From reading the guides, the U20 is aparently the most common chip to go. From looking it doesn't look to have been replaced (not socketed) but the LM339 has. I reseated it but no luck. The machine I don't believe has had a hugely hard life so it may have slipped through. I thought the clunking of the playfield on those damm hinges this morning I might have pinched something, but looked and looked and could see nothing. I tested the pin 3 (connector side) on buzz to earth and nothing. From googling it seems that the ground short error is never likely to be an actual ground short, but a 'something broke' error! More coincidence I think. But happy to get it out of the way now, the power board has been done, everything else clean.
The board will be out for a week as I won't be around (working up Marysville way)....
