Quoted from ThatOneDude:
So, I have an odd problem that I haven't debugged yet. On occasion, I'll hit a bumper or a slingshot or some other solenoid fires, and I hear a soft knock and get another credit. I haven't had to add credits to my Gorgar for months now. Anyone seen this before? Have I inadvertently set my replay to something ridiculously low?
If you, or you know someone who does, have access to a chip burner and some EPROMs you can try these tests out. In particular the solenoids test will allow you fire each one at a time and by doing so you might be able to diagnose the problem.
There's a complete set there so you can test the whole machine, including the CMOS and RAM chips, which could also be suspect.
Here's where you can get details: http://pincoder.reversion.ca
let me know if you have any questions or need more help