(Topic ID: 152588)

Williams F-14 Tomcat CPU Switch Matrix Error

By Glitchus

3 years ago

Topic Stats

  • 9 posts
  • 4 Pinsiders participating
  • Latest reply 3 years ago by GRUMPY
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#1 3 years ago

The F-14 Tomcat machine that my wife inherited has a problem with the switch matrix that I've isolated to the 11A CPU board, specifically in terminal 1J8-2. What happens is that when I jumper from that terminal to the terminals on 1J10 I'll get the whole row of 1J8 terminals that correspond in the switch matrix to the 1J10 terminal I'm jumpered to.

For example: I jumper 1J8-2 to 1J10-5, instead of getting the single switch "Left Center Ball Trough-13" I'll get the whole row from "Center Coin Chute-5" all the way through to "Left Drain 'Rescue' Kickback-61" or in other words, switches 5,13, 21, 29, 37, 45, and 53. And the same goes for every other 1J10 terminal I jumper to, I get the whole corresponding row of switches on 1J8 as being closed. Now I can jumper over to 1J10 from any of the other terminals on 1J8 and get the normal corresponding switch, it's just the 1J8-2 terminal that has this inexplicable problem, so there's not a short across the lines from what I can tell, which is why I think it may be an IC problem.

I've run the CPU diagnostics and everything checks out, so my current theory is that IC U40 (GD74LS244) is bad, as it's not included in the diagnostics test. But perhaps it could be IC U38 (MC6821P), but that IC is supposedly included in the diagnostics test.

So the question is, do I try to test/replace said IC's, look for the problem elsewhere on the board, or try to find a replacement CPU board?

If any pinball repair geniuses out there have any advice, I'd be happy to hear it before doing some invasive PC board troubleshooting and repair.


#2 3 years ago

What are the symptoms? Might be a good idea to share before you start replacing chips hoping to fix the problem. Sounds more like a bad diode in the matrix., but it's been a long time since I worked on system 11.

#3 3 years ago

Hey, thanks, I hadn't thought of a failed diode, I'll have to look at the schematic again....

Okay, there's no diode, but there are a series of NPN transistors which are in-line with each terminal, so perhaps the one on 1J8-2 has failed and there's bleeding across the rest of the terminals on 1J8. I'll have to check it out tomorrow, as it's pretty late now, but thanks for 'kicking me in the head', as I should have thought of that to begin with. I'll let you know what I find. Thanks again.

#4 3 years ago

I would check SCR6, SR15 and Q49 first.

#5 3 years ago

Will, do. Thanks for the advice. I'll be taking a crack at it again later today.

#6 3 years ago

Pre-drivers are usually the culprit with these errors, so check the transistor first with your meter in diode test mode. Just compare it with a working one next to it on the board. I think they're 2n3904s or similar NPNs.

#7 3 years ago

Could be a cap or transistor, probably the latter.

#8 3 years ago

It ended up being a bad transistor at Q49. Put a new NPN 2N3904 in, Checked Ohms across 1J8 terminals and got identical results (1.98K), put the board back in and ran the diagnostics switch test without error.

Thanks very much for all of the advice!

Now on the fix the next couple of problems I discovered while troubleshooting, such as; a broken solenoid spring, some broken ceramic resistors, a bad target switch, and a couple of burned out lights.

Thanks again!

#9 3 years ago

Sounds like you have everything under control.

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