you most likely have a short surface condenser on the mainboard, I would like you to comment on the main part number, you can tell yourself if you check continuity between the ground and the 21v pin will mark continuity, personally to find the short , I connect a source other than 6v not switched if not linear, with the main disconnected, I connect the 6v source between earth and 21v, feed and touch the capacitors the short one will overheat, do not connect the source long, connect and touch, disconnect, reconnect and touch, in this way and found the damaged capacitor.