U1 should be empty. How many beeps at startup?

According to the manual, U19 is the background rom. So when you put the sound board in earthshaker you got sounds and speech, but no background music?
How do you know the ribbon cable is good?

@uncivil_engineer If you're referring to the audio amp in the upper left corner, i think that's only for system 11, not 11A-C

This was a good Sys 11 BG music thread: https://pinside.com/pinball/forum/topic/williams-system-11-missing-background-music-investigation/page/2

Manual link: https://www.ipdb.org/files/1597/Williams_1987_Millionaire_Manual.pdf

whats the story on this resistor network? looks like it was replaced maybe? Are there any shorts between the legs?
How about under the battery holder, any corrosion?
any bad joints on the ribbon cable pins?

you don't need U1, it is the amplifier for 1J15, which you don't have. All your sound is amplified on the sound board. there are pictures on pinwiki of an 11a (not 11A) showing the populated U1 area, 3 LED rather than 7 segment LED diagnositc, all the same as TNT 215. You'll also see that the speakers are getting amplified by the soundboard on both 215 and 264.

http://pinwiki.com/wiki/index.php?title=Williams_System_9_-_11#MPU_Driver_Boards

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Can you see if any of those legs on the resistor network are touching? From the picture, seems like there is a bridge between 9-10 and 7-8 but hard to tell in that picture. Measure resistance between pins 2-9, ex 2-3, 3-4, etc and see if they’re all the same.

Src1 is the pull up resistor for the bus line going to the sound board.

Wow that picture looks a lot different than the first!

How about src3, that’s the other pull up for 1j21. Looks like pin 10 is broke on src2?

I think you would want to get the scope out next and see if you’re getting hi and low on 1j21 3-10, 11-13, 15, 16, and 18. Those are all the pins that get pulled up unless ground is sent from u41 and u42.

You could also check connection between those 1j21 pins and the corresponding legs on u41 and u42, see if the repair or corrosion caused something.

Bussed would be the replacement style if f needed.

That would probably do it!

Awesome! Glad you were able to get it fixed.