When you get a chance, post some pictures of the dead board for reference.

The amps are an analog circuit, so voltages could be anywhere between zero volts and the supply voltage.

That right there will cause no sound output. It's effectively disconnected the speaker.

Did you plug U14 back in to see if it was working after resoldering C47?

Hi,
Can you post another video that doesn't have the hum? I can't clearly differentiate the power on noise frequency from the hum frequency and how long the power on noise lasts.

Yes new pin headers and interface cable will help with some of the gremlins.

Are you using the factory Stern MPU board?

Stern didn't mute the amplifiers on the rev C-1 SB100 boards for some strange reason - you might need to install a capacitor to enable the power on mute, but we'll get to that.

The Alltek MPU board is the reason you're getting the power on noise. The reset circuitry on the factory Stern MPU board releases almost instantly allowing the CPU to disable sounds on the SB100 before you can hear them.
The Alltek reset circuitry takes longer to release meaning the CPU can't disable the SB100 soon enough to switch off unpredictable sounds on powerup.

You can observe this reset timing difference in the MPU LED behavior. Factory MPU board LED will quickly flicker on powerup, while the Alltek will seem more like a typical flash rather than a quick flicker on powerup.

How do you work around it? You need to create a delay enabling the amplifier mute circuitry on the sound board.
On the back of the SB100 sound board where the U6 chip is, try soldering a 10uF capacitor between pin 10 and pin 7 or pin 11 (both pins 7 and 11 are ground so choose whichever is easier to solder to). Positive lead of the capacitor goes to pin 10 of U6.

Make sure the J5 pin header on the Bally board is in good shape - they do typically get badly tarnished with dust, etc. since nothing is plugged onto it in Bally games.

Welcome back.

That particular capacitor mod I mentioned will not work on your sound board.

If you have parts, change the 3.3uF sound board capacitor at C56 (near U6) to something like 0.5uF. If this doesn't fix it then go the other way and change it to something larger like 10uF.

I don't want to say 100% certain (it can depend on the particular amplifier chips on your board), but you should be able to get it to be silent until the game starts attract mode and plays the welcome melody. It's a matter of timing with the mute circuit (which the capacitor at C56 controls).