I suspect some sort of short circuit.

If you really want to run down the problem you can swap parts from the right side to the left.

If that doesn't fix it, then it must be in the wiring. Honestly, I would probably look there first.

Couple of things:
The 50V solenoid circuit should be entirely separate from the 6v light circuit. If there was a short from the flippers to the GI, you would probably get bright bulbs and/or a blown fuse. You don't. You get dim bulbs, IMO indicating that applying the flippers drags down the whole transformer output, including GI. I wouldn't mess around pulling a wiring bundle apart.

1. If your EOS switch was dirty, or open all the time, it would likely result in a weak flipper, not dim bulbs on an unrelated circuit.
2. Ditto for the actual flippers switches.
3. Ditto for crappy soldering.
so switch filing, cleaning etc. is a bit of a red herring IMO.

You mention that upon further review, the right flipper DOES dim the lights, but only for a split second (likely until the EOS opens). This to me indices that functionally, your EOS on the "bad" side isn't working.

I think you said it was stuck open once upon a time. Again, this would not give you dim bulbs, but a weak flipper. So I think we can rule out the EOS itself.

>>What this appears to point to is that the coil is wired incorrectly. Such that the hold circuit is never working right.

When did this start? Why did you change the coil in the first place?

You may want to post pictures of your left and right flipper, including the wiring. If you have pictures from before the swap, post those too. If you don't have those pictures, consider it a lesson learned and take them next time. It is easy to get turned around with "left and right" vs "inside and outside" on flipper wiring.

>> Perhaps a helpful Old Bally owner can post pictures of their setup?

You could also reference the IPDB for help. There are pictures posted there of the flippers.

You bring up a point about the coil. NEW is not "known good". Take your coils and measure the resistance across the various lugs. Compare back and forth. You may have 3 coils to work with? I'm not sure what the reason was for replacing the first one. It wouldn't be the first time somebody replaced a good coil for no good reason. Look at the wiring on the coils itself on the old and new coils. Look carefully at the sizes and numbers of the coil windings attached to each lug. One lug will have only a fat wire. One will have a fat and a thin wire. One will only have a thin wire. Do they match? Match the lugs with the coils facing the same direction, not with the coils facing each other. Do the PART NUMBERS on the coils match?

As I and several others have mentioned, you may want to swap side to side with the known good side. IF YOU DO THIS BE EXTREMELY CAREFUL OF THE WIRING. Label the lugs on the good side FUNCTIONALLY. Power, ground EOS etc. Know where your wires go on the bad side. Be sure you hook it up right. I suspect it is hooked up wrong, so don't shortcut this step!

Post those pictures, and good luck.

Sh!t, MarkG you and I were typing at the same time. That'll teach me!!