In my attempts to troubleshoot missing background music on my Williams High Speed, I powered on the system with the MPU/Background Sound board ribbon cable backwards. This immediately resulted in the -12V line smoking and burning the trace off my power supply board. The fuse saved it that first time, but now when I put the cable back, the -12V line on the power supply board still burns up until I turn it off (before the fuse goes). Without the board's power connected, the MPU/display still boots fine and goes into attract mode.

I can see from the schematics that -12V is unregulated but I'm not seeing burning or damage on the background sound board side. Anyone have any suggestions or experience in this area? Thanks!

That’s what I thought but now I’m slightly more confused. The Q1 transistor on the board looks like it’s not testing properly, so possibly that’s the issue. I’m still a little hesistant to debug this since the board schematic in the High Speed manual is for a C-11030 and my board is populated slightly differently as a C-11029 and I can’t find a schematic or even parts list.

Also, am I correct in my reading that the only thing running off the -12V is the soundboard so that’s where the short must be? Everything seems fine with the sound board out.

Thanks so much for the response. I'm really "shocked" this even happened from reversing the ribbon cable. Did I just confuse the U1 DAC? C6 reads as 19.80nF on my meter (0.0198uF), so I guess it must be either the DAC or op-amp that coincidentally failed?

The only other thing I did was to fix the solenoid ground from 3J3 (power supply) and tapped it over to the ground on 3J4. This wouldn't have been the issue right? I'm apprehensive about putting this or any other sound board back in now...

I had purchased a reproduction power supply board that was missing ground to that pin, so none of my special solenoids (hideouts) would fire, but the relays were clicking, looking for ground. I’m currently rebuilding the +5V section on my original PSU, but honestly everything was fine until I messed with the ribbon cable on the sound board. It’s still fine now with the sound board out of the backbox.

Before I go replacing components on this board, could reversing the J1/J2 (CPU interface/volume control) wires have caused this? Since both are connected to U9 (MC1458 op-amp), it seems like getting this backwards is fairly easy since it's keyed in the same place...

I put the sound board back in an it doesn’t burn up the -12V line now. I’m almost certain I had the CPU interface/volume wires swapped in the wrong place (J1/J2).

I’m back to square one where I’m only getting sound effects and no music, but at least I’m not burning leads and fuses. Thanks again all.

I just tried an entirely new sound board replacement from Marco Specialties and I still don't have any background music. I have to assume there's something wrong with the MPU now.

Nevermind. Thanks again for your help. It turned out to be two bad traces from U42 through the battery area to the SIP resistor for J21 on the MPU.