Inside mine - a part 3?, 3rd board timed, upright -there is a volume control board which has pots for each individual sound effect, eg ufo, base hit, invader hit etc plus one for the master vol. I found a fair bit of info and schematics in web land, but was always a bit confusing as to which model was which. Let me know if you can't find anything useful and I'll have a troll through my info. Sadly my machine sits not working, but I grabbed the info thinking one day ....
