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Problem:
- Game is dead, no monitor sync.
Troubleshooting/dianosis steps:
- Since there was no good "sync" on my LCD test bench monitor I wante to test the timing circuits, usually I test the CPU clock first, then the video SYNC generated from the sync circuit on the game board.
- Tested clock signal on pin #6 of Z80 CPU, OK
- Test the SYNC line generated on the game PCB, this is the !COMPSYN signal coming from pin 15 on custom chip @u57, nothing, here is an issue we need to track down.
- Tested inputs to custom chips @ U57 all OK except pin #29, which was dead.
- Tested U17 (which feeds pin #29 @ U57, chip looks ok (it's a NOT gate and the output is the opposite of the input), but input to chip is dead, and it should not be.
- Followed back the input of this line which is generated from 72LS175 @ U19 pin #6, pin #6 @ U19 is dead.
- Followed the respective input to pin #6 @ U19, which is pin #5 @U19, also dead.
- followed back the input of pin #5 @U19 to pin #2 of timing PROM @U14. Pin #2 was actively changing.
- did continutity test from top of socket pin #2 of U14 to pin #5 of U19, no continuity! There should be continuity. The socket does look pretty rusty.
- verified continuity of underside of pin #2 to pin #5 of U19, continuity was good.
- removed socket @ U14 and replaced, reinserted timing PROM
- Game works!
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