Well, you solved your problem already in the post, as far as repair is concerned. Draw your own schematic diagram, based on what you have in front of you, and post it here. You say you have the ability to do that, so do it, as odds are it's unlikely you'll find the exact shematic you're looking for. Maybe it's not such a good idea to go plugging headphones into jacks before you know what is going on in the amp. Are you sure that it is a headphone jack? As far as making a plexi, that's a whole 'nother ballgame. Use the search function, there are several threads here on the topic of converting organ amps.