Hi friends. Got something wrong with the special solenoid circuit driving the left jet, which won't fire in coil test or when the playfield switch is closed. Grounding the blue lug on the coil to the rail does cause it to fire, so the coil is (I think) fine. I think I have at least two problems.

- Probing Q73 I find 0V on the emitter (pin 3) which is the ground path for the coil circuit. Normal.
- Q73 pin 2 is the collector, 25V as expected.
- Q73 pin 1 is the base, expecting 0V except when the coil is to be fired. <b>23V!</b>

I'm pretty sure this means Q73 is blown. There's a burn mark on the board by Q73 pin 1. I'm new at this level of diagnostic but if 25V was to pass through one or more resistors in this darlington package, it seems reasonable that we would find something less than 25V at the base.

Problem 2: with the playfield harness disconnected, 1J18-2 is shorted to ground. This is what comes back from the switch under the jet bumper. I think this means that if I replace Q73, I'll just blow out the replacement.

Someone before me has already worked on the board. From their rather ugly work and left-behind flux I can see that the following has been replaced:

- U39, switch matrix. Switches are fine.
- U50 NOR for flipper enable relay, right kicker, upper jet. These are fine.
- Q31. The lower playfield flasher domes are melted and now we know why.
- Q49, 1J8-2 switch driver. the board is out at the moment but I'm pretty sure that column is ok.
- Q71, 1251 bulb left visor. I think that one is ok.
- Q77, right kicker, fine in coil test.
- Q76, pre-driver for Q77.
- Q79, upper jet, fine.

...none of which gives me a clue about why 1J18-2 is shorted or what I should poke at before replacing Q73. What should I be looking for? I'll attach my notes in case that's useful. Thanks everyone!

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This is my situation and my plan. Thank you! I'll be back with the results.

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Of course you're right, thank you. Somehow I had myself convinced that a) I had picked the right alternative since b) Marco doesn't stock the 2N4401 specifically. I was wrong on both counts. Marco has the correct part.

- Q73 is replaced, old one tested bad.
- Q72 is out, tested bad. Readings below are with Q72 vacant and the board on the bench. I can't see this explaining my readings but I'd like to know if I'm wrong.
- ZR3 is replaced.
- Coil diode replaced. Doesn't seem like it was bad.
- Resistance to ground from 1J18-2,3,4: *18Ω*, 4.7K, 4.7K. Still not right.
- Resistance across R106, R94, R100: *158Ω*, 540, 545. Not awesome.
- SR20: resistance from pin1 to 5, 6, 7: *140Ω*, 4K, 4K.

So the SS3 circuit is still a hot mess.

- Am I wrong that the Q72 vacancy doesn't explain the inconsistent readings above?
- Do you agree that the anomaly in R106 and SR20-5 suggests that neither of them is at fault?
- Do you think C70 or (gulp) U45 are implicated?

Thank you

EDIT: Q72 replaced. As expected, no change to the above readings. I guess I'll order some NOR chips and some 0.1uF caps so I can check C70 and U45.

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Final update, hopefully. Resistance across pins 1-2 on U45 was 1 ohm. Should be around 5k.

Pulled U45, same resistance from pin 1 to 2. That NOR gate was blown, along with Q73 and Q72. By chance there was a socket under U50 so I socketed U45 and moved the 7402 from U50 to U45: anomalies above are resolved. Once I have a fresh 7402 for U50 I’ll reassemble and test. Not sure what caused the fault in the first place but there’s been a *lot* of prior repair on this board, so perhaps this is a last unsolved gremlin from an old calamity.