Hello,
The title says pretty much everything. Power supply holds fine until I plug the KPC board ribbon cable to the digital board. Fuses 3&4 blow on power up. I replaced the voltage regulator and the two capacitors beside it with no improvement.
If anyone here has an idea about how to try to fix this… The board is pretty simple. I can’t check the voltages coming out of the digital board since I couldn’t find any schematics. I checked the main power supply, everything seems fine.
If anyone is willing to sell me the whole board, feel free to hit me up.
I really don’t want to throw it in the trash 
Thank you !
If this is the simulator board, replace the large tantalum capacitor (looks like C10 on the blurry image I have) at the end of the large IC chip.
Tantalum caps fail in that they explode or do a dead short without any visible indicators. It can be replaced with a good electrolytic one.
Other than that check every component.
Around this 1990 time period was when several manufacturers had faulty circuit boards, i.e. they would go conductive over time, but not sure this board applies. if the above doesn’t work try to measure the conductivity of the bare circuit board. Its an outside chance, but do the tantalum cap replacement first.
Thank you for your advice.
In the mean time I was with The Soniq on the phone and he fixed the problem while I was about to buy a replacement board from him. The KPC board ribbon cable was just plugged in the wrong socket on the digital board. Yes I am an idiot and yes, I will take more pictures when disassembling something in the future to make sure I plug everything back in the right place.