TZ - here is the history. Hitchhiker Switch 52 acting up, turns out the arm is off on one side and it is wedged up against the side of the slot. No biggie. Of course, I went in with the power on, like I moron. On the way back in to install switch, my hand contacted the tab on the coil for the ball diverter/magnet/gate thing that stops you from going to the power PF when not lit. Yikes. Worst shock I have ever took from a pin. I was holding the switch while I did it. Game reset and came up with Check Fuses F115 and F114. Crapola.

F115 was blown - So I just went straight to U20 and U14 (they smelled cooked). New U20 and U14 installed, and the game booted up, but random coils were firing and I got a buttload of switch out and row short errors. Went straight to U18 and U19 and replaced them with (now socketed!) 339s. Now the game comes up "Ground Short Row 1 is Stuck Closed". But I already replaced these ICs. If the 339s were bad, would that have possibly fouled the new U20? I just don't think there is anything shorted to ground on the game. I know what I did in sending the high voltage up the switch matrix, but it went through me into the switch and up to the CPU. Nothing shorting now.

Should I throw another U20 in there?

Well, pulled the connectors to the PF off and get the Check Fuses 114 & 115 message. New U20 no help. Going back to U14 now. Going to socket new ones all around this time, but the problem is clearly on this board.

No dice. Back to the laundry list of shorted rows, clock broken, switch errors, etc.....

Crap

Jumping J207 to J209, I find that I get normal response and switch activation on each pin. Nothing out of the ordinary registering there.

After a good 12 hours and soldering in sockets like I never have, I found the issue. U20, U14 and all 3 Opto Board 339s were torched. Running smoothly now. If you run 50v into the Switch Matrix of a TZ, the under PF Opto Board ICs should be on your radar. Don't run 50v to the Switch Matrix in the first place, you jackanape.