This is an issue that has been bothering me for a while and I got the fix in last night.
Finally.
So, I'd like to share what I have learned.
TWD had had mystery ghost hits to the prison for the better part of a year now. Replaced optos, replaced opto boards, re flowed connections. Here is where it got strange. The drop targets would reset randomly during a game and with much less frequency than on the prison optos. So I am stumped. The game freaks out and goes into multiball randomly during games.
No Bueno.
As I am working on the game again last night I get the machine gun opto registering in the switch test and start to see the drop targets coming up when they shouldn't and when I unplug the connector to the 3 bank I notice that the optos on the ball trough go out. I make the connection. Checking the wiring on the connector to the drops (why is it ALWAYS the connectors...) on of the wires frays and breaks. I reconnect the wire to the connector after I twist and solder the wire together.
No ghost hits on either the prison opto or the drop bank now.
I used to be able to diagnose this more easily with the 8x8 switch grid from bally/williams and while stern's system is great, the vertical/horizontal relation to the switch grid is much different and not as intuitive as the older system. But even more important, knowing the connection between errors on the game will be more intuitive for me in the future. The error may not be the primary source of errors, it may be another link within the chain.
That's what I learned, I hope this will help you toon.
switch grid (resized).png