Another PCB on the bench from my friend Sasha: Taito Crime City. It’s a run and gun arcade game, that was released by Taito Corporation, in 1989; it runs on Taito B-System hardware, and it is a spin-off from Chase HQ, which had been released in the previous year.
The PCB was in very good conditions, clean and with no apparently damage, once the game has been powered up, we were been greeted with this …
An unstable and rolling image all over the screen, it’a a clear sign of a sync related issue, the game seemed to respond in the right way, it can be coined up, the audio seems working, also the players controls seems to work, i say “seems” because trying to play the game in this way produces some kind of “seasickness” … LOL.
… and what’s happening inside the game wasn’t so clear! But it’s time to investigate.
I looked at the sync signal on JAMMA harness and as we suspect it was completely stucked at ground level, an healty sync signal should tick instead. I traced back the sync signal through the PCB up to a pullup resistor connected to VCC (+5v) and then up to an output pin of an IC, an HEX buffer with open collector outputs, a 74LS07.
You can see the sync signal path marked in yellow colour, the signal is fed into the IC into pin 1 and then it comes out from pin 2, here is the logic diagram of the IC:
So before making any assuption about the possible causes of the issue we should point the scope over the input and the output of the IC to see what’s going on … the input is in BLUE (pin 1) and the output is in YELLOW (pin 2)
As you can se the input is correctly ticking at the right frequency (15.7Khz) while the output is stucked at ground level. In the output signal path the only discrete components were a pullup resistor, a ferrite bead and a capacitor and even if one of these components were damaged the sync signal shouldn’t be stucked at ground level, so the IC should be damaged and short circuited its output pin internally to GND (ground). Confident the issue was related to the IC malfunctioning i promptly remove it.
Testing it out of circuit confirm our suspects, the interested logic unit was failing:
and then socketed and replaced …
Powering up the board ends up with a perfect and stable image displayed on the screen! The sync signal now is ticking and healty as it should!