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.