- 2025
- Aug
- 13
A Workman Electronic Products N71-067 Power Supply
I picked this up at the Columbus Hamfest, mostly for the 1970s vibe it has. I love that round meter.
The inside is what you’d expect from this kind of device.
It looks to be a standard (reference) LM723 circuit with a series bypass transistor to step the current up. That’s also expected, it’s not like there was much else at the time. Note the 1977 date code sticker and the bread tie for the wires!
I can’t find a lot on this company. There’s one of the same name, but they are (were?) in Delta, Ohio - not Florida. Regardless, their website is broken and has a last date of 2004. There are a few other references here and there, but nothing concrete. I have to assume that this variant went away after the CB era ended.
Does it work?
No.
It’s supposed to be 13.8VDC, not 26. That’s probably the max that the device can put out, and the pilot lamp is just screaming here.
So what’s wrong?
The big transistor is my first check. It’s somewhat shorted B-E, but it has a resistor across those leads. The only way to actually check it is to pull it out - which will need to happen if the board is accessed, seeing as how the heatsink covers traces. The transistor checks fine once out of the circuit.
There’s a small buffer transistor as a driver for the series pass transistor. This one doesn’t seem good. But this one is right across the LM723, so there’s no way to tell which one is bad without pulling one out. The transistor is easiest, so we’ll do that. But, if the transistor is bad, the LM723 might be bad as well. I’m just going to order both parts and replace them both if one or the other is bad. I’m not as worried about the 2N3055 because those could handle a lot of abuse.
It’s actually surprisingly difficult to find DIP parts these days, so I turned to an old friend - Jameco. They still stock this kind of stuff, and if you order from someone else they’ll probably ship from Jameco. Check them out here:
Jameco Electronics https://www.jameco.com/
I ordered some replacements for all the semiconductors in this device at the same time, so we should be good.
As soon as they arrive, I’ll pull some easy-to-reach parts and see who is bad. I’m going to socket the new LM723 as well.
Stay tuned!
Next part of this series: https://wereboar.com … r-supply-the-repair/