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.
Saw someone at the TARC swap meet with one of these little Tektronix 222 DSOs for sale, so I bought it. Terrific timing since my Hitachi oscilloscope died a while back and I still haven’t gotten around to looking into it.
It’s little, and doesn’t have a lot of capabilities but I think it will do pretty much everything I need (at the moment). It comes with two scope probes that are permanently attached. It’s designed to be powered with an 8V SLA battery, but it looks like batteries with the right form factor and voltage aren’t readily available anymore. I did find a promising looking battery replacement project though. This one didn’t come with a battery, but a standard sized power jack lets you power it with wall power. It’s even got a RS-232 serial port!
The scope seems simple enough to figure out how to use without a manual, and I was even able to find a service manual online to download.
Here I’ve got one of the channels measuring the output from one of my EtherkitSi5351 breakout boards. I don’t remember what I have my RedBoard making the Si5351 do, but I seem to be getting a pretty good signal out of it.
One of my acquisitions from today’s TARC swap meet was a variable power supply. The person I bought it from acquired a bunch of these from a school district surplus auction. I’ve been wanting to get a variable power supply for the workbench, so I bought one of them for $20. Seemed like a pretty good deal. Almost grabbed a second one from him.
Banana jacks provide AC and DC outputs, and voltage for both is controlled by the knob on the left. Two meters show DC volts and amps, but if you’re using the AC output, you’ll need to measure it yourself.
The panel indicates the power supply will do 0-20 VDC and 0-25 VAC. With no load, the power supply topped out at 35 VDC and 26 VAC. This is an unregulated power supply, so any load is going to bring the voltage down.
Getting inside the power supply requires removing a total of 18 screws (6 on each side, 6 on the top). Seems a bit excessive to me, but I didn’t design the thing. Once the screws are out, removing the top exposes the innards.
Not much to it inside. There’s a big beefy transformer which accounts for almost all of the power supply’s weight. Voltage control is performed by the variac. A large (and loud) 120mm fan (lower right) provides cooling.
The key just turns a metal plate that flips the actual power switch on or off.
The cooling fan gets its own power supply so that it’s not affected by changing the output voltage. It’s just a 12V wall wart that gets powered from the AC input I think (haven’t traced any of the wires to see what goes where).
Plenty of modification potential with this power supply. I’ve got a small list of easy ones that I think I’ll make:
Replace the key with a regular switch
Switch out the banana plugs for 5-way binding posts and Powerpole connectors
Replace the fan with a quieter one
This seems to be a pretty sturdy power supply designed for the harsh environment of a high school lab. Everything inside looks to be in pretty good condition.