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.

Thanks

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.

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

Could be a cap or transistor, probably the latter.

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!