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Problem:
- Screen was just a bunch of "N"s with some random changes
Troubleshooting/dianosis steps:
- CPU was soldered on so first pulled up mame and ran in debug mode to see if that would shed any light on the subject. Found that the system initializes by filling the screen with Ns right before the game starts. Looks like it finished all the RAM writes, so looks like RAM is probably OK
- Desoldered CPU and installed a socket
- Used Fluke 9010a to do long RAM tests of 0x8000 - 0x83FF, 0x8400 - 0x87FF and 0x8800 - 0x8FFF all OK
- Pulled ROMS and computed FLUKE check sum using ROMident
- Put ROMs back into the game, and did a ROM test on the various ROM ranges with my Fluke 9010a. I Found that ROM @ A4, A5 and A7 all check out ok from within the system, but ROM A6 was giving a bad checksum with the Fluke.
- Looked at the control circuitry for the ROM selects, it was effectivily just an 74LS139 @ A3, chip had some white residue on the pins, and measuring with my oscilliscope was giving wierd results while doing a fluke READ (with loop) at the varios ROM memory ranges.
- Pulled 74LS139 @ A3 and replaced
- Game works!
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