Robotron ROM board
Inspired by the recent RGP event photos, I decided to bring in some of my games from storage and actually play them. First up was Robotron. Left it a couple of days to warm up then powered it up. Rug pattern ran ok but then got a ROM 5 error. Not good, but kind of didn't expect it to work first time. Got the CPU & ROM boards up on the bench.
First the easy check, reseat the ROM - no luck. Took the offending ROM chip out, checked against the MAME dump, all checked ok, so assume the ROM is fine. Just to double-check, swapped it out for a newly-programmed one, same problem. Looking at the schematics, the ROMs are selected by 74154 4 line to 16 line decoder @ 8E. I know the self-test at startup checks for the existence of all ROMs, so each of the ROM select pins gets selected for a short time. Except in my case the one for ROM 5, which is pin 4, no activity at all, the rest seem fine though. Swapped out the 74154 and now works perfectly
Defender ROM board
Flushed with my success at getting Roby working, I decided to attack a faulty Defender ROM board that I got in a joblot. Hooked up to a known working CPU board, got nothing, completely dead. Now ROM 3 contains the self-test routine, so decided to remove all the ROMs apart from 3. Then noticed, someone has put all the ROMs in backwards... Removed them all and replaced ROM 3 the right way round. Still nothing. Tested all the ROMs against MAME images, all except ROMs 2, 3 & 7 ok so burned new ones and replaced them all.
The game appears to boot up ok now, but LED 3 on the ROM board stays on constantly. The LEDs are driven from the PIA which is known to be a regular fault. Checked the pins with a logic probe and they were all over the place. Swapped out the PIA and all is well
Joust ROM board
Another one from a joblot. On power-up, rug pattern runs but then screen clears and some rubbish is displayed. Now the only chips on the ROM board that have anything to do with the screen display are the "special chips" (blitters). I don't know much about these so just a case of swapping them out for known workers in the hope it might work. Powered up again and now get the "All systems go" prompt. But the 7 segment display shows an "8" instead of "0", and the game won't accept coins. Coin inputs and LED are read and driven directly by the PIA on the ROM board. Running a logic probe on the LED outputs shows the PIA is not sending the right signals to the 7447 7-segment driver. As PIA's are known to be regular failures, swapped it out for a new one. Powered it all back up and all working
Inspired by the recent RGP event photos, I decided to bring in some of my games from storage and actually play them. First up was Robotron. Left it a couple of days to warm up then powered it up. Rug pattern ran ok but then got a ROM 5 error. Not good, but kind of didn't expect it to work first time. Got the CPU & ROM boards up on the bench.
First the easy check, reseat the ROM - no luck. Took the offending ROM chip out, checked against the MAME dump, all checked ok, so assume the ROM is fine. Just to double-check, swapped it out for a newly-programmed one, same problem. Looking at the schematics, the ROMs are selected by 74154 4 line to 16 line decoder @ 8E. I know the self-test at startup checks for the existence of all ROMs, so each of the ROM select pins gets selected for a short time. Except in my case the one for ROM 5, which is pin 4, no activity at all, the rest seem fine though. Swapped out the 74154 and now works perfectly
Defender ROM board
Flushed with my success at getting Roby working, I decided to attack a faulty Defender ROM board that I got in a joblot. Hooked up to a known working CPU board, got nothing, completely dead. Now ROM 3 contains the self-test routine, so decided to remove all the ROMs apart from 3. Then noticed, someone has put all the ROMs in backwards... Removed them all and replaced ROM 3 the right way round. Still nothing. Tested all the ROMs against MAME images, all except ROMs 2, 3 & 7 ok so burned new ones and replaced them all.
The game appears to boot up ok now, but LED 3 on the ROM board stays on constantly. The LEDs are driven from the PIA which is known to be a regular fault. Checked the pins with a logic probe and they were all over the place. Swapped out the PIA and all is well
Joust ROM board
Another one from a joblot. On power-up, rug pattern runs but then screen clears and some rubbish is displayed. Now the only chips on the ROM board that have anything to do with the screen display are the "special chips" (blitters). I don't know much about these so just a case of swapping them out for known workers in the hope it might work. Powered up again and now get the "All systems go" prompt. But the 7 segment display shows an "8" instead of "0", and the game won't accept coins. Coin inputs and LED are read and driven directly by the PIA on the ROM board. Running a logic probe on the LED outputs shows the PIA is not sending the right signals to the 7447 7-segment driver. As PIA's are known to be regular failures, swapped it out for a new one. Powered it all back up and all working
