Next was to solve the sound board issue. The fuse serving the sound board would blow on power up, so I replaced the bridge just in case ($3.00 part, and given the age of the originals, just worth the chance it was the issue. No good though, fuse was still blowing.

My electronics knowledge is weak, so it looked like sending this off to the experts. But I noticed a lot of continuity between the pins on the sound board amp, so something was amiss. This part is virtually unobtainable, and there's been a suggested mod to make an on board replacement using a modern amp. This was previously done to another of my system 80 boards, and while it works, it is a little ugly.