TECHNICAL UPDATE (Gottlieb System 1 switch matrix)
I think I've finally figured out the last technical issue that's been frustrating me over the past couple weeks. Drop targets.
During gameplay, sometimes the four bank drop targets would not score or reset, and sometimes they would. At first, I just assumed it was dirty or misaligned switches. I cleaned, recleaned and tested continuity on those switches. But no matter what I tried, it still didn't work 100%. I kept testing, and eventually found one *problem* target. Once the problem target dropped, no other target would score, and the bank would not reset. However, if the problem target was the last one dropped, the bank would reset OK. Having narrowed it down to one target, I initially thought it was being shorted - maybe the paper insulation was worn, but after taknig the leaf switch apart I found this wasn't the problem. So I went back to the schematics and found....the diode.
From Clay's repai guides:
All Switch Matrix Switches Have Diodes.
Because of the scanning of the strobe and return lines in the switch matrix, diodes (one way applicators) are used so there's no "cross talk" between switches. If there were no diodes on the switches, a single switch closure would make the computer think all switches on that strobe line were closed.
Gottlieb implemented switch diodes a bit differently than other companies. Nearly all other pinball makers mounted diodes directly on the switches themselves. Gottlieb however didn't do that - they mounted the switch diodes on small bakelite insulator boards under the playfield and away from the actual switches involved.
Bingo! ...that describes my issue exactly. So I'll be stopping off at Radio Shack on my way home to pick up a replacement diode. I'm 95% sure this will fix the problem, and will post my results here....fingers crossed.