Ms Pacman bootleg board

[HarrieD]

Hey you guys, we are in the........ damn, that's John's line.

Whatever, i need your help with something. I just bought an el-cheapo Ms Pacman bootleg board. I knew it was defective, the guy who sold it said that the board was booting, showed some pacmans on the screen and then reboots. Having watched almost all the vids on the one circuit channel (thanks Adam!!) this might lead to the conclusion there is something wrong with circuit which is driving the watchdog.
So far so good. I do repairs on MPU boards from pinball machines and i want to dive into arcade PCB repairs, that's the main reason i bought this bootleg board.

After searching the internet for schematics of this board i came up empty. Is there someone who by chance has got this schematic???

This is the board (a simular one)

[iankellogg]

There is pretty much a solid 0% chance of finding a schematic. the best you can hope for is they followed the official schematic close enough.

[HarrieD]

Thanks Ian for your quick reply. I was afraif of that I must say, even don't know if the roms can be found.

Will dive into it, most of the ic's are TTL so troubleshooting must be doable.

[iankellogg]

personally why bother?

A real pacman board is cheap. they usually sell for less than $80.

[HarrieD]

Just to get some experience on board repair. I got this board for 20 euro (23 dollar).

[iankellogg]

That's a pretty rough one to cut your teeth on. But the best of luck to you.

[TheGuru]

Its a really simple PCB with just ROM/RAM/CPU and logic so it's very easy to fix and a perfectly good example of something to learn on..... and as long as the PCB is in good condition it's probably worth spending time on it.
The watchdog means the CPU is trying to run the program but there's a problem, either the code is corrupt or it's waiting for something to happen and it takes too long. So the watchdog is working and you don't need to mess with it.
Check/re-seat ROMs.
Check for a clock at the crystal and on the Z80 at pin 6.
Replace the work RAM (the RAM near the Z80)
Check/replace Z80 and/or check/find logic fault. Start at the CPU and work backwards to get to the address logic/ROM/RAM etc inter-connections.
The CPU circuit on Pacman is not very big so if you follow the theory behind the Repair 101 videos by Adam you should be able to figure it out in time.