OK,
Have a Tehkan Star Force here for repair and thought I'd just document where it is so far. It has been a royal bitch!
Symptom: Dead. Stuck in watchdog. Audio amplifier heatsink gets red hot.
The audio issue is interesting because this particular board has 2 audio amps and double the audio circuitry compared to most of the board pics I've found online. I'm sure it's going to be a wiring issue with the JAMMA adapter I have so for now I've disconnected the +12v from the adapter to keep from burning up the audio amp chip.
Quick inspection showed 5 x Fujitsu logic on the CPU board and 6 x on the middle board. The 2 74LS27 chips on the CPU board had no outputs and were replaced. One of the 74LS138 on the CPU board had no outputs and was replaced. A second LS138 had chirping on all 8 outputs and was replaced.
One of the LS27 chips had a dead input. Traced that back through the PCB to the 2nd board and found a bad plated thru hole under a 74LS86 IC. Soldered some Kynar wire to the top and bottom sides of the thru hole to patch it.
Board would still not boot. Replaced a bad 74LS74 IC on the CPU board and now the board would boot if powered off/on a couple of times, but had graphics corruption. Reinstalled the 2 ceramic caps onto the new 74LS74 to fix some of the graphics corruption.
Replaced a dead 74LS27 on the middle board and 2 of the 4 Fujitsu 74LS175 chips with dead outputs to fix the rest of the graphic corruption. I went ahead and replaced the last Fujitsu LS138 chip to see if that had an effect on the boot issue, but it did not. That chip was fine and the board will still boot/play if power cycled a few times. Replaced the 10uf cap on the reset line to no effect on the issue.
Next I'll try tracking down one last missing gate input tonight, but I think that is relegated to the sound section. After that is replacing the EPROMs one at a time to see if that's the issue with the boot.
WHEW. Just wanted to write it all down and think about why it's not booting properly. The next step after the EPROMs will be looking through the logic gates on the CPU board with an oscilloscope to see if one or more has some malformed signals.
Have a Tehkan Star Force here for repair and thought I'd just document where it is so far. It has been a royal bitch!
Symptom: Dead. Stuck in watchdog. Audio amplifier heatsink gets red hot.
The audio issue is interesting because this particular board has 2 audio amps and double the audio circuitry compared to most of the board pics I've found online. I'm sure it's going to be a wiring issue with the JAMMA adapter I have so for now I've disconnected the +12v from the adapter to keep from burning up the audio amp chip.
Quick inspection showed 5 x Fujitsu logic on the CPU board and 6 x on the middle board. The 2 74LS27 chips on the CPU board had no outputs and were replaced. One of the 74LS138 on the CPU board had no outputs and was replaced. A second LS138 had chirping on all 8 outputs and was replaced.
One of the LS27 chips had a dead input. Traced that back through the PCB to the 2nd board and found a bad plated thru hole under a 74LS86 IC. Soldered some Kynar wire to the top and bottom sides of the thru hole to patch it.
Board would still not boot. Replaced a bad 74LS74 IC on the CPU board and now the board would boot if powered off/on a couple of times, but had graphics corruption. Reinstalled the 2 ceramic caps onto the new 74LS74 to fix some of the graphics corruption.
Replaced a dead 74LS27 on the middle board and 2 of the 4 Fujitsu 74LS175 chips with dead outputs to fix the rest of the graphic corruption. I went ahead and replaced the last Fujitsu LS138 chip to see if that had an effect on the boot issue, but it did not. That chip was fine and the board will still boot/play if power cycled a few times. Replaced the 10uf cap on the reset line to no effect on the issue.
Next I'll try tracking down one last missing gate input tonight, but I think that is relegated to the sound section. After that is replacing the EPROMs one at a time to see if that's the issue with the boot.
WHEW. Just wanted to write it all down and think about why it's not booting properly. The next step after the EPROMs will be looking through the logic gates on the CPU board with an oscilloscope to see if one or more has some malformed signals.


