Check U20 and U14. Check that the switch column wire is not shorted to ground

Heres more info.

The first thing I would do is again relative to the game's internal T.1 switch test diagnostics. And from there, determine if the problem is just one switch, a whole row or column of switches, or opto switches. Then branch off into more analysis. I highly suggest having the game manual when diagnosing switch problems. The inside front cover of the game manual shows the switch matrix rows/columns. This is important information when trying to figure out if the problem is a single switch, an entire row or column, an opto switch, a switch connector, wire colors, etc.

Remember the switch matrix is based on 8 rows and 8 columns (except Star Trek Next Gen, Indiana Jones, Twilight Zone which have 9 columns). If there's just a single switch not working, that's usually pretty easy to fix using this micro switch info. If a couple switches in a row or column don't work, often it's a broken row or column wire under the playfield which daisy chains the row and eight column wires between switches for that particular row and column. This can cause other switches in that row or column "down stream" from working.

In the case of a whole row and/or column of switches, or if the game is reporting a "ground row/column short" error, get the game into T.1 switch test and remove the playfield switch connectors from the bottom of the CPU board. Then do these tests: Switch Columns and Switch Rows. This will tell if the problem is on the CPU board (like a shorted CPU board ULN2803 chip). Or if the problem is just a bent and shorted diode on a playfield switch. Or an opto switch board(s) mounted under the playfield (if the game uses them, which most WPC games do).

If the switch involved is an Opto switch, that will require other repair ideas. Not all games use opto switches, but most do. And some era of games use more optos than others. For example, starting with Indiana Jones, all WPC games use opto switches for the ball trough. And games from Indiana Jones to Demo Man use trough opto boards different than later games (with problematic LM339 chips installed right on the trough opto board). To further complicate things, the problem could be the opto transmitter/receiver itself, or the under-the-playfield mounted board that controls the optos. Often disconnecting the under-playfield opto board (and the trough opto board on games Indiana Jones to Demo Man) will determine if the problem is playfield related, or opto board related. It's a good idea to disconnect the opto board(s) and see if the switch test T.1 reacts differently. (Tip: if it's Indy Jones to Demo Man, then reconnect the opto and trough boards one at a time to see if the problem changes, to help isolate which board is the problem.) Check here for more opto repair information.

Once the problem location is known to be on the CPU board, the playfield, or the playfield mounted opto board(s), that makes things a lot more managable to fix. What you're trying to do is break the problem down into smaller parts (as smaller parts are easier to test, diagnose, and ultimately fix).

If you hadn't noticed, diagnosing switch problems can range from pretty easy to very complicated! Sometimes it best to bring a professional in for these issues. But if you can keep your head straight and follow a systematic approach (as outlined above), often an "average joe" can fix a pretty complicated switch matrix problem. The remainder of this section will deal with individual parts of the switch matrix, how it works, and common problems & solutions to switch issues.

Dedicated Switches (Direct Switches).
The direct switches (or dedicated switches) are not part of the WPC switch matrix and are outside of the 8x8 switch rows and columns. Direct switches include the diagnostic coin door switches and flipper EOS switches. Since these switches do not go thru the CPU board's ULN2803 chip, if there is damage to the switch matrix, 99% of the time the direct switches will still work. This is handy if someone shorted 50 volts to the switch matrix, killing the switch matrix. In this situation, the direct switches will still allow the diagnostics to be run. Because the dedicated switches do not use the ULN2803, they are a bit more simple in operation. There is a row number associatd with the direct switches, but no column. Instead the row wire (orange) is connected directly to ground (black), which triggers the needed dedicated switch. Often if a dedicated switch does not work, the reason is a broken ground wire. The dedicated switches only use CPU chips U16,U17 (LM339) and U15 (74LS240), through CPU connector J205. An easy way to test the dedicated switches is to ground each pin of J205 while the game is in switch test mode. This will tell if the problem is in the CPU board or in the wiring.

The Chips that Control the Switch Matrix.
The switch columns are controlled by a single 18 pin ULN2803 chip on the CPU board at position U20. The switch rows are controlled by two LM339 chips on the CPU board at positions U18 and U19. The direct switch rows are controlled by two LM339 chips on the CPU board at positiion U16 and U17. These chip designations apply to all WPC generations.

On WPC-S and WPC-95 games, the ULN2803 that controls the switch columns on the CPU board is socketed. On all other WPC games up to 1994 this chip is not socketed. When a series of switches goes out, it tends to be the ULN2803 at U20 (all WPC revisions) that fails. Williams recognized this, and started socketing this chip with WPC-S. On WPC-S CPU boards, the ULN2803 chip is underneath the battery sub-board. ULN2803 is equivalent to NTE2018. If U20 dies "hard", it could also blow the 74LS374 at U14 (on WPC-95/WPC-S it's U23, a 74HC237/74HC4514 respectively) on the CPU board.

The LM339 chips that control the switch rows at U18 and U19 (all WPC revisions) tend to fail less often. LM339 is equivalent to NTE834. There are also two more LM339 CPU board chips at U16 and U17 (all WPC revisions). These two chips control the direct switches (coin door, diagnostics, etc). These do not fail often either.

There are also LM339 chips used on the under-the-playfield optic board (if the game has one) or in the ball trough boards on Indy Jones, Judge Dredd, Star Trek, Popeye and Demo Man. If any one of these LM339 chips fail (common on under playfield opto boards), the switch matrix will be confused. When there is a switch problem that can not be diagnosed, disconnect the opto boards and see if problems change. If they do, it's a good idea to replace all the LM339 chips on the under-the-playfield optic board(s), and use sockets for these chips. Remember that games Indy Jones, Judge Dredd, Star Trek, Popeye and Demo Man use trough optic boards with LM339 chips (WCS94 and later games don't have LM339 chips on the trough optic board). So these Indy Jones to Demo Man games have a second opto board with more potential chip problems.

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).

Check for Broken Switch Wires "Up Stream" (Switches are "Daisy Chained").
Since the switch matrix is a series of eight columns and eight rows, the playfield switch wiring is "daisy chained". For example, check out a switch matrix row in the manual. For row one, the playfield switch in column three row one (switch 31) is wired before the playfield switch in column four row one (switch 41). So if the row one wire breaks off switch 31, all the switches "down stream" (41, 51, 61, 71, 81) will not work! This is one of the simpliest, and easily overlooked, switch matrix problems.

Another hint that the there is a switch matrix wire broken; If for any particular column and row other switches work, this indicates there probably is not a problem on the CPU board! If there was a column/row problem on the CPU board, it would most likely affect all the switches in that column or row.

Broken switch wires can also occur at the CPU connector too. Make sure to look at the connectors. The wires can fatique and break at the connector, inside the insulation!

No problem and sorry for the delay i just logged in about 30 mins ago
and i havent been on the site all day

I had the same problem and did the same and now everything works!

Thanks!

Hey guys! I am in dire need of some assistance. I have a Shadow pinball WPC-S. Like a dummy I shorted the switch matrix to high voltage andhave been trying to fix it for about 3 months now. No luck, but here is the symptom.

I removed the plugs from J207 and J209 and when i use a jumper anytime i get to J207 Column 5, it shorts the whole row. All other individual switches work ie: 12, 34, 72, 82 etc. But anytime I use a jumper to do 51,52 53 etc it shorts the whole row and gives me ground error. The playfield should be taken out of the equation if I'm doing this with a jumper and no plugs on J206/7 J208/9. I did take out every fuse that i could see on the boards and my DMM shows they are good. I also replaced u18, u19, u20 and u23.

Any ideas guys? Thanks in advance for any help you can offer.

Jt