Disclaimer: I don't have access to a manual for this machine, so the following advices is general to early-mid SS machines

Resets are usually caused by one of two things, first is brownout detection, this is when the voltage going into the CPU (on the MPU) drops below the acceptable level, and to keep the programming from going into the weeds, the brownout detector resets the processor into a known state.

The other likely cause is the watchdog timer (if one is being used in this machine), this is a timer, that is periodically reset by the software, when this timer expires, it resets the CPU, this is so that if the CPU gets into a logical loop (aka stuck), it will be reset instead of powering off.

Brownouts: These can be tricky to diagnose, as it only takes a few milliseconds for the BOD (brown-out detector) to trigger. Sometimes a multimeter on the supply may not react fast enough to indicate a brown-out. You can try to "Rock the boat" by starting a game, and trying to draw as much power as possible, and see if you can coax a failure (remember coil power is not the same a CPU power, but the CPU power is used to trigger transistors/isolators which (down the line) do trigger the coils), generally, if you feel that brownouts may be your issue, you can check if the power supply has jumpers differentiating 100v-110v-120v-130v-etc. and bring it closer to the power at your location (measured from the socket, be careful to not touch the leads while doing so)
If you have an oscilloscope, it's single trigger mode can be (mis)used as a basic brownout detector, by setting the trigger just below the line level, and using a falling-edge single trigger, which should trigger if the voltage drops, remember that some voltage fluctuation is normal, but too much can trigger the BOD.

Watchdog: If the watchdog timer is triggering, it is likely that you have bad ROMs, or something is preventing the ROMs from being read properly. If you have the equipment, you can verify the romset against a known good romset's checksum. The romss are much more suspect if they have stickers that are peeling/scratched (assuming they are EPROMS[one E]) If you can see a small silicon chip through a glass/plastic window on the chip, the ROMs are most likley bad, and should be erased and re-written, or if not erasable, they should be replaced.)

I found a manual for this one, on page 1, the manual documents the procedure for selecting the input voltage, where the possible selections are as follows: [240,220,120,115]. My immediate thought is, if you already rebuilt the regulator board, you may simply have (relatively) low line voltage, and may need to use the 115 setting, instead of the 120.

If you don't already have the manual for this machine, a scan is available on the IPDB page for this machine.

Note: "Unregulated" was poor word choice for the previous post, I was meaning to communicate the fact that the voltage is not boosted if it is low, just dropped if it is too high.

If neither of the above give any insight, as you already have 2 more of this table, swap out the regulator board (the power board in the backbox) with another table, and see if the problem moves with the board. You may also want to try swapping the transformer and it's associated PCB, but that is major surgery.

(@Mods, sorry about the multiple posts, I kinda think of these additional suggestions after I've left the page for some time)

Make sure your flipper coil diodes are still good, working, and firmly attached.

the problem is probably not applicable to ROMs, since the poster is using an Alltek board.

Are the C23 caps on the driver boards you tried old?

Those things need to be fresh for clean power.

https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing.

I agree with Ken H, check your flipper diodes. They can look just fine from a visual inspection, but might be bad. It only takes one bad flipper diode to cause the reset. Had this issue on classic Bally a few times over the years.

You didn't mention this before. Do you mean the problem goes away with different SDBs installed?

logo.jpg

That explains it... I'd be willing to bet the flyback was getting into the driver board (and by extension, the MPU) and wreaking havok... Did you test the old diodes before you threw them out?