I have a T2 with two failed GI strings that I can't for the life of me troubleshoot. The two strings are the Green ("CPU") and the Yellow (Right Playfield).

Summary of current diagnostics:
There is power to J115 on all power pins 6-12.
All fuses are intact
Continuity from J115 to the Triacs, from the triacs to J120/J121, and from J120/121 to the fuse blocks all tests fine.
Continuity through the playfield wiring tests fine.

Now the strange part:
Voltage to Triac Q14 (yellow) and Q16 (green) is normal (7-ish volts)
Voltage at the J120 pin for the yellow circuit (pin 3) is 1.6 at brightness 1, 2.7 at brightness 8, and 2.3 when it's supposed to be off (when the test function is on a different string), and 0 when the connector is plugged in.
Voltage at the J120 pin for the green circuit (pin 5) is 2.0 at brightness 1, 2.5 at brightness 8, and 1.7 when the connector is connected although the lights don't illuminate at all.
I'll admit that I haven't tested every light in the right playfield, but I have tested the entire CPU string and there are no shorted bulb sockets or burnt out bulbs.
Additionally, both of these circuits lights up just fine when I jump AC power from one of the good circuits (left playfield, upper inset etc)

I had previously replaced the Q14 triac thinking it was the culprit with no success.
All of the transistors and resistors seem to test fine with the multimeter.

Does it sound like the 74LS374 is failing? I don't have a logic probe so I can't reliably test it.

Otherwise I am out of ideas--I can't see why there would be low voltage available to the J120 but not even a glimmer of illumination from the bulbs.

Thanks for your help!!

I haven't replaced the connectors...no signs of scorching but they definitely aren't new. I'll get some photos in the morning.

On the GI strings that work, I get 6.3V at the connector, about 5.7-5.8V at the bulbs when they're illuminated.

On the "CPU"/Green String, there is about 2.1V at the connector and 1.9-2.0V at the bulb socket.
On the yellow string, there is no voltage when the connector is plugged in.

On the good circuits I'm showing about 7.8V at MT1 and 6.3V at MT2. On the two bad circuits, I'm showing an erratic voltage at MT1 (the voltage jumps around within a range of 6.8 to 7.8) and 2.4 at MT2.

Side note: I'm trying to upload pictures (each less than 1MB) and I've let it sit for over an hour and they won't finish uploading. Is there something special I need to do in this particular forum for it to accept the pictures?

I think I'm starting to narrow it down. Both of those circuits are powered by the same wire from the transformer. I've tested the connector at the transformer and the voltage is a steady 7.9V. However, when I plug in the connector at the transformer and then test the voltage at the wire end that supplies J115, the voltage drops and becomes erratic. This is the only wire that is affected, so I'll try replacing it and its connections over the next few days.

Well I finally figured it out...turns out one of the wires from the transformer was loose at the 9-pin power connector. Replacing the connector and the female pins did the trick! Also replaced the IDCs with new molex.

Thanks to everyone's input and help!