You can order a replacement MPU ($195US I believe) for either cocktail or regular Game Plan games from:
Echo Lake Pinball Service & Sales
925 Marwin Dr.
Hinckley, Ohio 44233
Tips for specific games (from rec.games.pinball newsgroup):
Problems with the SWITCH CATCHER (PCB under playfield near left flipper):
>>> When I disconnect the Switch Catcher from the game (Supernova)
>>> everything works. When I attach it the game will not start. I have
>>> replaced the three major IC's and checked all caps, diodes, and
>>> Anyone have any thoughts?
>> Yes this can easily be fixed, If everything works fine when the switch
>> catcher is disconnected then this will fix it...
> The "switch catcher" Seems to control All of the switches that stem
> from the spinwheel at the top of the PF. It is located at the bottom
> left under the PF and has a 15 pin connector. There are the 4
> switches, 4 strobes, and 4 lines. It also gets 2 ground feeds and 1
> +5v power feed. With the switch catcher disconnected, the above 4
> switches do NOT work so leaving it off is really a bum option.
Switch Catcher - #02-70051B.
It looks like this circuit board is indeed designed to "catch" switches with short duration impulses using flip-flop logic that is tripped when a switch is closed and is reset when the next strobe pulse comes along.
I checked the PCB on my working Super Nova and found the following pins (counting from the front of the cabinet to the back) are all High (roughly 5.0VDC) #1, 2, 3, 4 (could be pulsing), 5, 6 (Vcc) 8 (High pulsing low), and 9 (could be pulsing).
Switch inputs are #1, 2, 3, and 5 - all should be High as the switches should be open.
The common./ground pin is (of course) Low - #13 & #12 (no wire to 12).
Pulsing pins are the Strobe Inputs - #7 (ST-0), #10 (ST-1), #14 (ST-2) and #15 (ST-3).
If any of your reading are different, then first check the PCB for cracked/cold solder connections to the long Molex pin row, then check switches for jammed closed, finally do the Diode Test on the TTL and LM339 gates...
http://www.flippers.com/service.html#tips (diode test)