Brian E. Brzezicki
Copyright 2015
Version 1.00 03/26/2015

Disclaimer
Use this at your OWN risk, do NOT sue me if it breaks your board... if your doing this your board is probably already broken though ;)

Game Board #1 3/26/2015

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!