Here is what looks to be a very handy guide on how these boards work .

http://www.techdose.com/projects/Stern-Pinball-SB100-Sound-Board/346/page1.html.

From page 4 of this link

"Test points TP2, TP3 and TP5 should be roughly 2.6v DC.. this is because the multivibrator output at these test points is a 50% duty cycle.  If your multimeter can measure frequency (Hz) you could also check the frequency here, but you would need to have an idea of what frequency to expect and they will vary depending on the game so easier to measure for 2.6v.  If the test points measure very low (say 100mv or something far less than 2.6v) then it's likely the LM324 IC or 4013 for that particular multivibrator circuit.  These test points should always read around 2.6v as long as the sound board is receiving power, regardless of whether you've started a game or not."

But , the above suggests the LM324 (U17) but you have already replaced that .

The symptoms you describe sound like only 1 of the 3 multivibrator circuits is working .
I am guessing that maybe 2 of the 3 4013 IC's are bad ??
 Really hard to see the diagram on the 1st page ? of the link but it looks like U8 and U9 ?? could be suspect ? 


I also notice in this link he mentions the frequency can be adjusted via the pot in each of the multivibrator circuits.

Actually handy for me to find this link because I recently acquired a Playboy which I think uses the same board .

Anyway, good luck with sorting it out