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Fake voltage regulators

Repairing a symmetric regulated power supply

A great find on the sidewalk

I was walking down the sidewalk when I came across an abandoned power supply and a bag full of components in front of an antique store. The dealer told me that the device does not work. One might wonder why he left it on the sidewalk in this case... Anyway, I grabbed the whole.
(note: the pictures below were taken after the fix).

On first inspection, it's a well built DIY creation. Solid folded aluminum housing, all cables tie-wrapped, voltage regulators mounted on generous heat-sinks with all the fittings. The only complain would be on the big capacitors; usually, they are glued on the PCB to avoid humming.

However, we see the project was never completed, since the digital voltmeter (DVM) is not connected internally. Also, withing the components in the bag was another voltage regulator, a LM317K in a TO-3 package.

First test

After a more in-depth inspection (transformer, fuse, wiring, card), I did a first power-up to no load (without regulator); no problem. Then I connected the positive regulation; the potentiometer started to smoke after 3-4 seconds. Immediate stop.

My guess was a short or a component mounted backwards. So I traced and checked the schematics, nothing was wrong except the pins VIn and Adj of positive voltage regulator were swapped.

My suspicion went towards this regulator.

I removed it from the heatsink. The marking looks good:

but the other side is suspicious:
Why are E and B engraved on a voltage regulator? E stands for emitter, B for base, which are the typical pins of a transistor. Then it became clear in my mind the LM317 is fake, actually a recycled transistor. Which explains why there was another one in the bag of components, why the pins were swapped and overall, why the project was never completed.

Temporary fix

For now, I replaced the LM317K by a LM317T in TO-220 package, mounted on the internal heatsink (which I guess was intended to a 7809 to power up the DVM) and everything works fine. I will chase for a real LM317K to complete this project, but I have a nice power working supply for my workbench. Great!

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