I'm spinning my post in the Pinbot club to a separate topic, since it actually concerns general Sys11 behavior and I'd like more exposure to be sure I'm on the right track.

I'm working on a project Pinbot that required attention to the Special Soleniod section (among other things). All four coils associated to U45 were acting up: the left sling and left pop would lock on, the left visor flasher section was totally fried, and the bottom pop would not work at all. There were several issues going on but the short of it is, for the left visor flasher (Q71) I had to replace the TIP122, 4401, and 5W resistor. I also replaced U45 with a new 74LS02.

Now there's no more issues at power on or attract, all tests pass, and I can start and play a game. But after about two minutes, the special solenoids act up again. First time this happened, the left sling began acting erratic by pulsing more steadily until it locked on its own. I powered down before any real damage was done. After a few minutes I turned it on again and all was fine, so I started a new game to troubleshoot and again after a couple minutes, this time the bottom pop exhibited the same behavior.

Each time, only one function acted up and only its specific associated transistor was hot. But what gets me is why the different ones each time, and after a couple minutes? Nothing else on the board looks or smells suspect, and apparently it "recovers" when powered off. So it's not a steady-state fault.

Could this be a mere symptom of needing to replace those other TIP122s outright (conventional wisdom says "no")...or something else (like the 7402-now-74LS02, at U45) randomizing the symptom upstream?

After doing some reading on my IC at U45 hunch and it turns out, the 74LS series is probably not a good sub in this application. Unfortunately it was all I had for a 74()02. The LS series draws low power and runs at high speed... sounds great right.... but it only supports a lower max current draw... oops, the special solenoids would especially be more than it likes.

So now speculating: could that explain why things get wonky after a short time in play... AND why the fault is not a hard-lock state... AND why it manifests on a random circuit (maybe the one most in use as it happens)? I'm catching the problem before the IC goes totally poof, but I'll have to get a standard 7402 and see if that solves the issue.

Any experienced techs care to chime in?

Hmm, that does sound kinda similar. And unfortunately maybe not a straightforward issue to fix...? Regardless, I'm pressing on:

I remembered I have a roached Sys6 driver board, and those have two straight 7402 chips. And I recently bought a Hakko FR300 desoldering iron, so what better reason to see if I can now harvest old parts like a pro? So I tried and successfully removed the 7402's without a problem. I swapped them into my Sys11 U45 socket and... things got interesting!

So with a 7402, during the test the left kicker "tries" to fire once: you hear it buzz, then nothing. No fire, no movement, no clicking, buzzing, or anything. After a few moments its transistor still is not even warm. Of course, I harvested two 7402's from that old Sys6 board... maybe I messed one up, or it was already bad...? But I tried both and got the same result.

While the used 7402 was installed, I let my daughter play a test game that lasted a few minutes. Aside from the dead left sling, everything else was fine. Nothing ever shorted, stuttered, or locked on. Kicker transistor did not get warm.

Afterward I put the new 74LS02 chip back in... and the left kicker came back to life!

If I install the standard 7402, it goes dead again.

I do have some new 7402 and 74HCT02s on order from GPE, and I'm interested to see what results those yield... but meanwhile this is some food for thought. To wit, given what little I understand about IC design: if the LS02 is a low-consumption version which can't handle the sustained stress, but otherwise allows the rest of the left kicker to fire, does that point to weak components elsewhere in that circuit? Where should I focus?

Well it's been long enough now that I think I can share the resolution... and it's kind of a wierd / funny thing.

To be clear, the behavior remained that every time I put the 74LS02 in, it would act weird after a time... but I'm now running with those old 7402's I pulled from the Sys6 board. And what fixed those? An extra grounding screw on the board!

Yeah... when I was first troubleshooting, of course I didn't put all the screws in, because a few are quite the PITA to reach if I'm not positive I won't have to yank the damn thing out again. I think I only used 3 or 4. But when I had to tidy my workspace a bit, I put a couple more in so as not to lose them meanwhile. Turned the game on to test something else and the kicker started working. I made NO other change other than the extra screw!

Of course we've all read the advice and legends about boards sometimes acting weird without all the screws. Yet as it happens, I've had games missing several screws that acted just fine but whaddayaknow, I just learned it the hard way. And since I've played the heck out of the game since without the posted issue, I can say: screw those boards!