Bench power supply. Bin or repair

[David Hess]

I picked up a power supply on e-bay. Its a Powerline electronics 30V/10A. It looks ancient.

The older it is the better it is, because it will be easier to fix.

[james_s]

That looks like a real nice beefy power supply, if you don't want it then sell or give it to someone who does. In working condition that's worth a couple hundred bucks or so. As-is it's worth whatever someone will pay for it.

[Andy Watson]

That looks like a real nice beefy power supply, if you don't want it then sell or give it to someone who does. In working condition that's worth a couple hundred bucks or so. As-is it's worth whatever someone will pay for it.

+1 for getting it working again.

[AlcidePiR2]

These linear power supplies are usually easy to fix. One of the most common problem is dried electrolytic capacitor. But do not rush to change them all. Some may still be in good shape, especially the large ones.

You should be able to find a schematics for a similar one. Is there any opamp ?

I suppose yes, at least IC102 and the one above. Please make a detailed picture of this part to see the markings of the opamps (or supposed).

[AlcidePiR2]

A simple google search gives this thread with schematics and parts of LAB 521 which should be quite close.

   https://www.vintage-radio.net/forum/showthread.php?p=827131

Opamps are probably also 741.

[m3vuv]

is there a 731 lurking in there or just opamps?

[robert.rozee]

It appears to work above 4V, but dropping the voltage below this results in zero current.
Is there anything I should be looking at as a quick fix, or is it destined for the bin?

1. the four power transistors bolted to the large heatsink at the back: carefully remove the nuts and bolts (two per transistor) and clean the rust off all nuts, bolts, AND the transistor casings where the bolts make contact. the transistor cases are generally the collector, and any bad connection here will cause mis-behaviour. you should be able to do this without any need for desoldering. make sure not to leave out any insulators (on the bolts) you remove in the process.

2. test. if still faulty, replace the two large electrolytic capacitors. as a temporary test, you can just solder (smaller) replacements across them.

3. if still faulty, replace other electrolytic capacitors.


the unit is well worth repairing. even if the electronics was fried beyond repair, it would still be worthwhile rebuilding the power supply.


cheers,
rob   :-)

[CharlotteSwiss]

throw away? Don't do it, but if you decide to do it leave the gps coordinates of where you will leave it 

[Wallace Gasiewicz]

If it works and delivers a current to a load over 4 volts (meaning you can power something with it), it basically works, meaning the big caps, transformer and rectifier diodes work.
Since it does not work under 4 volts, the control components are in question. The big potentiometers could just be dirty or otherwise wonky, as previously suggested, or maybe one of the big control transistors (even just rusty screws as mentioned), or some other little cheap component.

[JacquesBBB]

I agree with Wallace.

If its working over 4V,  the power transistors are OK. You can clean them, but it will only be cosmetic.


I have fixed dozens of   such linear  PSU. Most often, the power transistor are OK.  When they are fried, there is no output.
Most often also, the big capacitors that filter the input  line are OK, or do not affect the functioning of the unit.

The problems comes with the small electrolytics capacitors  associated to the opamps  that cannot then regulate properly the  output.  In some cases the opamp could be damaged. But
first  verify the capacitors.

For units that stayed for a long time on the shelf the potentiometers can pose problem. In some cases ,just turning them a few time fully CW, CCW will fix the problem.

There may also be a pot on the board that can be adjusted to change the minimal voltage of the unit.  There seems to be at least one in your pictures.

[metertech58761]

If those are 4-40 or 6-32 nuts / bolts, those should be cheap enough to pick up a couple dozen or so at the hardware store.

If you know how to run an ohmmeter, I'd also start by checking the continuity of every bolted connection and correct everything that has a high-resistance connection.

That may go a long way to restoring full functionality.

[wasyoungonce]

It looks a bit crusty on the pwr transistors but easy enough cct to fix.    Please fix it!  It will serve you well and the next person for life easily.   Good linear PSUs.....are an absolute must for any good bench repairer.

Looks to be some commonality between their PSUs
https://www.vintage-radio.net/forum/showthread.php?p=827131

edit: beaten in link
2N3055 pwr transistors, LM723 op amps basic diodes.......easy enough and cheap enough to repair by replace.