6 Slot – Memory test on power up giving error
Address Write Read
00008000 5555 55FF
Checking with Mame, the memory chips in question were only small, so looking at the board revealed two DIL 5814’s. Chip select signals to these were toggling as expected, so I needed to identify which ram chip was which. Using the test probe I shorted pins 11+12 on one chip, powering on the board now gave read as 51FF so I was looking at the wrong chip. Replacing the other (with a 2018-35) sorted the problem.
One of the guns on a Time Crisis 2 was no longer registering shots on the screen. Following checks on wiring, cleaning lens, etc. it appeared that the optical sensor (opto) was dead. This is a small PCB in the gun, 2421961100, pictured above. I couldn’t find any schematics or information about the parts used,…
Boots up, and plays, but incorrect sprites – most noticeable little bug changes into ‘END’ at one point. Check 74ls138 at 8n, controlling graphics banking, and pins strobing. Replace chip and now uses correct sprites. Quiet missile sound and explosion sound. Both of these effects use the op-amp chip, LM324 at 7T. Replace this chip…
Boot up with garbage, quickly clears screen and then nothing. Used Fluke with Z80 pod – read rom OK, and write ram OK, but timeout reading any Ram location. Check fluke status and ‘wait’ always set. Check pin 24 of Z80– stuck low, trace back through 7H,7C,6C1 to 6B1 pin 2 – always low, although…
Totally dead, no screen display (blank white screen). For this I used the Fluke Z80-S2650 adapter to check out the ram and rom, and located two errors. The rom mapping was incorrect, with a large range of locations returning random values. Examining the board (since I have no schematics), all of the chip select lines meet up…
No display – When move board, display flickers – go around board pushing each chip in turn, when press on 2E picture appears. Pin 16 rusted, only just in place. replace chip – picture back to normal. No star display. Board had been hacked to play an alternate game, pin 15 on chip 2B had…
No Sound. Signal at output of SN76477 pin 13, follow through to input of sound amp, then from output until we reach electrolytic capacitor used to decouple connection to speaker. Signal going in, nothing coming out – replace capacitor sound back to normal. Only drawing a few pixels of player balloon sprite. Following the schematics,…