Working on a Doctor Who, and I've run into a strange issue. Hoping someone can point me in the right direction.

Current Symptoms:

- Game sometimes thinks the coin door buttons are being pressed, randomly, even in attract mode. It seems to be infrequent, sometimes it happens every few minutes, sometimes not at all for a while. If the door is closed, I get a message saying to open the coin door to use the coin door buttons. If the door is open, it will trigger the test menu.

- During gameplay, when I hit the lower right ramp and trigger Switch 37 ("Score Bottom Ramp"), the flippers will all trigger and hold for a second or so, and then return to normal. All 3 flippers do this simultaneously. Sometimes after this happens, the game also thinks someone is pressing a coin door button again as well, but not often.

MPU has no obvious corrosion issues, but I swapped it with another Doctor Who MPU from a known good game, and the same problems exist. Just in case, I replaced U19 (Quad comparator) and U20 on my MPU, no change. I unplugged J205, but the game still thought someone was occasionally pressing the coin door buttons.

Anyone have any thoughts as to what might cause this odd behavior? I've never run into this issue before. I can post a vid if it helps, just let me know.

If I disconnect the coin door connector from the interconnect board at the front of the cabinet, the game still thinks the buttons are being pressed, so I don't think it's the actual buttons. I do have a spare, brand new interconnect board, I can swap that in if you think it would help.

Okay, I'll go take a look at that.

Question: Wouldn't disconnecting J205 eliminate the wiring/interconnect board/coin door as the source, since the behavior occurs with that unplugged?

Nothing noticeable upon first inspection. For kicks I also swapped in the new coin door interface board I had on hand, no change. I also booted the game with nothing at all plugged into the coin door interface board and it still thought the coin door test/menu button was pressed.

I don't know if it matters, but the coin door interface board in the game is a -1 revision, because this is a re-import. I don't think that's an issue, but figured I'd mention it just in case.

I've never done that, but I could see if I can set that up to try. Could you explain in more detail what you mean, please?

Is there a chance this is a ribbon cable issue? I could swap cables with one of my other WPC games if you think it's worth trying that as well.

The power driver board? I actually had that rebuilt because the game was having the standard WPC reboot issues. I was stopping by Allan D's place anyway, so he went over it and bulletproofed it for me.

Understood, I appreciate the detailed reply. I used the generic "bulletproofed" term people toss around, but to clarify, I had the board checked by a highly respected tech that I (and many others) know well. He has both the knowledge and the actual manufacturer test equipment to go through these boards thoroughly. In this case the standard work was done to fix known issues (and potential issues). I know opinions vary on the suggested rebuild or replacing of parts on these PD Boards, but to summarize what I recall:
Voltage regulator replaced
BR1 (which was poorly replaced in the past by a previous owner) replaced
C8, C4, C2 replaced
Headers inspected, tested and replaced as needed

After that, the board was installed on a test rig and run through various tests, with no issues, restarts, etc. Voltages were checked/tested.

I actually spoke on the phone with the tech today, and shared a video of the odd behavior. He was surprised, and we discussed possible causes. Like you did above, he cited the 12V regulated as a likely culprit. He asked me to check it again on the power driver board, which I will do shortly. He also asked me to verify the wiring/connectors associated with the ramp switch, flippers, etc.

I will start by checking the 12VR at the PDB.

Okay, so some interesting testing. Maybe this will help with diagnosis.

Note: Coin door connectors were disconnected from the coin door interface board at the time of testing.

I hooked up my meter to TP3 (12V Regulated) on the Power Driver board and powered on the game. The initial reading was 11.88, but after about 5 seconds (as the game finished booting and went into attract mode), the voltage started vacillating between 9.47 and 11.90 VDC.

Upon starting a game, the voltage immediately locked in at 11.88 VDC steady, no fluctuations.

I manually triggered Switch 37 and the meter reading immediately plummeted. Reviewing the video I made, it looks like it drops to around 6.92 to 7.01VDC and then goes back up to 11.88 steady about a second or so later. The first couple of times I hit the switch, the flippers all triggered as noted in previous posts. After several switch triggers, the flippers stopped this behavior. The +12VR voltage drop upon Switch 37 activation also changed slightly, only dropping to around 8.04VDC each time.

I proceeded to check every other accessible switch, and only one other caused the +12VR to fluctuate, switch 47. It caused the same exact drop as Switch 37, although it was not every time (probably about 75% of the time). Otherwise, the +12VR it stayed at 11.88VDC, regardless of switches triggered. I will test again to verify my findings, but wanted to report.

I noted that Switch 37 and 47 are in the same row of the matrix, although I'm not sure that matters in this case.

Disconnecting Switch 37 seems to have eliminated the voltage drop issue. The diode was removed and tested good. Ordering a new switch, and I'll continue to investigate the wiring, etc in the mean time.

Dang... new switch didn't change anything. And even stranger, if I play with the switch disconnected, everything seems fine, but if I connect it, trigger it once (causing the voltage drop one time), then disconnect it again, the problem persists on a different (related gameplay-wise) switch. If I never trigger it, or leave it detached (thus never triggering it), it never has a problem that I can tell.

So bizarre... not sure where to go from here.

It only triggers Switch 37 in test. Honestly, the game does all the "normal" stuff scoring/switch wise in test and gameplay aside from this weird voltage drop and flipper triggering thing. Taking it to a local experience tech and repair guy next week to have him take a look. He's stumped as well, and intrigued.

I wanted to follow up and say that I believe this issue is resolved. I will perform further testing in the coming weeks, and circle back if that is incorrect.

After extensive general testing with a very experienced technician, it was noted that the feature lamps were significantly dimmer than expected during the all lamp test. When the connectors for the playfield inserts were disconnected from the power driver board and the game was booted up, the regulated 12v fluctuations disappeared.

After going through and looking for things like shorts, grounds, wiring issues, etc., we went back to the beginning and started with the incoming voltage from the transformer. Working our way from there, we checked the 18 volts source incoming at the power driver board. When we had the board on the Williams test bench, and the oscilloscope hooked up, the display suddenly showed what appeared to be AC instead of DC. Manual pressure and manipulation of the associated components caused things to return to normal briefly. That obviously seemed strange, and after some quick checking it was determined that one of the large capacitors and associated bridge rectifier were likely the issue. The components themselves were fine, but someone in the past had removed them, and in the process accidentally removed several through-holes on the board. Following the repairs to correct that issue, and reinstall of the capacitor/bridge rectifier, the 18V incoming was corrected, and subsequent downstream regulated 12v became rock solid.

TLDR/Short Version - If my understanding is correct, the incoming 18V was not being properly handled due to damaged through holes on the capacitors and bridge rectifier associated with the process of supplying regulated 12V. Because of this, the draw from things like the feature lamps would pull the 12V down too far (below the 5V), fooling things like the MPU comparators into seeing what they thought were closed switches, thus triggering items like the flippers.

Hopefully that makes sense, it was the first time I ever seen an issue like this.

What's interesting is that the most likely reason the issue existed is because someone in the past followed one of the guides to "replace all the 15,000 uf caps" to try and fix reset issues. The caps themselves were not the issue, the person that removed them previously did so poorly, damaging the board/through holes.

As for requiring a technician, I'm actually a tech at an arcade with hundreds of pins and games, but it's great to have access to someone with even more knowledge and experience, plus better tools and test rigs. I love going there and being able to plug boards right into the Williams, Bally, etc original text rigs. So much fun, and educational. I learn more every time I visit him. He had fun as well, because this was something new he'd never seen. It's the mysteries and solving them that often drive us!