I recently used the MPU board to test in another pin trying to narrow down a problem. This may or may not been the cause of this problem. Since then I started to get errors at boot up. Check Opto Switch 81, 82, and 83. With this being a switch matrix problem, I first ruled out the MPU using a Pintech Switch Matrix Tester. I then started looking over the playfield looking for a short of some kind that used the wires colors White/Brown, White/Red, and White/Orange.
This led me to the Accelerator Board Assy. Here I found that several leds where out, pulling J5 allowed the leds to come back on again and the lights to disappear on the switch edge test. This lead me to believe that the problem was on the Accelerator board. Here I deducted that U1 LM339 was bad, so I added a socket and replaced U1 with a new chip.
This did not change a thing. So now I am scratching my head, trying to figure out what I may have missed. Any ideas? Dont think that its a shorted circuit, but I could be wrong.

Well, I changed out U4. Still in the same boat. lol

Also changed out U20 on the MPU board. Not there yet....

I can take a q-tip to the optos and I can see on the switch edge test they will go out. I also de-soldered the White/brown, White/Red, and White/Orange wires hoping that the short would disappear. But that didn't change anything.

Guess I need to pull the white wires out one by one to see which one is causing the short.

There are 3 leds that are out when J5 is connected. I pulled one by one the wires and each time an led came on. But the errors keep coming back even with the wires are unplugged from the connector.

How do you test for a bad opto?

I also just added 2 more errors...switch 84 and 85

No I didnt. I did test them with a volt meter and they only read in one direction. But I will change them out to see what happens.

Not on this pin

Well I think that the opto tested good.

Yes I can see the leds turn on when I close an opto.

Ill do that this evening.

I tested the opto again using the link above. On the black housing opto I have 5 volts till I covered up the white housing. When I did that the voltage jumped up to about 13 volt. On the white housing I got 1.3 volts constance.

So it looks as if the opto are working within the normal range.

I must have done something right. The only error that I get now is the 84 switch.

Well all my switches are working now. I have no idea how they started working again. I had the supercharger running in test mode, but once I stopped it did not want to come back on. Looking to see if I blew a fuse.

Yep, I did blow a fuse.