This must be the most complicated on/off switch ever devised! I think I understand this circuit some now. In order for the AC current to be able to flow through the triac, the resistance needs to drop in the photo-optic isolator, and this only happens when the light is on. The light is controlled by the flip-flop, in which it is only possible for one one transistor to be conducting at a time. When current flows through one transistor the other one is cut off. The low side of the light is connected to the collector of one of the transistors. When that transistor conducts, the collector voltage drops and current flows through the light. Which transistor is conducting changes every time there is a triggering pulse. Now now how that 2 transistor triggering circuit works still confuses me, but it doesn't matter because I got it working now.
The problem really was just loose transistors. The channel down not working I fixed with touching up the alignment. (Why it all of a sudden needed realignment I don't know) And I just pulled that random extra electrolytic, I have no idea what it was doing in there.
The 2 further problems I still have are that the volume control now seems to have only 2 steps and not 4. And while I could get the convergence nearly right-on, the I could not get purity perfect. I think I can put this down to the fact that so much of the plastic of the purity ring unit disintegrated when I pulled it off the old CRT, so that when I tighten it around the new CRT neck, it isn't centered. But I have a new purity ring / blue lateral magnet unit on the way. I'll look into the volume control problem when I get to redoing the purity and convergence adjustments with the new unit in there (which might not be until summer).
Putting the CRT in was majorly difficult. There's this sort-of metal envelope which surrounds the back of the CRT and holds it in. But neither attaching that to the CRT first and then putting it in the cabinet, nor attaching it to the cabinet and then trying to slip the CRT in will work.
This is the procedure that eventually worked for me:
There's 2 straps which screw together which clasp the CRT in the metal envelope. Undo these. Attach the metal unit just on the bottom loosely to the cabinet. Then slip the CRT inside. (being very careful not to scratch the front of the CRT on the back edges of the thing - it's harder than it sounds) It won't go all the way in, at most I could get it about 1" from the front. Then attach the top of the metal unit to the cabinet, loosely. Now comes the hard part - you have to pull those straps close enough together that they will sort of lock in place. This took me several hours and I cut up my fingers on the straps doing it. Once this is done attach the metal CRT holder to the cabinet completely, and then CRT will be pushed all the way to the front by tightening the straps.
Is there an easier way to do this? There must be!
Thanks for the help on this. And a pic of it working...