[FONT=verdana,arial,helv,helvetica][SIZE=-1]Another way to test the opto receiver is using a DMM. (We talked about that a few paragraphs above.) First block the opto transmitter with a piece of black electrical tape or some other object. The game can be in attract mode or in switch test, it does not matter. Now put the black DMM lead on ground (the metal side rail of the game works well). Put the red DMM lead on one leg of the opto receiver (gray wire). One opto receiver leg should show 12 to 13 volts DC, and the other opto leg should show close to zero volts (orange wire). Keep the red DMM lead connected to the "low" (zero volt) opto leg. Now shine a flashlight into the opto receiver. The DMM should now go to 12 volts DC, and when the light is removed, go back to near zero volts. If this does not happen, the opto receiver is bad. Or if 12 volts is seen on both opto receiver legs, the receiver is bad (or there is direct light shining into the opto receiver). Note as discussed above opto receivers do wear out, and instead of showing zero volts, may show 2 or 4 volts. If they get above 2 volts, than it's time to replace the opto receiver.
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Opto Board (the Opto Receiver and Transmitter Tests Good, now what)?
If the receiver tests good with the penlight flashlight, and the transmitter tests good with the infrared sensor card, there is one more thing that could be wrong. This would be the opto board. Usually before messing with the opto board I make darn sure that the optos themselves are not the problem.
I typically do this (for non U-shaped optos) by taking a new opto (receiver or transmitter), and holding its legs to the back of the opto board. For the transmitter I can check it with a digital camera or an opto sensor card. For a receiver I can test it with a penlight (or the other tests given above). Unfortunately if the opto board has a problem, these tests may not work...
Most of the newer WPC games have a seperate board mounted under the playfield called an "opto board". These have some LM339 voltage comparitor chips and diodes and resistors. If this board fails it can really confuse the game. Also games Indiana Jones to Demo Man usually have an opto board under the playfield AND the trough board is essentially a second opto board. Both these board have LM339 chips, which can be problematic. (After Demo Man starting with WCS94, the trough opto boards no longer have LM339 chips, as these were all moved to the under-playfield mounted opto board. So the trough opto board becomes less of an issue.)
There are many different 'flavors' of these opto boards, so it's hard to give an exact test for the opto board. But there are some general things that should be looked at:
- Make sure the CPU board is not the problem. I always put the game in switch test T1, remove all the connectors from the bottom of the CPU board, and cross each switch column/row (this test is described in the switch matrix section). This rules out the CPU board as the problem. I always start there.
- After eliminating the CPU board as a problem (and reinstalling the CPU board connectors), I remove all the connectors from the opto board and make sure the switch matrix test T1 operates cleanly with no errors (other than the missing optic switches). If problem free, then the optic board(s) is starting to look like the problem.
- Don't forget Indiana Jones to Demo Man games essentially have two problematic opto boards: the under PF mounted board and the trough opto board. Games after this WCS94 and later trough boards do not have any LM339 chips on their trough opto boards, so these trough boards don't implode like the Indiana Jones to Demo man trough boards. Also the trough opto boards on this series of games daisy chain 12 volts to all the other opto boards under the playfield! So if you have a bad .100" connector on the trough opto transmitter board, it can interrupt 12 volts to the other under playfield opto boards. This is particularily a problem on Star Trek Next Gen.
- Opto board LED - there's a red LED showing the opto board has power. It should be on.
- Opto board has many IDC connectors. It is not uncommon for these connectors to get a wire broken/pulled, causing an opto (or set of optos) to not work. To test this (game off), pull one female connector just slightly off its male header. Then use a DMM and check for continuity from one male header pin to where the wire goes. Repeat for all pins. No continuity, and you have an IDC connector problem (very common).
- Check the back of the opto board and make sure all the male header pins do not have cracks in the solder where they attach to the circuit board.
- Using a DMM set to diode function, make sure all the 1N4004 diodes on the opto board test correctly. They should read .4 to .6 in one direction, null in the other.
- Check all the resistors with a DMM.
- Check the traces from the header pins to the resistor/diodes. It is not uncommon for a trace to be broken or burned on an opto board. Actually this is a very common problem.
If everything checks out, that only really leaves one thing left: the LM339 chips on the opto board. I generally replace all the LM339 chips (and use sockets!) on the opto board (there are usually two to four of these chips on the opto board). Unfortunately the LM339 chips are not that easy to test, since they're dealing with voltage levels. But as long as the voltage levels on the outputs of the LM339 are stable (not pulsing and not fluctuating), the truth table for the individual comparators can be tested with a DMM (inputs) and a logic probe (output).
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