I reviewed the whole thread this evening, since it's gotten rather long. I notice that the regulator 12BH7 was working initially, so the lack of heater voltage now is probably something simple - hopefully, just s single bad connection. Until we get the heater power restored, there is no point in looking for other problems, in fact it's quite likely there aren't any.
I can't tell from the picture whether all four 1N5820 diodes are pointing in the same direction. I'm looking at that because you read 7vAC between power transformer terminals T4 and T5 (or perhaps where they connect to the PSU board, with the same labels). There should be more than 9vAC between those two points. I do not actually think this is the problem, but it's best to be certain. If you still get 7 volts, I'll have to ask if your meter is accurate - test a fresh 9v battery (on DC) to confirm.
There should also be around 9vDC at the forward ends of each of the two diodes that lie nearest the output terminals on the board. I specify both of them in case one is not soldered well - like the large chips, these diodes can absorb enough heat to slow the soldering process.
The terminals on the board labelled 6.3vDC should have 0 volts on the negative terminal and +6.3v on the positive terminal. If the positive voltage is zero, then it may be worth the trouble to attempt to measure the middle leg of the '1085 regulator - be careful there, since it is probably only accessible with a probe, and you don't want to slip and short the pins! Pin 3, which is the one closest to T4/T5, should have 9vDC on it.
I'm afraid those are all the test points that seem to me to be accessible within reason. Make those measurements, in that order, and we should be able to make a good guess as to where the problem is.
