Some days ago I bought on Ebay for cheap an untested original Sega Pengo PCB, today it arrived:
Buying a board as untested is always risky, you could ended up to have a not working one on hand and indeed mine was.I was greeted by a solid black screen when I powered it up, the main Z80 CPU didn’t reset properly.As schematics suggested the /RESET is generated by a counter 74LS161 @IC23 so I went to probe it and found it was missing clock signal.At this point I started to suspect some trouble in the timing signals generation.The Pengo manual has a troubleshooting section which suggests to check some iC in case the picture will not appear:
When I went to probe a 74LS74 @IC66 :
I found its outputs pin 5 and 6 were stuck high.I put a my fingers on it and it was really burning hot.Also comparing it against a good one with my HP10529A logic comparator confirmed my suspicions:
Once removed, the IC failed the out-of-circuit test:
Fitted a good IC fixed the board completely, no further issues were found.