Power Designs Model 4050 Repair

[cspearow]

I just bought a Power Designs Model 4050 power supply. 0-40 V @ 5 A. It weighs a ton.

The output voltage is fine, but the current limit has a problem. It shows 5 A when it's actually sourcing 150 mA, and it current limits accordingly.

This shouldn't be too hard to figure out, but I can't find a schematic anywhere, for the 4050 or similar units from Power Designs. This seems odd, because it's a pretty popular line of power supplies.

Does anybody have a secret source for Power Designs schematics or service manuals?

-Carl

[mzacharias]

Just a guess - maybe a burned resistor?

[cspearow]

I think I found the problem.

 There is a wirewound power resistor, marked 0.25 ohm. It looks like a 25W tubular ceramic. I'm pretty sure it's the output current sense resistor.

I measured it with a 4-wire meter, and it measured 24 ohms. I removed it from the unit, and then it measured 26 ohms. Then I applied some power to it, and it seems to have changed to 0.5 ohms. Freaky.

There is no physical damage that I can see. I wouldn't think such a failure would be common. It doesn't run hot.

I couldn't find one exactly like that, but I found a different style on Digi-Key, and it's on order. We'll see how it goes.

[SeanB]

Probably has a cracked connection to the internal resistance wire coil. Very common on power resistors after they have had a lot of thermal cycles, though this one with 6W or so of dissipation at most should not have failed, it probably had a manufacturing fault where the crimping operation nearly cut through the wire and this eventually fatigued open, the high resistance being from the last piece still joining the 2 sections of wire.

[cspearow]

The power supply is running now.

The only problem is that the current meter is off. It reads about 3 A when the output current is 5 A.

There are several unmarked trimpots on the board, but frankly, I can't be bothered trying to figure it out with trial-and-error methods, so I closed it up and that's that.

$41 out the door (plus some time)  for a 200W bench power supply. I can live with that.

[SingleBitError]

I recently restored a 4050 that I bought at auction a couple years ago. After having no luck finding a manual online I found one on ebay. Spent more for the manual than the 4050.

I finally got around to scanning the manual to PDF. I uploaded the manual to the KO4BB site. Should be out of quarantine soon.

[bitseeker]

Thanks for uploading that, SingleBitError.  Some models are rather obscure, especially when it comes to documentation.