This time will solder brass washer on underside,each end, solder just
between washer and board , not standoff touching surface.