Electrical Troubleshooting
Ok, so going back to my pinball troubleshooting skills, I first traced the wires from that solenoid to the ribbon cable connector, and in comparing it to others, it matched pretty well at about 2.2 ohms. (Yes the other end of the ribbon cable was still connected inside, but printer was powered off). This made me confident the solenoid was hopefully good and not burned up.
Next I opened up the back of the stand and found where the ribbon cable connected to the logic board. I unplugged it (just slid it out/down), and in tracing the pins back, did another measurement to find that both pins seemed to be connected, now showing about 3.3 ohms. At least now I know the ribbon cable was good there and that there must be a problem on the board. I suspected a bad solder joint, considering how old this thing is (42+ years!).
Now that I needed to dig into the logic board, I needed schematics, and found them on DeRamp’s web site HERE. Each solenoid is driven by its own driver circuit based on an op-amp. The first thing I attempted to do was trace the ribbon cable connection to the main transistor leg, and huh, I couldn’t find a connection. Then I followed the trace back on the schematic, and realized that each driver circuit has its own 2 amp “picofuse” to protect the solenoid. Hah! I tested the appropriate fuse, and found it was open.
Of course, it’s never that easy. I clipped on a glass 2 amp fuse, and sure enough it blew nearly instantly when I turned the printer on, so there was still an issue with the circuit. Transistors all checked out good, and the rest of the passives visually looked fine, and knowing my past experience with old op-amps (hint: they fail, a lot!), I turned my attention to the op-amp, an LM301A, which are now very obsolete.
I compared the input voltages to the next circuit over, and as far as inputs, everything checked out and matched the other circuit. The output however, feeding the first transistor, did NOT. Where a good circuit had ~1.2v dc, this showed…-17vdc! In addition, I checked and that transistor was pretty warm, not burning hot, but definitely not a happy camper.