My Xenon blew the PF fuse before and it was a shorted diode on the drop target reset coil. Sometimes these just blow from excessive playing. Also I have noticed the fuse clips on these become weak. Loose connection equals extra heat. I always replace them when doing a swap. If you're not getting 0 ohms on any coils the problem is probably something else. You should try to figure out which coil you heard kick on.

Are all the mechanical parts moving freely? No binding?

Not sure what my next step is. Do I pull the solenoid board and check for a shorted transistor? Would that blow the fuse at startup? Could the board be at fault at all?

Do I desolder one lug from each coil, then resolder one at a time until I find the culprit?

Something else I'm not thinking of?

You might have a bad solenoid diode.
1) Unplug the drive connectors.
2) All coils ref. yellow wire.
2a) Unsolder the one driver wire to each coil.
3) test
3a)Does 1amp fuse blow?
No, plug drive connectors back in
4)test
4a) Does 1amp fuse blow?
No,
5) Carefully retouch return wire back to each solenoid
-- one at a time.
5a) Does 1amp fuse blow?
5b) Put game in solenoid test, hold one return wire to each solenoid
5c) Does solenoid fire correctly?
5d) If so, resolder wire back to good solenoid.
6) Repeat process for each coil, till bad solenoid is isolated.
7) I had to do this process on a Bally Eight Ball. One thumper bumper had a bad diode.
7a) Replaced bad diode and repaired bad transistor on driver board.

You have the two connectors on the left of SDB. and the bottom right connector.
The two right side connectors stay in place.
1) once disconnected test cabinet/playfield coils.
1a) if you are good at reading schematics... then
b) do the bottom left connector solenoids, if all good, then, test
c) the top left connector associated solenoids... if all good, then
d) do the bottom right connector... this should have most of the playfield coils.

If a coil fired before the fuse blew, put in a new fuse, lower playfield, then briefly power up to see which coil fires. Then look at the manual to see which transistor on the SDB drives that coil and test the transistor.

https://www.pinwiki.com/wiki/index.php/Bally/Stern#Testing_and_Replacing_Transistors

BigAl56 and vec-tor are referring to the same thing. Disconnect the connectors from the solenoid driver board (SDB) that run to the playfield coils to prevent the coils from activating at power up. These connectors are J1 (upper left of the SDB), J2 (lower left of the SDB) and J5 (right bottom of the SDB).

The four coils you've noticed that locked on are controlled by the following SDB driver transistors:
Q9 - Right Pop Bumper
Q10 - Middle Pop Bumper
Q11 - Left Slingshot
Q13 - Left side Saucer

These driver transistors are all pre-driven by the U3 transistor array chip on the SDB.
You have to determine if the driver transistors are shorted or the U3 transistor array is faulty.
Disconnect J1, J2 and J5 from the SDB then power up.
Below are good readings for the U3/Q13 circuit. Note you will not get the 43V on the metal tab of Q13 since the coil is disconnected but the other voltage values you measure will tell more of the story particularly if U3 pin 11 measures a few volts. You're only concerned with the "off" voltage readings.

SDB_Coil_Drive_Voltages_Q13.png

Set your multi-meter to DC voltage. If your multi-meter isn't auto-ranging set it to the 20V scale.
In most cases when you're measuring voltage you hookup the black meter probe to ground. You can use the "GND" test point on the solenoid driver board but I prefer to sit the probe tip tightly under the ground braid running along the bottom of the head to free up a hand.
With the red meter probe, touch the points on the solenoid driver board referenced in the chart above. Note down the voltage readings and come back to us.

SDB_Q13 (resized).jpg

Heh, I only marked pins 1, 8, 9, 16 on U3 so you could see how the chip pins are numbers not expecting you to take readings on them. But good you did.
So the voltage reading you got on pin 12 is bad related to the side saucer, and the voltage on pin 8 is also bad regarding the middle pop bumper. These are U3 faults.

If you feel like more measuring:
pins 2 and 3 of U3, relate to the right pop bumper
pins 4 and 6 of U3, relate to the left slingshot
pins 7 and 8 of U3, relate to the middle pop bumper (pin 8 already measured far out of spec so is bad)

These will tell us if U3 is responsible for causing the other said coils to lockon at powerup.

These are the readings you should normally get on U3 when the momentary coils are inactive:

Pins 3, 6, 8, 10, 11, 13, 16 should all read around 0.9 volts.
Pins 2, 4, 7, 9, 12, 14, 1 should all read around 0.3 volts.

The results are out of spec causing those coils to lock on. Pins 3, 6 and 8 are sort of inputs that must never go above 0.9 volts. Your readings exceed it and thus U3 is a goner.

It's a CA3081 transistor array, basically it has seven simple transistors inside it. The schematic shows them as separate transistors.
Order a replacement and swap it.

What's the chance the little 'm' came on the meter display indicating the reading was actually 300mV (milli-Volts)?

Side Note:
Bally and Stern owners are entering a new area of the RCA CA3081 chips failing.
I got stuck trying to trouble shoot a Stern game from this area...
Long story short, a second coil fried... traced it back to one CA3081.

https://nvram.weebly.com/transistors.html

Andrew sells replacement boards to be able to use discrete transistors if you can't get 3081's. Great plains electronics shows the 3081 out of stock but that might be my browser.

That Marco socket is suitable, no harm in adding one.

The typical failure is the base of an internal transistor goes open circuit. Three of which happened here.

Carefully measure the voltages on pins 9 and 10 of U3 and report back. There's no solder whisker shorts on those socket pins?

This suggests there might be an open circuit to pin 10. Possible trace damage at the pad connecting to the socket pin.
Pin 10 is connected to one leg of resistor R69 - I don't have a board in front of me to check which leg but measure if you have zero ohms continuity from pin 10 to either R69 leg.

If you don't have any luck, there's a somewhat unused one that connects to test point TP7 which you can borrow without any ill effect. It's the one at R72.

Correct.

I measured them long ago when I made that table above helping someone with a flipper relay issue. But the voltages can be somewhat deduced when you understand how the components work.

Diodes will generally have between 0.6 - 0.8 volts drop across them when conducting. Same for the voltage across "base" and "emitter" pins on a simple bipolar transistor when used in switching circuits like this - in this case they go as high as 0.9 volts on U3.

The output driver transistors in these circuits is actually a "darlington" transistor which contains two internal transistors that result in high current gain amplification. The Bally schematics do not properly indicate these transistors - what's circled in red below is what the driver transistor actually contains inside it. Because there are two internal transistors in series, the max base to emitter voltage doubles to around 1.2 - 1.4 volts when it needs to be switched on hard.
When you understand these diode/transistor voltage drops, you can then map out the expected voltages in the circuit.
When the transistor is turned on hard the voltage between collector and emitter drops to around 0.3 volts as it's "switched on". when you take away the voltage from the base to emitter, the collector essentially becomes an open circuit as the transistor switches off.

SDB_Flipper_Schem.png

It's not always going to be the case that you measure a resistor in circuit and it reads its true value. This is because circuitry around the resistor has its own resistance in parallel which reduces the measurement.
So if you measure a resistor and it's lower than marked it's likely (not always) surrounding circuitry is affecting the reading. However if you read a resistor and it measures higher than marked, then you need to look more seriously at the resistor as being open circuit/out of spec.
When resistors go out of spec there is usually signs of thermal fatigue (looking burnt).

If you're getting 480 Ω with the resistor isolated on the bench then your multi-meter is likely faulty. Note when measuring resistors make sure your fingers aren't touching the meter probes; your body will affect the resistance reading more so with higher value resistors and how moist (conductive) your fingers are.

It's a bit high, maybe the black meter lead didn't have good connectivity on the ground point.