Funny, I'm waiting for a reply on about the same issue...this one is just a bit healthier than yours. I need to get the meter on the test points and look into the cap values, etc, but I'm pretty fresh to DMM and such.

Wish I could help, but thankful your post is here, too. Best of luck. I'll be researching other's posts until someone has some sorta guess as to which components go first, most often or makes things "slow" or intermittent...

I don't know how similar/different the -50 sound board is compared to the -51 but this site helped me to track down which chip went bad in my HG. http://www.pinball4you.ch/okaegi/rep_soundold.html#programmable1

If that doesn't help and no one assists, barakandl may be able to repair it or he has a new repro https://pinside.com/pinball/market/classifieds/ad/46360

Eric

They are not really worth the time for me to fix the originals anymore now that I am selling repros.

Since you get some sound out i'd assume the CPU is running at least somewhat and the amplifier stuff is OK

What does the button on the -51 sound board do? Does it play a noise? You can push the sound board NMI button and then watch the connections between the PIA and Sound generator for activity. The chirper in my logic probe will play the sound fairly clearly on some of the sound generator pins. I always do that to see if the sound board computer is running OK.

Next up I would look at the input signals....

The MPU sends four or five signals to the sound board. These inputs are buffered and inverted by the 4049 chip. Make sure all of them work (use sound test). Watch with probe input connector, through the 4049, and to the sound generator.

If you have digital sounding faults when pushing the NMI button, swap the ram, pia, rom, sound gen one at time with known good to see if there is a change.

These boards are pretty straight forward. Assuming the connectors and caps are ok. The magic is all done by the four big chips, not a lot else to go wrong. If the 4011 was bad no sound (no reset, no rom cs). If the sound is really soft and can't be adjusted, lm3900. No sound but computer is working, introduce hum into the main amp... if nothing, replace tda2003

Is one, (newbie,) chasing a dragon of sorts to think he could get the tools together to diagnose/fix these things?

Assuming there is a lot of reading, investing and time in general, I'm wondering what kind of hell I'm up against if I stay frugal and use this as a learning opportunity.

(The board in question works, just gets "interrupted" when multiple sounds at once, etc. Thumping is slow to return.)