That is by design there's supposed to be resistors under the PF near the flashers that keep each flash very SLIGHTLY on (although you shouldn't be able to see it brightly....) - the theory at the time was that with the flashers pre-warmed, they would last longer when the higher voltage pulse went through them.

If they have been replaced with LED's this might be an issue because LEDs are ON or OFF, and the warming voltage might be enough to light them. If they are regular flasher bulbs, the resistors might be out of spec and are in need of replacement.

How much voltage are you getting at the light socket?

What resistance (ohms) do the resistors read with your meter?

Hmmm, have you tried different flasher bulbs that you know work properly? I ask because you described as an old klunker. I can imagine someone just throwing in whatever bulbs they had laying around. What type bulb is printed on the glass globes?

The globes with nothing printed on them are suspect. You have no idea of the "type" of those bulbs. They could be for/from anything, automotive, boats, ect.

Now, your voltage question is a complex one... It should actually be much higher than 13.2V (more like 28V when not under load) but let's not worry about that... Sounds like you tested for "AC" volts, not "DC". No biggie.

My strong suggestion is to remove the "grounding" wire on one of those small warming boards and see if that resolves your problem. (I think it's the wire furthest to the right in your picture above). And then tape it up or add heatshink to it so it doesn't touch other stuff. If that works, remove the grounding wire on all those "warming" boards.

The grounding wire (actually the entire resistor) is not really necessary, it only makes the bulbs pulse brighter when pulled on. You could remove those large resistors but that's a lot more work.

Now, this also assumes you have the proper bulbs installed.

The technical reasoning is better explained here in this thread:
https://pinside.com/pinball/forum/topic/removing-warming-resistors

There could be pinball purists that post under this like "no, don't do that" and OMG no... blah blah, just ignore those fools.

Suspect solenoid output circuit 15 Flash Lamps signal is stuck on:
Unplug 2J9 on the driverboard (Lower left bottom plug) to check if they are now off.

also check the resistor 8R8 is a 330 ohm 2Watt and 8R7 1ohm 1/2Watt
Bulbs are #89
IMG_20190927_191426092 (resized).jpg

Are you sure the bulbs in the flasher sockets are incandescent bulbs? If they are LED bulbs, they will be on all the time until you take the warming resistor out of the circuit.

If they are incandescent, what happens when you go into solenoid test? Do they get brighter when they are firing?

Confirm you have #89 bulbs in there. Take the bulbs out and look around the sides and bottom of the metal base for numbers printed or stamped.

Suspect Driverboard solenoid output 15 transistor fault:

Replace Q43 Tip 122 and Q42 2n4401 as a set.
re check flash lamp function.
IMG_20190928_180522723 (resized).jpg

No change after replacing replacing Q43 Tip 122 and Q42 2n4401 as a set:

Check Ic 4 Pin 11 using a logic probe.

Board issues...

I would still suspect that you could have some other bulbs besides 89 bulbs installed. Have you tried swapping in flashers from other sockets in the game that you know work properly?

Did you ever try to disconnect one side of the large resistor(s) at the warming boards?