You can't check a fuse by looking at them.
Take them out one by one with the power off and check them with a meter.

Turn the game off. Switch it back on and quickly hold the flipper buttons in. At about the count of 6, you should hear a CLICK from the backbox, indicating the flipper enable relay fired. Do you get that click?

IIRC, the small flipper is transistor controlled like any other playfield solenoid and that pulse is normal (both of mine did it).

Check/rebuild J4 on MPU and J3 and J4 on SDB. Check for burnt traces leading to and from the flipper enable relay on the backside of that board. Check for cracked solder joints on the backside of the board where the relay solders in.

Always check the EASY things first, as many people get ahead of themselves.

After the fuses, check and reseat ALL connections in the backbox.
Early Bally machines of this age can be effected by surface corrosion to the pins and the connector themselves especially if you never upgraded them from the originals.
As another mentioned, cold solder joints are also VERY COMMON, they may need reflowed.
Check the flipper solenoids as well UNDER the playfield, you may have a pawl leaf switch issue that could have caused a "lock on" and a fuse popping, due to "hold" power being left on too long.

You just noted another problem with the sound board with the high pitch (sometimes a "squeel"), which happens due to these causes (connection seating), but sometimes its related to other problems specifically related to component mounted circuit boards, but more later.

Golden rule: If you smell smoke, see burnt traces, or stop popping fuses immediately, TURN IT OFF!

There are adjustment pots on the sound card that you can try adjusting.

General info FWIW.... all non decoded solenoids will give a brief pulse during the associated PIA's power on self test. This includes the flipper relay and any solenoids that may be enabled for longer than a brief one shot. Knowing that you can use it as a quick visual queue as to where to start at.

"Carefully" crank the potentiometer back and forth a few times, sometimes things get a little "sticky" over time.
Settle it out, test on a reset on in the sound test menu.

I find lots of TMS5200 speech chips end up sounding like chipmunks. Specially on the one s where the IC legs corrode out big time... I wonder if that gets up inside the IC package and damages something.... changes resistance/capacitance somewhere.

I was thinking maybe the crystal...

Crystal is for the CPU clock. Speech clock is set by a resistor/cap (r9 i think). You can change the resistor value and get some different speed effect, but it doesnt always get you the results you want.

It's possible you may have a feedback electrical short coming from the play field to the CPU, this can cause all sorts of havoc with both the lamp and switch matrix. You need to start testing voltages

Make sure the fish paper insulator is between the start button and switch blade.

It is a ground modification, but done in a very lazy/sloppy way.

https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing

Start at the rectifier board in the bottom. Check those test points. The solenoid board then regulates some of the rectifier board voltages. If you had a cheap scope you could measure the ripple on the 5V. You could also use the AC setting on your volt meter to see if that large cap is doing its job, its been replace at some point. The high voltage one looks original.

Like your driver board, is it another Stern part rather than a Bally?

That might be more of a hack than a mod. I would bet on that the male/female ground return pin burned up and that is how they fixed it instead of dealing with the connector itself.

The line in fuse is not drawn near the rest of the fuses on the rectifier schematic. it is off to the left and mislabeled as F1 (should be f6)

I always repin all those female connectors. It's a sucky job, but if you want those old Bally games to be totally bulletproof, it has to be done.

Next time the overvoltage light comes on, measure the voltage with your meter.

Embryon should have the cabinet rectifier board with square plugs. Not the rectifier board in the head with single line molex 1.56" plugs.

A considerate thought.

All these problems you reported so far, indicate one thing.
You need to replace and repin ALL you primary female connectors, or you are going to continue to have problems.
Games of this age require this if you want reliability.
Everytime an owner unplugs and replugs a connector, eventually the contacts fail.
You are not solving the problem, just delaying the inevitable.
You cannot troubleshoot a problem with components and voltages in the future, if your connections are bad.
Its going to happen as capacitors dry up, resistors deteriorate, diodes heat burn, etc, etc.