Thomas/LA3PNA suggested I look at the input and reference voltages going into the μA723 regulator, so while I was at it I looked at all the pins in the empty socket. The only unusual thing I found was about 0.1V where the output pin of the regulator went. I’m guessing the voltage regulator probably didn’t like having that much voltage on its output pin, and that’s probably what was killing them.
Pin 10 went to the anonymous red transistor type thing, which in turn was connected to the pass transistors and Vcc.
Consulting with Thomas again, he said it was probably a Darlington transistor or something similar that failed. The μA723 output turns on this transistor, which is then able to provide more current to turn on the pass transistors than the μA723 alone would be able to.
Took the transistor thing out and connected the μA723 output to the base of the pass transistors and everything worked! Got a stable 13.7V at the output of the power supply, and I could turn it off and on again without any problems.
Found a TIP31 power transistor in my collection of parts and put that in place of the dead red transistor and it looks like this power supply is back in business.
It’s a pretty ugly soldering job, but I think it will hold up. Still need to test it under a load though.
Spent some time poking around some more, but it’s been sitting on the table since then. Last week I ordered some μA723 voltage regulators to fix someone’s Astron RS-35M power supply. Since I had a few extras (ordered 10 of them), I popped one into this dead power supply. Fortunately it’s socketed, so replacing it was pretty easy.
Plugged the power supply in, turned it on and much to my surprise, the power supply seemed to be working again! 13.7V at the output and seemed pretty stable.
Thinking everything was good again, I turned the power supply off and unplugged it, put the cover back on, plugged it back in and turned it back on.
Poof, back up to 27A and no voltage.
Went back in, put in a new μA723, turned it on and it was back to 13.7V. Left it running for a few minutes, turned the power supply off, turned it back on a few minutes later and it was back to 27A and no voltage.
Well double crap. So it looks like there is a deeper issue with the power supply that’s causing it to kill the voltage regulator.
Off I went to the last remaining Radio Shack in my area (a franchise store, also known as Hurricane Electronics) to see if I could find some fuses and a replacement lamp. Found some replacement fuses easily enough (35V, 20A), and much to my surprise, replacement bulbs that were the exact same style as what was already on the power supply.
Replacing the light was easy enough, but took a bit longer than expected. The original pair of wires for the light kept breaking when I tried to put the light back into place, so I ended up just replacing the two wires with some 18 gauge stranded wire I had. Once the light was back in place, I plugged the power supply in and on came the light. Yay!
Then I replaced the fuse, turned it back on and was greeted with the meter telling me there was 13ish volts. With my DMM, I read 13.3 V DC at the meter.
I tweaked the pot at the control board to bring it up to 13.7 V DC.
So it looks like the only problem with the power supply was the blown fuse. Now to see how it works with a load applied.
I’ve finished tracing the power supply schematic out to the control board. I’ll go through it once or twice more to make sure I’ve got everything right, but I think I’ve got a fairly complete schematic of the power supply now.
Here’s the schematic so far.
I did run into an anonymous transistor type part with a red case that I wasn’t able to identify. Whatever markings were on it have been rubbed off, so there’s no way to really identify it anymore.
The base of the transistor (I’m assuming it’s the base anyway) is connected to pin 10 of the 723 voltage regulator while the emitter is connected to the bases of the power transistors. The collector goes to pin 12 of the 723 and the collectors of the power transistors. Checking with schematics for my Astron RS-35, there’s a TIP29 transistor in roughly the same spot, so I’m reasonably confident this mystery part is an NPN transistor of some kind.
Tracing out the control board was a bit tricky, but I think I managed to get it right (I think I’m missing the diode on the schematic though). I’ll need to recheck where the wires coming off the control board go in the rest of the power supply.
Quite impressed with the way this power supply was put together. Everything is soldered or screwed together nice and cleanly, and the wires are nicely dressed and bundled together with zip ties and string.
Except for the blown fuse, I don’t see anything obviously wrong with the power supply. I think I will look into replacing that AC input plug though before I try applying power to this thing.
I’ve traced the power supply schematic past the SCR and out to the power transistors that are attached to the big heat sink. This part of the schematic might be a little questionable because a lot of the wires are covered up or obscured by other components and a little harder to follow.
The schematic so far:
I’m up to the control board now. It’s small, but has a lot of wires coming in and out of it. Fortuantely it’s all point to point wiring on perfboard, so I don’t have to follow any PCB traces.