WPC switch column out - Fixed

[Fleetwood mIkea]

I installed LEDs in a new to me Dr. Who and now it will not play.
The message initially was 'Pinball missing 3' (its a 3 ball game, meaning it can't find the 3rd ball). All 3 balls were in the trough so it was potentially a switch issue - nothing to do with missing balls, so I replaced switch 3. Then, occasionally the slam tilt would engage and the machine would just display 'slam'. The slam tilt is disconnected. I ran switch edge tests and it turns out that ALL switches in that column (column 2 on the switch matrix - Green/red wire) are out. These switches include slam tilt, coin door closed, ticket opto, #24 (doesn't exist; always closed), trough 1, 2, and 3, and the outhole.
I also replaced the U20 chip on the board with moderate success, each of the switches now registers in both the edge test and the single switch tests, but the same message remains and it will not go past 'searching for pinballs'. It now does a 15 second countdown, ejects all 3 balls, then displays 'out of order'. I should note that prior to swapping the U20 chip it did not countdown at all, it just showed 'searching for pinballs' and '15' but never counted down. Progress???
I have searched all over for a shorted wire in that particular column, so unless there is a broken wire inside the insulation I can't see one. Would it be worth replacing all of the diodes on all of those switches, just in case?
The other thing I am considering is replacing the U14 chip as well (not socketed, ugh) because I have read that if the U20 fails the fault could continue on up the board. The board itself looks relatively new and was probably replaced in a previous life.
At this point I'm not sure if I have 100% determined if it is a board vs. under playfield issue; I have only visually and not electronically eliminated the PF as the problem. I have read a few guides on doing this but I'm not sure I understand the 'jumper method' for example.
Any thoughts, advice or info is appreciated.

 

[DRANO]

Might be helpful to know what game and/or manufacturer you're working on.

I've had several of these issues and I always seem to find a broken or disconnected wire. Somewhere. Sometimes it's really tricky to locate, but using a DMM to test continuity between various points might identify it if a visual won't.

Bad diodes can cause all sorts of switch issues too, but the they wouldn't read properly in switch test. They might incorrectly read as other switch names because a bad diode would confuse the game.

Also, make sure you've covered the easy stuff. Make sure you're using real balls and not your finger during switch tests. Your finger may press the arm further than the ball. Also, are switch contacts clean?
Sorry if this sounds basic, but I've found that we often jump to more complicated fixes before checking the obvious stuff.

Good luck

 

[Fleetwood mIkea]

Thanks Drano- great tips. I think I may just start over with the super simple stuff- connectors, wires etc. I may have missed something when I went over it the first time. I did find one diode from a rollover switch that may have intermittently contacted a light socket, but it was on a completely different circuit and correcting that did not help. Not sure if that is related or not. It’s a Dr. Who -Bally machine.
Thanks again!

 

[DRANO]

Hah.. I missed the Dr. Who note. See, we all miss the simple stuff

Your issue 'could' be in the board but that's usually the last resort. Make sure all the cables are correctly seated too.

 

[Fleetwood mIkea]

Alright- back to basics- I’m back at it tomorrow night- wish me luck!

 

[Fleetwood mIkea]

Sooooo.....
Turns out my first instinct was correct- and Drano was correct that it was something simple- human error.
I somehow replaced the U20 chip with the wrong version- a 2804A instead of a 2803A. I’m blaming age and eyesight, but it was probably just simple inattention at the electronics store.
Swapped in the correct chip and she’s up and running!