PCB Repair Logs The Simpsons
Forum Thread: The Simpsons PCB Repair
Symptom: No sound, but sound amp okay
Diagnosis: Noticed that when going into the self test and doing a ROM Check the two mask ROMs for the samples at 1D and 1F were never being tested – the test seemed to skip those, so it tested the rest of the ROMs and put up ‘OK’ with a checksum, but never put up a checksum for 1D and 1F or even an ‘OK’.
Played the game and checked the schematics and found no address or data line activity on those ROMs. The addressing comes from the surface mount Konami custom ‘053260’ at location 3E and the inputs to this chip were good, just no output. So had to replace the chip with one from another Konami board of that era, then the sound worked perfectly.
Cure: Replaced faulty ‘053260’ Konami custom chip at location 3E
Solution: Going into the test menu I did a ROM check and since it's quite in depth I was inclined to believe it's result and it threw up a bad mask ROM at 16L.
I didn't see any noticable damage to the traces that ran from it but I did notice some scuffed tracks near there which made me suspect one was broken. Metering the tracks they turned out to be fine.
Next port of call was to try and work out what this was equivilent to (if anything) in the EPROM world. Turns out the pins matched a 8mb (512kx16) EPROM 27C800 (I knew it was an 8mb ROM since it was on the board).
Knowing roughtly what I was looking at I got out the logic probe (rapidly becoming my favorite tool) and started checking the address lines. I didn't have to go too far before I came across the first address A17 which was completely dead. This should join up to the first pin on the next sprite ROM in the line as all the other addresses did. I couldn't veryify for 100% since the track went off under a logic IC but when I tried shorting them:
Success! I assume a through hole had somehow failed, though how I've no idea. Regardless been playing for a bit now and all is still well.
Notes: I thought while I was at it I may as well convert this four player board to a two player since I don't have a 4p cab so burned the 2P ROMs from the MAME ROMset and off I went