(Topic ID: 6166)

switch column not working

By Harvino

12 years ago



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#7 3 years ago
Quoted from pinmike:

Row or Column "Ground Shorts" and the U20 Chip.
The CPU board's U20 chip ULN2803 is a common failure point for the switch matrix. If the game is reporting rows or columns as shorted to ground (especially multiple shorted rows or columns), often this U20 chip and downstream the 74LS374 chip at U14 (on WPC-95/WPC-S it's U23, a 74HC237/74HC4514 respectively) on the CPU board are usually the problem. Rarely the CPU board's LM339 chips fail too, where U18 controls rows 1,2,3,4, and U19 controls rows 5,6,7,8 (but replace U20 first followed by U14/U23, and then look at U18/U19 last). Another thing to remember is there's LM339 chips on the under-playfield mounted opto board(s). If there was a 50 volt coil power to switch short, often the LM339 chips on these opto board(s) can fail. Disconnect the opto board(s) and see if the problem changes. This will isolate the problem to a particular board. Remember Indy Jones to Demo Man uses a different trough opto board that has its own LM339 chips, and these can fail too (in addition to the *other* under-PF mounted opto board on these games).
After Replacing CPU Chip U20, the Fuse does not Blow, but Many Switches show in the Test Report.
This is very common. The CPU is confused from the blown U20 switch matrix chip, and will report many switches as "bad" in the test report. To "unconfuse" the game, go into diagnostic, and select the first switch test (T.1, switch edges). Using a pinball, manually activate the switches that came up in the test report (see the game manual for their location, if they can't be found). The switches should report correctly on the display in this test mode. After activating each switch once, exit the diagnostics, and the game should work normally. Alternatively, if the game will allow it, just play a game! This is often all that is needed to clear the test report.
More on Ground Row Shorts and Other Strange Switch Problems.
Switch ground short errors are often the most confusing problem to find. One may think that if the game is reporting a switch ground short, that a playfield row switch wire has somehow been shorted to ground. Unfortunately this is rarely the case! More often it is some other problem (usually a bad U20 CPU chip, or a bad LM339 chip on an under the playfield opto board, especially if the U20 and U14/U23 CPU chips has already been replaced).
If you are a skeptic and want to believe the switch "ground short" message, there is an easy test for this. Power the game off and remove the row and column connector plugs from the CPU board at J205, J212, J206/207 and J208/J209. Then using a DMM, check for continuity between any switch row or column wire to ground. Chances are really good you will find there is no ground short. If you do find a short, then the wire will have to be traced from the CPU board connector to the last switch in the daisy chain.
Now that we know there really is not a playfield switch grounding problem, we can do some further testing. Disconnect all four switch input plugs from the bottom of the CPU board. Put the game into switch diagnostic test T.1, and none of the switches should be activated (except for switch 24, which is "permanently closed", as discussed above). If a whole row of switches is activated, that would mean that row's LM339 is bad. If a column of switches are activated, this means a bad U20 chip. If just one or two switches are activated, plug the four bottom connectors back in and disconnect the ribbon cable that goes between the CPU and the power driver board. If the switch matrix confusion clears up, the problem in on the power driver board! This could be U7 and/or U8 (WPC-S and prior) on the driver board, which are 4N25 opto issolators used for some of the direct switches.
To isolate the switch problem from the playfield, it is a good idea to use a jumper wire to test the switch matrix, right at the CPU board row and column plugs. The diode is optional and not needed if all the playfield switch connectors are removed from the CPU board. This is procedure is described later in this section, but here's a couple pictures below. Also don't forget a ground short or other wacky switch behavior could be caused by a bad LM339 chip on the under-PF mounted opto board(s).
If a particular row or column of switches does not work using the above jumper wire/diode test, chances are good the CPU board U20 chip (or possibly U14/U23) have failed. Battery corrosion on the CPU board can also cause these problems, so keep that in mind too (any green parts on the CPU board is a bad sign).
If the above jumper wire/diode tests show all switch rows/columns as working, then the problem is located on the playfield. This usually has to do with the under-the-playfield opto board(s), as used in games Twilight Zone and later. There are LM339 chips used on the under-the-playfield optic board (if the game has one, and most games Twilight Zone and later do use at least one). If any one of the LM339 chips fails on the under-the-playfield opto board (very common), the switch matrix will be confused. This is often reported as a "ground row short", or other weird and sparatic switch matrix errors.
In this situation, put the game in switch test mode, then remove the power plug on the opto driver board under the playfield. Usually there is one large plug that houses 12 volts and all the switch row and column wires. Remove this plug. Remember opto switches are normally closed until the opto light source is blocked, and is shown as a "box" on-screen in the dot matrix switch display. So removing power to the opto board should make all the opto switch "boxes" change to "dots" (and the switch test report may quickly go "bonk" for each opto switch that now triggered from closed to open, when the opto board's power was removed). If a ground row short clears up after doing this, then there is a LM339 problem on the opto board.
Since the optos themselves and the opto board runs on 12 volts, at this point make sure you have 12 volts at the opto board! If it is 11 volts or less, this can cause sparatic problems and weird switch reports. Low 12 volts is usually a BR1 bridge or LM7812 voltage regulator problem, as discussed above. Get this fixed before proceeding.
If 12 volts is good and present on the opto board, I generally replace all the LM339 chips on the under-the-playfield optic board, and use sockets. These are cheap chips, and they are not easy to test with a DMM, so I generally just socket and replace them. These chips can be diagnosed with a logic problem, but often I just replace them wholesale.

Sorry to resurrect, but I found this extremely helpful even eight years later. Happened to find out that my LM339 chips are bad which had me go through the entire post that you made since it was the last thing to check. I also learned a lot about how to diagnose on the way to figuring that out. Looks like someone had already made sockets for them on my opto board so I put in an order for some replacements

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