Just to share the information here for everyone, this is from the readme of the L-2 Firepower ROMs

Firepower was unique in that it made use of the both the "PROM"
sockets and the "Game ROM" socket for holding game software.
Using both sets of sockets allowed Firepower to have an extra
1.5K bytes of game program.

Note that the original configuration of chips requires that MPU board
jumper J4 be installed and J3 be removed. There are orange labels in
the cabinet and on the board noting this fact, but we state it again
in case those labels have disappeared over the years.

Replacing the ROM and PROMs for Firepower is difficult because the
bipolar PROMs are hard to find, and a device programmer capable of
burning them is even harder to find.

If the original chips are missing or damaged, the better solution is
to use the "FPCOMBO" ROM data programmed into a 2732, and make the
board modifications outlined below.

SYSTEM 6 MPU BOARD MODIFICATION FOR USING "FPCOMBO" GAME ROM
------------------------------------------------------------
*********************************************************************
* *
* !!!!!! THE USUAL BIG DISCLAIMER SECTION!!!!!! *
* *
* If you don't know how to work on circuit boards, don't even *
* think of doing this modification. If you've been meaning to *
* learn how to work on circuit boards, don't even think of doing *
* this modification as your first project. Get someone else who *
* is already skilled at circuit board work to do it for you. If *
* you are concerned about maintaining 100% originality in every *
* way on your machine, don't perform this modification. While it *
* has been designed to be easy to "undo" should the need ever *
* arise, it will permanently alter your circuit board where one *
* trace is cut. A circuit board modified in the described way *
* will no longer work in any other games unless the modifications *
* are undone. This is meant to provide a way to keep a Firepower *
* running, where the MPU board is expected to stay in a Firepower *
* forever more. *
* *
* While every effort has been taken to ensure the accuracy and *
* and viability of the Firepower combo ROM and the circuit board *
* modifications described below, this solution is provided with *
* no warranty nor support from Williams Electronics Games, Inc. *
* *
* Your use of this information is at your own risk, and is *
* provided solely as a courtesy to enable you to continue enjoying *
* your Firepower for years to come. *
* *
*********************************************************************

1. Remove any Game ROMs (IC14, IC21, IC22, IC26) from their sockets.

2. Remove any jumpers at J3 or J4 (to the right of IC15, a 74LS139).

3. On the bottom of the board, at IC14 pin 21, cut a notch in the
heavy circuit trace on both sides of the pin. Removing some of
the existing solder on the trace may help. The idea is to get
rid of the +5V connection to the pin, while still leaving an intact
solder pad at the pin itself.

4. On the back of the board, run a small jumper wire from IC14 pin 21
to IC30 pin 14. IC30 is generally not installed; it is the 16-pin
chip outline just to the right of IC14 and just above the interboard
connector. Solder the wire through the hole at pin 14 of the empty
IC30 location, and to pin 21 of IC14, making sure not to short it to
the other side of the trace you cut at IC14. This brings A11 to IC14.

5. On the back of the board, install a 4.7K ohm, 1/4 Watt resistor
between pins 20 and 24 of IC14. With the resistor leads bent at
90 degrees at a comfortable distance from the body of the resistor,
they should line up at just the right spacing to solder one lead
to each of those pins. Again, make sure not to short the pin 20
connection to the jumper wire at pin 21, or the cut trace edges
around pin 21. This provides a pullup for the active-low chip
select.

6. Install two small-signal diodes (example part numbers are 1N914
and 1N4148; the same sort of diode used in the switch matrix)
at locations J3 and J4. The cathode (banded) end should be towards
the TOP of the board for both diodes. This allows either of the
two Game ROM address ranges to drive the active-low chip select
at IC14.

7. Program a 2732 with the "FPCOMBO" ROM data and install at IC14. If
the game doesn't already have working Green-label ROMs at IC17
and IC20 you should of course install those too.

I saw one guy recently who didn't actually cut the traces on IC14, and just bent the leg at pin 21 up and ran a jumper wire from the leg over to the pad.

Personally, I don't find it's difficult to back the cut traces out with a little bit of a solder bridge. But, odds are, once someone puts one of these boards in a Firepower, it's gonna be there for life anyways.

Those adapters do sound slick... but for those of us who enjoy working on the boards...

Well, I hope plan A works for you, there's nothing more satisfying than fixing it yourself! If the game is at least partially working, often times new sockets, roms and interconnect will fix things up nicely.

There are plenty of great mentors with buckets of experience with these boards on pinside, if you have a problem that stumps us, I'll be surprised.

You likely won't find a PROM3 unless someone has a good used one laying around. The actual chips are almost as difficult to find as a programmer that can program them - obsolete.

If you can't find a Canadian resource, I just did a set of Firepower boards for a customer this week.

I already shared my opinion of machined sockets / socket strips vs twin leaf - I'm a fan of the twin leaf personally. I'd say go with whatever choice you're most comfortable with.

You'd remove the batteries if you did nvram, correct.

I typically do a cr2032 lithium coin cell

Coils are not related to GI. Completely different circuit in the machine. Maybe you could have shorted coil voltage to the GI circuit somewhere and blown out all the bulbs, but that's pretty unlikely.

It's possible on a neglected game that all the GI could be burned out, as bulbs age, they start to discolor and eventually silver and eventually burn out - the resistance goes up as they wear out.

Go through all the connectors and fuses - potentially you've got a connector problem on the playfield GI.

I wouldn't replace a transistor on the driver board unless it has failed.