Repair: Capcom CPS1

Status: Successfully repaired on 5/24/2023

I didn’t take as many pictures of this repair as I should have – sorry about that!

In for repair was a CPS1 main board with no sound.

First, I tested the B + C board on another CPS1 main board.  The sound worked fine there, so the sound issue wasn’t from a bad ROM or problem with the ROM board.

Next, I probed the OpAmp and final amp with an audio probe.  I wasn’t able to pick up any audio with the probe, so amplification wasn’t the problem.

Next, I probed the Z80 Sound CPU’s lines with my oscilloscope.  I confirmed that the Z80 was receiving Power, Ground, a Clock signal, and that Reset was getting triggered at power up.  All of those signals checked out, but the Z80 seemed to run for a second and then halt.

I probed all the Address and Data lines between the Z80 and the edge connector for the ROM board – they all checked out.  I suspected that the Z80s RAM was bad – that would cause the Z80 to start program execution but then crash.  I desoldered the RAM from the board, but it tested fine out of circuit in my memory tester.  I installed a socket and then reinstalled the same RAM chip.

Next, I desoldered the Z80 CPU itself from the board, installed a socket, and tried another Z80 CPU.  That was the problem – the sound started working properly with the other Z80.

By the way, an excellent resource for starting out on any CPS1 repair is this writeup by PCBJunkie:

His photo of all of the chip functionality labeled is fantastic – hopefully he won’t mind me sharing it here.