Solved Taito space invaders freeze then reset

[patzik]

I have a 3 stack taito space invaders that developed a problem. The game starts with only the scores and credit on the screen then freezes. Then after about 3 seconds it resets.
I swapped boards, and the problem is on the cpu board. I ran the testrom and this runs fine revealing no issues. The point it freezes and resets is very consistent, and ram and rom test ok, so what is the code doing at that point that freezes the pcb causing the watchdog?

 

[philmurr]

Can you post a video of it resetting. I had what sounds a similar issue to this when I was making the multigame and it was due to watchdog timing.

Also what version of code is it running?

 

[patzik]

Had a couple of hours time today, so took a look at the pcb. Assuming the address and data part of the cpu pcb was ok, I focused on the other parts, and signals on the 8080. After some time having swapped 2 chips with no improvement, I decided the shotgun method might be much faster. Trying to understand the schematic (in which this part of the pcb was not correct) scoping the board etc, took quite some time. I decided that there were about 5 or 6 ic’s that were probably cause and socketing these so I could test them would be the faster approach.
Took 20 minutes, and succes! It turned out to be the 74ls174 in position 44 in the corner of the pcb that had one bad output.since the board was only running a fraction of a second each time this fault was near impossible to find.