With power on no doubt, which shorting a single lamp socket to a switch, which both are different voltages. Always suggested to change bulbs under the playfield with power off.

Test F115 and F114 with a meter.

If not a fuse, I'd go with this.

LTG : )

Starting point is always a fuse. Without power, nothing happens. But it still blows, yeah, likely U20.

This happens when you remove the connector to the coin door interface board. This is where switch #24 is located. When the game can't see switch #24 as closed it assumes that there is something wrong with the +12VR. This should disappear when you reconnect J212.

Wouldn't this indicate that the issue is **NOT** on the MPU, so replacing fuses and ICs not help in this case?

Unless I'm misunderstanding what @op is saying, I guess...

My ground row short went away when I replaced u20 and it was socketed to make things easier.

If you take the plugs off and the short goes away I do think it’s on the playfield. Maybe make sure nothing has been moved causing a short. Inspect the area for anything touching that’s not suppose to.

In my past experience, yes - since the chip is fried and likely has gates shorted to ground. BUT - I am NOT an expert on that chip..

Consider disconnecting A-16807 - the "Custom Opto SW10 PCB Assembly". Reconnect your CPU connectors (J206 and J208) and disconnect J5 on A-16807. This will isolate the opto board. If the source of the ground short is on the board it should disappear.

Disconnecting J205 won't change the situation. Those are a bunch of orange wires that are direct grounded switches - not part of the matrix. You need to disconnect J5 on A-16807 - NOT J205 on the CPU board. This will isolate the A-16807 board from the switch matrix. If the source of the ground short is from the A-16807 board then it should disappear. J5 is a bunch of mostly white wires, some green wires, a black wire and a gray wire.

No. See the above. Disconnect J5 on A-16807 while leaving all the other connectors on the CPU board connected.

Yes. I'm overly cautious and I usually like to be 100% sure before proceeding. If I were doing this diagnosis I would disconnect J5 and see the problem disappear. Reconnect J5 and see the problem re-appear. Disconnect J5 again and see the problem disappear. Doing nothing else to the machine and seeing that kind of result leads me to conclude the problem is on the A-16807 board. That's a custom 10-opto switch board. You CANNOT replace it with a generic 10-opto switch board from another machine. This one for TZ was only used in TZ.

If you're comfortable with PCB diagnostic and repair I would bet that one (or more) of the LM339 ICs on the board is bad. You can purchase one @ https://www.pinballlife.com/10-opto-replacement-board-for-twilight-zone-a-16807.html if you just want to replace the board with a new one.

The old board can be repaired. I would not throw it out. If you decide you want to throw it I'd be interested in it.

The ICs were NOT socketed from the factory so someone has done some work on that board. Check the previous work to make sure that the circuitry (continuity) is correct.

This is a good reason for turning off the game when changing anything on the underside of the playfield, including changing the #44/47 bulbs. It is a very small risk replacing #555 bulbs, but still if you have a tool in your hand, turn the game off.

Words of wisdom. You should never have your tool in your hand, when working on a pinball machine.

LTG : )

... What about someone else's?

TMI

LTG : )

Hey now, you started it. I was just takin' it to the next level. Sheesh.