That switch that you showed in the photo is not one of the scoring switches for the drop targets themselves. The drop target scoring switches are located on the part of the target bank that is located underneath the playfield. The switch shown is merely a switch that was designed to score 10 points if the ball strikes the white rebound rubber when some or all of the targets are down.

Thus, the fact that this switch was closed all the time, resulted in continuous power being applied to the 10-pt relay in the backbox (the N relay) and it probably remained pulled in.

With the 10-pt relay continuously energized, the solenoid on the 10-pt score reel should have been continuously pulling in the plunger and thus, the score reel was incapable of advancing any further (until the plunger released and permitted the ratchet mechanism on the score reel to reset).

Lee

I would bet the coil of the N relay got mildly hot.

That relay coil (A-9375) only has to move a small plastic yoke with 4 thin metal switchblades and thus, is not required to deliver a lot of physical force. The A-9375 has a (relatively high) resistance of approximately 15 ohms, which limits the current through the windings to about 1.6 amps, which is enough to heat up a small gauge wire if left applied for a period of time, but isn't really a short-term threat to burn up the wiring.

Lee

My post didn't quite go far enough to explain #3.

When the N relay is energized, there is a NO switch on N that should close and lock in power to the N relay until the score reel moves a full step, and then an NC End-of-Stroke (EOS) switch on the ten's score reel opens, which is designed to cut power to the N relay. Since the N relay was continuously powered on by the misadjusted switch behind the rebound rubber, the score reel EOS switch then opens just far enough to try and cut power to the N relay, but power is immediately reapplied from the misadjusted switch. That "mechanism" likely caused any vibration you may have witnessed. The score reel plunger was trying to relax and let the ratchet reset, but could never get there, because as soon as the plunger started to release, the EOS switch would close again and power would be reapplied through the N relay.

NC = Normally Closed; NO = Normally Open
Lee