Do you have acid damage on your mpu? Are the connectors tight or can they be pulled off easily?
I usually have to repin all bally connectors.

Looks like it depends what MPU roms they used.

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Does this 3rd sound board have new caps? Low volume points to failing caps, or bad pot or amplifier.
AS-2518-51 – Space Invaders, Nitro, Silverball, Rolling Stones, Mystic, Hotdoggin, Viking, Skateball, Frontier, Speakeasy, BMX, Gold Ball
• 2 ea. 1uF 50V Axial Electrolytic Capacitor (C5, C13)
• 2 ea. 470uF 25V Axial Electrolytic Capacitor (C7 & C8)
• 1 ea. 4700uf 25V Axial Electrolytic Capacitor (C15)
• 1 ea. 1k ohm volume pot (RT1)
• 1 ea. TDA2002 or TDA2003 – power amplifier (U9)

Caps
https://www.greatplainselectronics.com/proddetail.asp?prod=BALLY-S51E-KIT
Volume pot
https://www.greatplainselectronics.com/proddetail.asp?prod=3386P-1-102
Amplifier
https://www.greatplainselectronics.com/proddetail.asp?prod=TDA2003AV

You'll have to email Ed to see if he's got the parts. Otherwise, http://www.taydaelectronics.com/ may have everything, except the caps won't be axial; they mostly have radial.

Jerry Clause
Gameroom Service
54230 Bradshaw
New Baltimore, MI 48047
[email protected]
586-725-8032

not sure who you had do your board work before, but this is the guy I use for issues beyond my ability.
I don't like to mess with sound boards beyond capacitors, pots and roms.
Jerry is very reasonable and does excellent work.

Maybe send him your 2 faulty boards and keep your quiet one for now.