I did end up building a solution for this, but I won't derail this thread with the finer details.

Good luck with the project! One piece of advice would be to make the firmware easy to update in the field. With something universal there's always room for more features (or fixes)!

It would refuse to run without it, but I assume we're talking about "faking" it with the output of the display interrupt generator. When you pipe the display interrupt generator output over to the zero cross detector input you're not exactly running without a zero cross as far as the software knows, you're just running the zero cross detector faster than it would be in the game, thus "faking" the zero cross. Approx. 120Hz vs. 300-400Hz.

This stuff will work too on the bench with the zero cross "faked". The only issue would be driving a lamp driver board as already mentioned. Using the MPU isn't really the best solution for bench testing lamp driver boards anyway.

The easiest solution is to find a transformer that puts out approx. 20V and run the output through a rectification bridge and connect to TP3. Clay described this in his old guides. It's also pretty easy to use a ~$1 microcontroller to pulse the PIA input at the right frequency, as you mentioned. That will solve the issue of the game code acting weird with the faster zero cross, but it doesn't really gain much for bench testing because the test code will generally run fine at the higher zero cross speed. If you want to test attract mode or game play, that can be flaky at best with the faster zero cross. Display,lamp, and solenoid tests work fine, but will run faster. The display test is a little annoying running on an MPU-200 with faked zero cross. Connect a bright LED display and you might have a seizure.

It's easy to do and there should be a good boot-loader and a bunch of examples with the Microchip MPLAB software. I have worked with 18F4550 and 18F2550 with boot-loaders for firmware updates and it's really easy. I would go the route of adding a programming button which you hold down when you power it on to enter programming mode. Could be one of the existing switches. That's easy to do and there are examples in the IDE. You could also do it in software and set a bit in the internal EEPROM which is checked on boot, but that's a little more complicated.

The only thing to be careful about is licensing because you would need to use the Microchip software to load the new image into the microcontroller and you're using their USB ID which may not be allowed.

That's good info.

I built a mini version of the solenoid driver which connects to the MPU to test the PIA outputs which drive the solenoids. Not what you're describing, but might be interesting to some people. It uses the same decoder as the driver board and drives LEDs instead of solenoids.

Bally-Stern_Mini_Solenoid_Driver_(resized).jpg