Hey gang, first time i've ever had to post in this thread!
I purchased a mostly working Fish Tales several weeks ago, I didn't really play it other than a bit during my inspection at the sellers house. I brought it home and immediately started tearing it down and restoring it, which is now done. I finally got to play it and found a weird issue that took the fun right out of it...

What's happening:
When you alternate boat ramps enough times to light monster fish, then hit the spinner (or left loop switch manually), which collects the bonus... The game crashes and resets. Hitting the spinner or loop switch at any other time during gameplay operates and scores normally, and all switches report flawlessly in all switch test modes... I can recreate the crash by manually hitting the switches with my finger 100% of the time. I can even recreate the issue using a matrix tester, so its not any of the switches or diodes on the PF! The power supply checks out perfect, and I also have a kahr board for testing, and that makes no difference either.

What ive done, after being certain that all switches, diodes, wiring, etc are good:
I swapped the FT ROM into a CPU board from another WPC game, and the glitch is gone, so ive definitely narrowed the issue down to the CPU board.

The CPU board has no hacks, its minty clean and shows no sign of any component on the board ever being replaced, the U20 is even still soldered in.
I pulled the Asic chip, cleaned the contacts and reseated it, I also pulled and reseated U6, no change.
I replaced the MPU (U4), no change.
I cleaned all the ribbon cable pins and replaced all the ribbon cables, no change.

Aside from this one scenario glitch, the game plays 100% perfectly as far as I can tell, there are no issues with audio or the DMD either.
I have been working on Williams system 11 & WPC games for nearly 31 years, and I have never run into anything like this before!

I would really appreciate some advice, thanks in advance!

Hey Cheddar, I thought of this too during my early troubleshooting efforts, not only do I get no resets in coil or flasher test modes, I was able to manually cause the reset with the flasher circuits disconnected from the P/D board.

Thanks for chiming in, its certainly is a possibility... I did do a RAM clear early on, but i'd be very interested if you would elaborate on your theory about this?

This manual has all that... https://www.ipdb.org/files/861/Williams_1992_Fish_Tales_English_Manual.pdf
But not really a true schematic... Pages 54 & 55 are useful

Thanks for chiming in Roamin! I tend to agree with you and YeOld. My game is a very early version (# 114) and has the 2064 RAM. I have a RAMTRON chip on the way, along with the latest ROM (because the game has a prototype ROM in it now) from Matt's Basement Arcade, which should be here by the weekend. I will update with the results... Fingers crossed!!

Ok so, I got the RAMTRON NVRAM chip and installed it, and the issue is apparently gone! I could no longer manually cause the issue and I played a dozen games and no crashes or malfunctions of any kind! As an added bonus, I now have a socketed RAM slot and no batteries required!

Thanks to YeOldPinPlayer & Roamin, you guys apparently nailed it!