I have been working on a few System 6 MPUs for a couple Firepower machines I'm rehabbing and am having a similar failure across all of them (3 to be exact). So at least the issue is consistent across MPUs.

Each board will boot fine and go into attract mode without issue. I can go into audits and check coils/switches/etc. and everything is good. When I hit the start button, the ball will kick into the shooter lane and that's when the displays start displaying garbage, and shortly there after the board completely crashes. I've been able to duplicate this same behavior without power to the coils, so it is unrelated to the ball being kicked into the shooter lane or a coil firing. I have had this same issues across multiple (modified) MPUs and multiple Solenoid Boards.

When I depress the Diagnostic switch on the MPU it does show a PROM fault (Bottom LED is lit).

Per the Combo Rom instructions I have installed two 1n4148 diodes (band up) at jumpers J3 and J4, installed the jumper from from IC14 pin 21 to IC30 pin 14. Cut the traces on both sides of IC14 Pin 21 and verified that it is disconnected on both sides and installed the 4.7K ohm resistor across pin 20 and pin 24. Note: I have used 1/2 watt resistors as that's what I have on hand but that shouldn't matter.

New EPROMS, Sockets, etc. I thought maybe it was some faulty RAM and have changed out the 6810s and 5101 with the same results.

Maybe I'm missing another jumper someplace? The board pictured did have some previous work done but otherwise it's in good shape.

Usually these issue winds up being some silly mistake on my end which is likely the case with this as well but I'm currently out of ideas so any help is appreciated.

pete_d - All three boards in the same cabinet. I've tested an after market board set (MPU and Driver) that I keep handy (Kohouts/Pinball PCB) for testing and it was able to boot and play fine so my assumption is it isn't the cabinet. To that end, I"ll test the modified boards in another cabinet and see what transpires.

I was able to test in another cabinet and had the same results so it isn't a cabinet issue.

Prior to doing the mods I verified the boards in a factory test fixture and they all worked fine. They boot and pass each one of the tests (Switches, displays, solenoids, lamps,etc). They will still boot the Williams test program when installed and pass the tests with the modifications done.

Just something about U14 it isn't liking. So what I'll do next is use one the Rom for U14 provided from Pincoder and verify it's reading/writing things alright and see what happens. I've never actually used this test so it'll be interesting to see and I appreciate the reminder it exists!

pincoder - Your ROMs are extremely helpful for diagnosing problems and can't thank you enough for releasing them. I actually use yours (along with Leons) to start testing/repair before I even get this far.

ChrisHibler - I really hoped that was the issue but I buzzed out the traces and they are not connected. I also verified the pull-up resistor isn't shorting anything as well.

Oh yeah, I was in on some of your original testing so I still am using the outdated stuff, heck you hadn't come out with the Adapter yet. Man has it been that long?! Stupid Covid screwing with my mental timeline.

At any rate I'm curious as well as I haven't ever used that specific test yet so hopefully tonight and will report back.

Update: I tried burning two different Images (one set of 2716, and then doubled up for a set of 2732). Placed the 2732 in I17 and I14 - Bottom LED flashed indicating failure. Anytime a 2716 was placed into I17 the displays would display 654321 through each display and It wouldn't give the LED flash pattern as outlined in the documentation. I tried placing a 2732 in I17 and a 2716 in I14 and the bottom LED flashed indicating failure.

I think I'll try to undo the 2732 modifications on one of the boards and see how that goes.

My apologies, this post is a bit confusing with all the numbers.

Next Update: I went back and un-modified a board and was able to boot a ROM set for Gorgar. Re-modified for the 2732 combo ROM at IC14. Ran pincoder tests via message above. The first 3 passed however the 4th test failed. I'm not entirely sure that I Burned the correct ROM images on the final test.

Final Test - I burned 12-rom-IC20-PATT2048 onto two 2716s and placed them in IC17 and IC20. I do not have a file named 12-rom-IC20-2048 so maybe that is the cause of failure. Since this ran Gorgar earlier I'm surprised that it didn't pass the final test although, again, I'm not sure I got the ROMs correct.

Yeah I'm pretty sure I didn't pay close enough attention on my earlier tests, hence the weird numbers in the displays.

All 4 tests passed - Found the file up a page in the directory - sorry about that. But hey we passed all the tests, probably first time in my life

On the positive the board now boots and I can play a game so great success!! I wish I could definitively pinpoint what that issue was but it was a combination of double checking IE: Redoing, my previous work and changing out the 5101 CMOS. I have terrible luck with that stupid chip. I don't know about anyone else but i can test it and it'll be fine, put it in a game and all kinds of trash comes up. Who knows.

At any rate I'll mark this one as successful and take a look at the other boards and see what I can find.

Thanks all for the advice/input.