In the middle of a good game (of course), the left sling spontaneously stuck "on", with the coil energized.
I switched off the game pretty quickly. The coil was & a bit whiffy, but it appears to be OK.

Note that, as soon as the machine is powered up (& in attract mode, even before a game is started), the coil energizes.

- Sling switches are not stuck shut--the have reasonable gaps & behave fine in switch test.

- Assuming/hoping it was its power board transistor that failed in an "on" state, I replaced the transistor.
Nope, left sling still energizes the moment the game is turned on.

- My next test was going to be disconnecting the power board's J9 connector ("Drive Transistor Line Connect" for high current solenoids: flippers, pops, slings) & using jumpers to swap the left & right slings, to see if the problem moved to the right sling or stayed at the left sling.

To my surprise, even with J9 disconnected, the left sling energizes when the machine is powered up.

To my caveman mechanical engineer brain, this tells me that the problem is NOT in my PCBs, because if I disconnect the coil from the PCB, it can't be the PCB that's telling the coil to fire.

- So, is this a playfield wiring short circuit? If so, which wire shorting to what other wire causes this behavior?

- Or is the coil's diode bad?

A few manual pics attached for reference, from when I was deducing likely suspects on the PCB.

Thank you,
-Jason

(My 7yo boy has been playing the snot out of this game for the last couple months in quarantine, & he's getting really GOOD & now knows the ruleset better than I do. I want to get him back up & running--it sure helps out while I'm trying to be productive working at home!)

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Gunnut40 , thank you for the reply.

From what I observed, I don't see how it could be the transistor. Please confirm if I'm missing something:

- I replaced the power board Q13 left sling transistor with a new one. While I was in there, I also replaced the right sling transistor & upper flipper transistor. All 3 were the same P/N & came from the purchase. The other 2 transistors behave just fine. It would be quite unlikely that the Q13 left sling transistor was bad & also its replacement was bad in the same respect. Agreed?

- I completely unplugged the J9 connector (which connects the transistors to the coils) & powered on the game. The coil is still stuck energized. I don't see how a bad transistor can energize a coil when its transistor is disconnected. Logical?

That's why I suspect an issue in the PF rather than the power board.
Rational?

Thank you,
-Jason

Greetings, Programs.

WOOF! WOOF! WOOF!

That's the sound of me barking up the wrong tree.

One of the clues was that the left sling coil would lock on energized even if the cable connecting it to the power driver PCB transistor was UNPLUGGED.

After better grasping that the coil is supplied +50VDC from J10 & that the wire going to the transistor grounds the circuit, yes indeed, there must have been a short between Blue/Green & GND (yes, the manual had a typo: should show connector J9, not J8).

I have 3 Great Lakes Modular SPL-KIT Star Post Light Boards under star posts at this sling, configured to flash when the sling coil fires.
I desoldered them from the coil lug & the coil started behaving!
But those light boards were still on solid as if the coil were energized.

Culprit!

I took out those light boards & saw that one of the light boards' GND wire had frayed & was touching the neighboring coil solder pad.

Short between GND & coil Blue/Green wire indeed!

Fixed up that solder joint, blobbed a little hot glue across the solder joints on all 3 boards to insulate them & prevent this problem from re-occuring.

Soooo, 4 transistor & 1 diode replacements just for "fun."

End of Line.