Hey folks, thanks in advance for the expert advice I know you'll be able to provide!

I recently acquired a High Speed (yay!) but quickly realized the upper left pop bumper wasn't working (boo!). I've been working hard trying to get this sorted out and am having some trouble. After a bunch of troubleshooting, where I'm at now is the coil locks on immediately on power up; everything on the game works fine when this coil is disconnected. Here's the history of my troubleshooting thus far:

-- Suspecting a bad transistor, I inspected the MPU and found a super-toasted burned out transistor at Q77.
-- Replaced this transistor & powered back on. Very soon, I was greeted with that electrical fire smell as the transistor burned out again.
-- Dove under the playfield and found an incorrect coil was installed at the probem bumper. I disconnected and removed it, wanting to install the proper coil.
-- Installed a new coil at this location (and one other, which had a diode that looked unhappy). Pulled MPU again, tested transistors for Special Solenoids, all came back good except Q77 and the pre-driver transistor at Q76. I replaced them both and reinstalled the MPU.
-- Powered on, and the problem bumper location locked on immediately, so I shut it back down.
-- Disconnected the problem bumper just to verify everything else was fine. Started a game and now a second bumper was out, the third was intermittent, and both slingshots were out. Argh!!!
-- Pinging some smarter friends for input, they advised to look for shorts. I inspected and didn't find any. Friend came over to help, he didn't find any shorts either.
-- We pulled the MPU again, and this time found two transistors were bad -- Q77 again and now Q79 -- and one was suspect, Q69. The pre-driver transistors all measured good.
-- With multiple items dropping out, we suspected the logic chip, so this time we replaced both 7402 chips at U49 & U50. We also replaced Q77, Q79, and Q69, as well as their associated pre-drivers Q76, Q78, and Q68, respectively.
-- We powered back up, and the problem bumper locked on again. Disconnected this coil, and started a game, and the other bumpers and slingshots have come back to life.
-- We pulled the switch assembly off the bottom of the playfield to look closely and verified again there were no shorts here. The capacitor and resistor on the switches appear to be good; they measure the same as the ones on the other bumpers. The diode on here was returning different readings than the others, so I replaced it but that made no difference either.

So that's where I'm at. The upper left pop bumper locks on immediately when powering up. When that coil is disconnected, (as of now) everything else on the game plays fine.

Your expert guidance is appreciated. While I have some limited experience troubleshooting SS games (primarily via some work on a 1977 Power Play and a 1989 BK2K), I still feel like an amateur, so I won't be offended if you help me walk through some of the basic stuff as necessary. Thanks in advance!

CPU is System 11 (D-10881).

With 1J18 disconnected, the coil still locks on when I power up.

Yes I do. It's been a while since I used it so clear instructions would be very helpful, thanks!

...but we just ascertained that the coil locks on even while 1J18 is disconnected.

The only thing I can do right now to stop the coil from locking on with the power turned on is to disconnect the coil itself.

Let me know if I did anything wrong here. Test conditions:
-- 1J18 connected
-- Problem coil disconnected
-- Game on, in attract mode
-- Probe set to TTL
-- Probe black on ground
-- Probe red on 5V
>> Probing U50 pin 1 yields 'LO' light

Ok, so circling back:
-- Problem coil connected
-- 1J19 disconnected
>> Coil does NOT lock on when game is powered up.

Logic probe Test 2 of U50 pin 1 ... conditions:
-- 1J19 disconnected
-- Problem coil connected
-- Game on, in attract mode
-- Probe set to TTL
-- Probe black on ground
-- Probe red on 5V
>> Probing U50 pin 1 yields 'LO' light

Yes, switch has sizable gap. I continuity-checked the switch and verified it only sees continuity when the switch is closed.

When you say "the base of Q-76" ... Q76 has three legs. Should this test be probing one in particular?

Update: Logic probe test of Q76. Conditions:
-- 1J19 disconnected
-- Problem coil connected
-- Game on, in attract mode
-- Probe set to TTL
-- Probe black on ground
-- Probe red on 5V
>> Probing all three legs of neighboring pre-drivers Q74 & Q78 for reference yields readings of HI-LO-LO.
>> Probing all three legs of pre-driver Q76 yields **no beeps/tones/lights at all!**

NOTE: Pre-driver Q76 was brand new as of last night when we replaced (among other things) U50, Q76, & Q77.

Ok thanks for clearing that up for me! As described above, I'm getting no reading at all from any legs on Q76 (others nearby do show LO on middle leg). U50, Q76, & Q77 were all replaced last night.

Using continuity check on my multimeter, I find:
>> U50 pin 1 to Q76 base = beep (continuity). Yes, this trace is on top of the board.
>> Q76 base to R103 leg = beep (continuity). Yes, this trace is on the bottom of the board, but I was able to check it from the top by touching the exposed legs.

Confirmed, reading 0.0 - 0.2 for both.

Closer inspection (see photo) shows that we were being verify conservative with soldering (on the bottom side), so as to not end up with big blobs. As such, we didn't have much flow-through to the top of the board. Maybe this connection isn't adequate? I'll have to pull the board again and add more solder if you think that might help.

Screenshot_2018-05-28-13-31-30 (resized).png

Ok. I'll try to get to it this afternoon and will update you then. Thanks again for all the great help so far!

Educational question, in the meantime -- I see now (thanks to you) on the schematic where it identifies the pin number that goes from U50 to Q76; but how do you determine that the logic probe should return a LO indication?

THANK YOU GRUMPY!!!!!!!!!!

I'm not really proud of the work, but I got more solder on that pre-driver and NOW THE BUMPER WORKS!!!!!!

I guess it's hard to say definitely what the specific problem(s) actually was/were, but we replaced enough parts to (hopefully) be certain this problem is done and overwith now. I just put a few games on it, and had no issues whatsoever (except a light that went out and a spinner switch that needs adjustment, lol).

This is such a huge relief. I've had this game for a few weeks now and have been dying to start shopping it out, but wanted to wait until I got it fully working. Now the fun can begin!!!

Thanks to everyone who helped! I sincerely appreciate it! This community is the best -- between the colleague who sold it to me and was trying (remotely) to help make things right, the friend who came over last night to help me with additional teardown & troubleshooting, and you, Grumpy, for helping me through the home stretch. Thank you all!!!

Glad we could help!