- 2025
- Sep
- 3
A Workman Electronic Products N71-067 Power Supply - The Repair
After poking around in the power supply for a little while, it was apparent there was something going on right around the LM723 and it’s driver transistor. Since one had to be pulled out to test the other, I pulled the driver transistor. It’s shorted C-E, so that’s the problem - the LM723 can’t regulate a voltage when the regulator’s driver is driving the output to the max.
That’s an easy fix, it’s a S8050 transistor. I had to place an order for some other things, so I ordered a batch of transistors.
It was easy enough to install
However, since the LM723 probably was quite stressed from this failure, I decided to replace it as well. I just cut it out and pulled the pins from the solder pads, then removed the solder with wick. That 1970s phenolic board smell! It’s going to get socketed just in case. (Finding these in DIP is getting kind of difficult!) This was a device where they only left copper where it was needed, so it came out easily.
Since the big filter was quite old, I replaced it as well. Here’s a shot of the completed and re-installed board:
And, it works!
So what all was replaced?
The S8050 driver transistor for the series pass regulator.
The LM723 Regulator IC
The main filter capacitor.
I did dial the output back a little to 12.8VDC, just because I want this on my bench for other things.
A relatively easy repair, but there’s not much in this thing to go wrong. Next up is the Heathkit AG-7 that’s been kicking around for a while - it’s going to need more than I thought, as well as a Viz (RCA) power supply with a designed-in fault.
Previous part of this series: https://wereboar.com … /[url]