If you're using one of the cheap Chinese PSUs, don't. They are a bad day waiting to happen. Please don't risk hurting yourself with those cheap nasty PSUs, never mind the risk of damage to the board you're working on.
Get a Suzo Happ arcade PSU, Mean Well bench PSU or a non rubbish (something 80+ rated at least) ATX PSU (ATX is the safest option)
Electricity is no joke, at the very least a shock will hurt like hell.
Those 2 customs are exactly where I would be looking of the fault is definitely on the B board