Grab a piece of wire with both ends stripped. Hookup one end to ground and touch the other end at:
1) Main lamp driver board, bottom right leg of SCR Q18
2) Main lamp driver board, in connector J2 pin 20 (where the Grey-Black wire is)
3) The Grey-Back wire lug at the lamp under the playfield on the left near the small aux driver board heatsink (AS-2517-67)
4) Under the playfield on the left near the small aux driver board heatsink pin 3 of the connector (where the Grey-Black wire is)

In the four tests above, when does the lamp near the AS-2517-67 leftside aux driver board come on?

Note the four lamps in the doodlebug area driven by the AS-2517-67 are 12 volt lamps (921 type), not your standard 6.3 volt lamps.

You probably didn't get good contact in J2 pin 20 with your jumper wire. The test before it on SCR Q18 is upstream from that connector and since it worked, you were getting continuity through that connector pin.
During those 4 tests did the doodlebug lamps also light? Have you checked those doodlebug lamps to make sure they are ok and are 12 volt lamps?

Another thing to test is SCR Q18 itself. Grab your jumper wire and hookup one end to test point TP3 on the main lamp driver board. Touch the other end of the jumper wire on the upper middle leg of SCR Q18. The lamp at the left playfield aux driver board should light. This is a manual way of activating those small SCRs on the main lamp driver board (connecting the middle leg of the small SCRs to TP3).

Your resistors measured good. The schematics list R1 as being 330 Ohms - the parts list says it's 300 Ohms?? A board I have here has a 330 Ohm resistor.

Disconnect that AS-2518-67 board and measure the resistance between the three legs of the Q1 SCR. Also swap the meter leads around when measuring the resistance. If you get any near zero ohm resistance (short circuit) measurements it's likely faulty.

Solder a wire across that burnt trace, install some proper 12V lamps in the doodlebug area and see how you go.

SDB J3 pins 13 and 25 are connected to each other. It's a loop back wire that's there to provide 5V power to the logic chip on the SDB which selects solenoids. One of the recommended mods for this board is to solder a wire between those two pins on the back of the board for redundancy.

Someone may have cut the wire as a test because the board might be locking on coils at power up. Anyway, temporarily jumper test point TP1 to TP3 and see what happens.

SDB_TP3.jpg

Those two cut brown-white wires should be connected to each other. It should work provided the terminals in the connector are ok.
Original wire length is about 5 or 6 inches.

The Aux Driver board with the heatsink (Part number AS-2518-67) is for flashing the four 12 volt "921" lamps in the doodle bug area during bonus collect.

The Aux Driver board without the heatsink (Part number AS-2518-68) is for flashing the GI. It controls the Triac mounted on the rectifier board base plate.

Schematic says #921 lamps. #912 will work fine (lower wattage).

FireBall_DoodleBug lamp.jpg

Why do you need to replace so many switches? have the contacts lost/damaged their gold plating?

The MPU board "Activity Switch" is for clearing audits back to zero in book-keeping mode. It's the separate "SW33" on factory MPU boards. It's not a reset switch.

Also add capacitors to the flipper end of stroke (EOS) switches which will extend their life and soften flipper high current spikes to reduce the phantom switch matrix issues. Note the capacitors need to be high voltage.
There was some recent discussion about adding capacitors to the flipper button and EOS switches here:
https://pinside.com/pinball/forum/topic/reducing-arcing-on-early-ss-pin-flipper-eos-and-cabinet-switches

The three Odin/Wotan standup targets (back left of the playfield) are also supposed to have capacitors on their switches.

That's expensive. I think the originals were about 5 or 6 amps (can't remember). If you're replacing, go for a 10 amp unit which are half the price and while you're at it you might as well replace the MOV (Metal Oxide Varistor) connected to the filter.

A Fireball II I worked on earlier this year either had the caps on those switches or clearly had them cut with leg tails still on the switches.
See the Fireball II playfield switch matrix schematics below showing the capacitor locations and also the note saying they are 0.05 microfarad. Use 0.047uf caps 50V or higher - it'll fix the intermittent fast hit misses.

Fireball_II_Playfield_SwitchMatrix.jpg

The self test numbers for the solenoid coils have no correlation to the driver transistor Q numbers (on some early Sterns they do, but not Ballys).

The Fireball relay is driven by driver transistor Q17. What's the exact problem you're trying to fix with it?

So the right slingshot coil isn't activating?
What happens if you very briefly ground the metal tab of driver transistor Q16? does that right slingshot coil fire?

Possibly. Has the MPU board suffered from battery leak corrosion?

Did you get any schematics with it?

Do you mean the slingshot coil is locking on? If so, isn't it burning up?

If I remember correctly, one fuse is for the standard playfield coils, the other fuse is for the doodlebug ball magnet. The schematics don't list the second fuse and it's been some time since I've seen the real thing to confirm.

I think our terminologies are confusing each other with the word "lock". When we say a coil is locking on we mean it activates immediately at power up and stays activated - if the fuse doesn't blow, it eventually cooks. Do I understand correctly that when you say the "board locks it down", you mean the coil is essentially dead (not activating) because the Alltek SDB has disabled it due to a malfunction (shorted transistor) it's sensed?

The Callahan Mutha MPU board pictures in the below thread indicate it's pretty similar to the Bally board - reset section looks the same.
https://pinside.com/pinball/forum/topic/evel-knievel-with-revision-12-muthapcb-not-booting-up

R11 operates at max power - Bally underspecced it. On top of that, the Callahan board uses a physically small version of that resistor so it runs even hotter having less mass to dissipate the heat.
I always replace it with a 3 watt version. You can find them on ebay.

So when does the slingshot coil become disabled?
The coil has a diode on it? The diode is good? (not shorted) and it's installed the right way around? - banded diode side towards the yellow wires.

Not a dumb question at all, I was going to suggest a 470 ohm resistor. R11 was there as part of the charging circuit for the original Bally spec nicad battery. Your board doesn't have a battery in the charging circuit so you should be able to raise the resistance of R11 to reduce is power consumption.
Let's presume there's 9 volts across that resistor; an 82 ohm resistor will consume 0.98 Watts. A 110 ohm resistor will consume 0.73 watts.
You generally want the wattage rating of the resistor to be at least double what it's consuming.

Note that resistor won't be the cause of your intermittent startups - to diagnose that you'll first need to determine how many times the LED on the MPU board flashes on power up.

So long as you have the yellow wires on the coil lug where the banded side of the diode is, then you might have a shorted driver transistor on the solenoid driver board (SDB).

If you want, try swapping the two slingshot wires at the J5 connector at the SDB (pin 14 is left slingshot, pin 8 is the right slingshot). If the same transistor gets disabled you know it's faulty. If the fault follows the coil you know it's not the SDB.
If it is a suspect driver transistor get in touch with Alltek, their warranty might cover it. That is unless you want to replace the driver transistor yourself..

This is going to be the typical Bally phantom pop issue caused by high current surges to the flippers being picked up as noise in the switch matrix. The only true way to fix it will be to separate the flipper wires in the harness...

From memory, those transistors are surface mount. You comfortable dealing with that? Note Alltek uses MOSFET transistors, they're different to the transistors used on the Bally SDB.

Ok, good to know. barakandl posted a mod a few years ago to get rid of some of those reset section components when using NVRAM, and that Dallas chip on your board is essentially a NVRAM - it has an internal lithium battery from memory with a limited life (but it's many years).

Hmm, it's still consuming at 0.8 watts. Your original 82 ohm resistor must have been consuming nearly 1.4 watts! No wonder it was cooking.
That 16.59 volts on the front side of R11 is a little high. Is the transformer in your game wired for the correct and up to date wall line voltage?

I normally measure about 5.8 - 5.9 volts at the end of R11. Don't worry about it being 5.68 volts.

Can you post a picture of the wire voltage jumper block next to the transformer? the block is where the mains fuse is.

Yes, a metal film resistor is perfectly fine to use.

The voltages you listed across the new 150 ohm resistor were: "16.59vdc at the front of it, 5.68vdc at the end of it".
16.59 - 5.68 = 10.91 volts across the resistor.
Ohms law says current is voltage divided by resistance:
10.91 / 150 = 0.727 amps through the resistor
Power is calculated as voltage times current:
10.91 * 0.727 = 0.793 watts (I rounded it up to 0.8 watts)

Hope that makes sense.

I need to see where the two top orange wires are located on that connector to determine the transformer input voltage selection.

JLBINTN_TransformerVoltage1.jpg

The transformer's configured for 115VAC.

Slightly less current draw/strain on components. But you might prefer the extra 4% power in the flippers leaving it at 115V

The solenoid wire for Q5 comes from pin 10 of the J2 SDB connector. That J2 SDB connector is designated for cabinet wiring. So that 3 pin plug allows you to separate the playfield harness from the cabinet harness to pull the playfield out separately. You didn't mention exactly where those other two wires are coming from for me to comment, but very likely it's a similar situation (they probably come from connectors designated for cabinet or backbox).
There's a few Bally games with that extra connector.

MPU J2 is designated to the playfield switch harness. Having said that, the schematics show two wires from that J2 connector running to the cabinet for the little demon kicker post button. It's a bit weird since MPU J3 is designated to the cabinet but for some strange reason that little demon kicker post button is mapped somewhere in the switch matrix that isn't supported by that MPU J3 connector.
The doubled MPU J2 wires split off into the cabinet harness.
BTW, that connector in your machine isn't the factory Bally fitted connector. If you look at the gallery in this thread, that connector looks a little different.

[Edit] This is what that connector looks like from factory:
https://images.pinside.com/5/9b/e2/59be2de85a5f04c5b4347894a8270fcc2432e4af.jpg

Ok just wanted to make sure you were aware.

Sounds like you have issues in the switch matrix. I'd suggest that you run through the switch test mode for diagnosis.

Raise all the drop targets and remove all playfield balls from the game.
The game should report "0" in the ball in play display indicating no closed switches, if not you have a stuck switch that needs investigating.

Using the switch identification table in the manual (print page 17), activate each switch one at a time but do it in REVERSE order listed in the manual - i.e. start at switch 40 and work your way in reverse sequence to switch 1. Important that you do it in reverse order because the game only reports the lowest switch number that's closed.
Leave drop targets down after you've activated them and leave balls at the corresponding outhole switches after activating them.

Look for switches that don't respond, and switches that report the wrong number.

You can give new EOS and flipper button switches that are tungsten (high current) contact based a bit of a clean with sand paper to remove any protective coating, otherwise you might get what you saw on new installation.
Suffice to say, never use sand paper on gold plated (low current) contact switches.

No issue at all using 1N4004 - the earlier Ballys till about '79 used 1N4004 diodes on the switches..

That's not right, it should be the same as all other switches with a diode. It might cause problems when certain drop targets are down.

Did you manage to separate the wiring for that switch? For test, just run a new wire direct from the MPU board to the button switch (while disconnecting the old wire in the harness). If the problem goes away you know it was caused by electrical noise from the flipper current.

Which playfield wire did you segregate? If you're rewiring the wire to the little demon solenoid that won't buy you anything. The issue is switch matrix wiring picking up noise from the flipper wires. The wire to segregate is the white-green wire coming from MPU J2 pin 8 although it runs to a few switches on the playfield so could get difficult/messy- the worst part (noise wise) probably being the last section running adjacent with the flipper wires to the outhole switch.

My Fireball II had two playfield fuses too.
Ballys of that era have a 1 amp slow blow fuse for playfield solenoids (except the flippers). The Fireball II schematics lists that playfield fuse as 3 amps slow blow - it was increased from 1 amp to 3 amps because of the higher than usual current draw for the doodle bug magnet.
It's probably safe to presume a decision was made to ensure all other playfield solenoids were still fused with a 1 amp slow blow and that the doodle bug magnet should have it's own dedicated 3 amp slow blow fuse.

Is that Little Demon still being a Little Devil every now and then?

My guess would be no.
Medusa which came out a few months after FireBall II also had that same post between the flippers (on Medusa it's called the Shield of Gods post). Medusa has red flipper shaped inserts that are lit under transparent red flipper bats. So Bally was playing with lights in that area but they didn't light the post between the flippers.

In that case try putting a 0.047uF capacitor across that little demon button switch in the cabinet.
BTW does your switch have a diode on it? The cabinet schematic doesn't show one.

Bally Parts Catalogs are downloadable from PinWiki:
https://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Parts_Catalogs

Eg: you'll find the flipper buttons in the 1980-1 catalog electronic PDF page 33 (paper page 63).

The four lamps around the doodle bug area are 12 volt lamps connected in series to the 43V solenoid power. What you're seeing is factory and was done for higher intensity flashing. So if you're replacing those lamps, make sure you use 12V units.

See schematic here:
https://pinside.com/pinball/forum/topic/fireball-ii-club-members-only/page/8#post-5039003

Nope.
It's a 100 ohm 5 watt wirewound pot.
https://www.jameco.com/z/5W-PW-24H-20S-B100-Jameco-Valuepro-24mm-Linear-Taper-Wirewound-Potentiometer-100-Ohm-20-5W-1-4-D-Slotted-Round-Shaft-20mm-L_140514.html

Nice work.
You can open the original pot and give it a clean. There are four tabs holding it closed that you bend up a little so it disassembles.
Give all the swiping arms a little bend so they have more tension and clean the original gummed up grease you'll see.

Either cleaner will do. Once it's clean the pot will return to good service. Just remember to give the swiping arms a little more tension too.

It might have been a white lithium grease that turned brown from dust/dirt.
I don't bother replacing it, that way no gunk can build up again. This is a pot you usually set and forget so lubrication isn't important.