My dead oscilloscope is still dead. After spending some time poking around the innards, I’m starting to think the issue is on the high voltage side that provides power to the CRT. I’ve checked the voltages at connectors where the PCB is labeled with voltages, and those match up. It seems like the rest of the scope is probably working except for the CRT side of things.
I think at the moment, the scope is beyond my ability to fix. Guess I’ll shelve it for now and maybe go back to it when I’ve gained a few more skill levels and higher level items.
Undoing four screws on the sides of the scope let me slide the top cover off to reveal the innards.
The top circuit board looks to be the power supply board, and probably a few other things. Undoing a bunch of screws holding the top board down and a bit of fiddling around (discovered the board is on a hinge) let me lift it up to reveal more of the scope’s innards.
Three screws hold the cover of the power supply section.
A few dust bunnies inside, but overall everything looked to be in fairly decent condition (aside from not working).
First look around the inside didn’t reveal anything obviously wrong. No blown caps or scorched areas. Whether this is something I’ll be able to repair or not is still up in the air. This scope is probably going to be spending a while on the bench.
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.
Finally got around to replacing the old corroded soldered-in D cell in the RCA Voltohmyst with a new battery today.
Snipped the wires off the battery and into the trash it went. Had to drill out a rivet to remove the battery holder clip.
The battery holder is a little on the big side and just fits into the space vacated by the old battery and clip. Soldered the wires onto the battery holder and fastened it to the Voltohmyst using some double sided tape.
Now it’s got a new battery that can be replaced whenever it’s needed.
Next thing to do is go through the manual and read up on how to calibrate the meter.