With the game powered off and the dmm set to diode test, red lead on ground and black lead probe all the solenoid driver transistor tabs that are used for the sound board to see if any are shorted to ground.

Logic probe test would be to put the game in rolling solenoid test and watch with the probe on the 4050 buffer on the sound board. Normally they will be pulled high but sink low when the driver board transistor is triggered. Watch each toggle from resting high to briefly low when it's turn comes up in solenoid test. You can follow these signals all the way to the PIA.

Yeah that transistor is probably bad. Remove the connector and see if it clears reading short.

Best to use the diode test for that check, but resistance test showed a dead short. Two clicks on dmm counter clockwise.

if you are getting noises when removing plugging in the connector i'd have to think one of the inputs is pulled low otherwise the sound board would not respond.

So you isolated the problem to the mpu/driver and not the sound board right?

You found a shorted solenoid on your driver board up above. Perhaps that solenoid is still locked on because the predriver or whatever is damaged / locked. There are certain input patterns that call for silence... perhaps your shorted input is the one that silences the board????

need a logic probe before i can really tell you to go on. I think you need rolling solenoid test mode and monitor every input signal (with a wire attached on SB) and follow it through the 4050 buffers to the PIA. The inputs should be pulled high and then sink low briefly during its turn in solenoid test. See if one is permanently low.

use dc volts but the fact you got a different reading on q39 probably means it is turned on. Is that the same transistor you found shorted and replaced up above? Perhaps the circuitry behind that transistor is bad and locking it on.

Pin one is bottom left and counts counter clockwise to top left.

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The long edge of the triangles are the inputs to the 4050 buffers. When the game is in rolling solenoid test the input pins should be sitting high (5.0v) and then sink low for a moment during their turn during solenoid test.

The point of the triangle is the buffer output. Since this is a non inverting buffer with no enables, the output will match the input always

Q39 is stuck turned on or has a collector to emitter short.

B side ports on a 6821 only drive to TTL high, I don't think there are any pullups on the pia ports. But still I am surprised the blanking (ic4 pin 9) is that low tho and that the 7408 output is only driving 2.2v. I think the blanking should sit closer to 5v... maybe it getting dragged down somewhere.... blanking can be a a pain because it goes to so many places.

This is where a scope or logic probe is nice... is it 2.2v because P8 is oscillating and that is the avg voltage or is it a flat line 2.2v because of a blown chip.

Guessing the PIA or that 7408 is bad.... i would probably do the 7408 since you had a shorted transistor.

And again very odd a solenoid transistor for the sound board would be bad... hrmmm .... i wonder if a connector mixup happened at some point in the past.