Things are certainly looking grim for my hopes of completing restoration before July is through. I have replaced every shorted part, and I still don’t understand what’s going wrong. I am letting my Retrochallenge friends down!
The problem really is that I am simply out of my league with the +5V regulator. I can fake my way through digital electronics quite well because much of it feels natural to me. And since everything I do is low frequency, I have the luxury of more or less ignoring capacitance and inductance, and I can pretend that transistors are nothing more than electrical switches. It makes life so easy.
Understanding and repairing this power supply, though, that’s another matter. When it comes to analog electronics, I can understand a basic emitter-follower amplifier alright, but anything more complex than that leaves me scratching my head. Even with the circuit description in the manual (which is quite good) I don’t really “get” it.
So far my understanding of the problem is that the regulator is not switching. It should be generating a sawtooth wave at high frequency, which eventually gets smoothed out to +5V by an LC network. But the switching isn’t happening. I have now replaced every transistor in the circuit, and it is still not switching. I either screwed something up, or one of the other parts (possibly one of the diodes, or one of the zeners) that I THINK is good is not good. I don’t want to go full-on shotgun and replace every component, that’s ridiculous!
Still… I do have a backup plan up my sleeve. I located (and bought) a spare regulator board. I got a very good deal on it, only $45. I consider that a very reasonable price for my sanity. It should be here next week, and I’ll keep it as a spare if I actually DO get this board working. And if I can’t, at least I’ll have a known good board to use in its place.
I’m a little down about it, but at least it’s a learning experience.