I had a similar problem with all of my Twilight Zone optos working intermittently. The problem turned out to be a failed 100 uF capacitor on the under-playfield opto board that had leaked electrolyte and eaten up a trace on the board. I replaced the capacitor, jumped the bad traces, and now I'm firing on all cylinders.

I've attached the drawing of the Demolition Man opto board from the manual. You should use a multimeter set to DC volts and measure the voltage across capacitor C1. I suspect you'll find that the reading is 12 V while the optos are working and 0 while the optos are not working. If you don't have 12 V there, that's your problem.

You should inspect the area under and around capacitor C1. If you see any signs of leaking or corrosion, that's bad! If you wiggle the capacitor and it doesn't feel secure, one of the legs may be corroded.

You could also use a mutlimeter to measure the capacitance of capacitor C1. It should read somewhere in the neighborhood of 100 uF.

demo-man.png

Nope. It definitely could be a bad capacitor / bad trace. That's what it was in my case. Start with a visual inspection of the board around the capacitor. Any signs of leakage or corrosion mean I'm probably right.

You can read of my troubles and eventual solution here: http://pinside.com/pinball/forum/topic/all-twilight-zone-optos-go-out-intermittently

Did you measure the voltage across the capacitor? I suspect you'll find no voltage there when the optos are not working.

Part of your circuit is probably not getting power. Measure for 12 V on the opto board. Do you have it?

I already explained that as descriptively as I could in a post above.

With the game on, and meter set to read 10 V DC (looks like it's around 7 o'clock on your meter), put the two leads across the big 100 uF capacitor on the opto board.

Try to test this first when the optos are working (may be easier said than done). You should measure 12 V when everything is working.

When the machine is on but the optos are not working, it would be very valuable information to know whether or not 12 V is present on the opto board. In my case, I found that 12 V was missing.

This indicates that your opto board does not have 12 V, which is what I suspected. Next, try measuring the power coming into that board from the power driver board. (Measure DC volts. Red lead on pin J3-1 on the opto board. Black lead on pin J3-3 (ground). Refer to the drawing I included in a previous post.)

I suspect that you'll find that there is 12 V coming to the board even when the voltage measured across the capacitor is 0.

Are the optos working when the capacitor measures 12 V?

You need to try to measure those two voltages when the optos stop working. I know it's easier said than done. I'm reading up on the Judge Dredd opto board now.

The manuals for Demolition Man and Judge Dredd indicate that they both use the same opto board: 7 opto switch board assembly A-15576

I suspect that you could switch them. I also think the problem would stay with the opto board, so you'd move the problem to Judge Dredd. But hey, that's certainly progress!

You got me. It sounds unrelated to your opto boards. To fix it, I'd start with reseating your ribbon cables.

So DM's opto board worked fine in Judge Dredd? And JD's opto board still had problems in Demolition Man? If so, you proved me wrong, which is good progress. That would mean the problem is not the board.

The problem must either exist between the boards (power interface/CPU) and the opto board, or between the opto board and the optos. I can't take a good guess as to which is more likely.

Next time you can get Demolition Man to act up, try again to measure the 12 V coming into the opto board.

I'd say that means to problem lies in the playfield wiring. (My first guess is a broken wire at one of the optos.)

It would be helpful to know if the opto transmitters or the opto receivers are failing.

The opto transmitters (white plastic side) normally emit infrared light all the time. You can test those by using a digital camera (the camera on your phone should work nicely) because they can see infrared light. If you can see using the camera that the transmitters are emitting a purple-looking light, then they are working. If they're not - then they're not.

The opto receivers (black plastic side) can be tested using a small flashlight (incandescent for sure, not sure about an LED flashlight). In switch test, shining the light on the opto receiver should cause the switch to close. Removing the light should cause the switch to open.

More info here: http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Opto_Testing

I imagine that one of those tests will pass and one will fail. What do you say?

dm-playfield.png

The connections between the optos are daisy chained. There's a wire going from the board to the first opto, then to the next opto, and on...

It really only makes sense if the chain is broken between the board and the first opto in the chain...

I'm sorry to hear that. I know how much of a pain in the butt these problems can be. And intermittent problems are the WORST!

My Twilight Zone opto problems came back this week! I knew right where to look, and it turned out to be another bad trace to the capacitor that I replaced. Unfortunately, you've ruled out your opto board as the problem.

In the future, when you get motivated to check it out again, the last post I made about testing the emitters and transmitters is where you should pick up. That should divide your problem in half.

Good luck! I hope you don't actually give up!

Wait, are the trough optos malfunctioning, too?

I was assuming it was just the optos on that 7-opto board: bottom popper, elevator hold, top topper, chase car 1, chase car 2

We need an expert in here. I've just been pretending.

The power for the optos comes from the power driver board connector J118. 12 V from J118-2. Ground from J118-3.

Measure the voltage from J118-2 to J118-3 on the power driver board. You obviously have 12 V there when everything is working. Try to measure it when the optos are not working. Try this with J118 connected and disconnected. That should isolate the problem to board or playfield.

The DC motor for the cryoclaw also uses 12 V power from these pins, so you could see if that works (in test mode) while the optos are out. You're just looking to see if the motor will move at all.