WPC opto issues…

I’ve had an ongoing issue with my World Cup Soccer ‘94 – at certain times in the game, it will push balls into the plunger lane when it shouldn’t. That points to some sort of problem with the sensing of the balls in the trough at the bottom of the game.

WCS is a WPC – I think that means “William Pinball Controller – and one of the nice features of the software that runs the game is that it has some test software that lets you visualize the state of the switches in the machine:

In my testing, the trough switches work fine when there is a ball in them, but if there is no ball in them, they cycle between open and closed. That is likely to be the problem. I verified that the trough LEDs and opto are functioning correctly and I’ve done the usual “unplug and replug all the cables” approach, so it’s time to dig further.

I did a post on pinside – which yielded no responses – and then pulled out the 7-channel opto board. It looked fine, but when I put a VOM on it, I found 10 VDC and 2.7 VAC. Not good.

I should mention that this machine had some issues on the power driver board, so I took an opportunity to recap it, so I knew that my supply voltage was fine. which meant the most obvious culprit was the 100 uF cap on the opto board.

Except it wasn’t; replacing that caused no real improvement. Which led me back to the power driver board where I found similar voltages at the test point. I pulled the board, found no continuity between the – terminal on C30 and ground, which meant I damaged the through-hold plating with my recap and it just took a while to fail. I added a jumper between the cap terminal and the adjacent ground trace – it would have been trivial to add a trace there, but for some reason they didn’t.

Put the board back in the game, everything works fine. Later I found a note in the WCS repair guide that said, “if the optos are malfunctioning, it’s usually a connection on C30”. Yep.

So, what do you think ?