Wonky BK Precision 1743B power supply

[Debunque]

Hello - question from a newby here.

I have a BK Precision  1743B power supply (CC and CV, 0-35V, 0-6.5A) that mostly works, but has glitches. This is an analog supply that makes loud clicks, suggesting it uses a relay to switch between transformer coils.

First problem: it takes a while to stabilize after turning on.  The voltage and current dance around.

Second problem: Now it won't to go the top of the 35V voltage range.  Turning the coarse and fine voltage adjustments all the way up takes it to about 25V, and then it crawls up slowly.  Because of the crawl-up (now at 28V) this suggests that it isn't a problem of the final transformer coil not being activated.

The voltage and current readings are basically correct, agreeing with a BK multimeter to a couple of percent.

The unit is a bit rough with the casing a somewhat rusty from humidity.  So a bad contact is a possibility.

So my question is ... is this this behavior (lack of stability, can't reach highest V) characteristic of some known problem, like a bad electrolytic cap?   If it's just a cap, it's worth repairing.  If it involves more digging around, then maybe a new supply is in order.

Jon


edit - there's one big old-fashioned electrolytic cap rated at 50V that is at 42V when running, but dips down to 20V at heavy current draw.  The other caps are small.

[mojoe]

I don't know the history on your unit, but if it is rusty, then I'd certainly open it up and poke around inside. First, I'd spray some DeOxit on the adjustment pots. If you don't find any physical damage inside, do at least check the filter caps (discharge them first). If none of that works, then a little more work will be required. See if you can find a schematic.

The fact that it mostly works is reason to keep it. New linear bench supplies are expensive.

[Debunque]

So there's only one big cap as I noted in my edit.   It seems OK.  It sparks violently when shorted after turning the unit off.

Yes, linear PSs are expensive, especially at high amps.  This one goes for $475 new.  However, Amazon is selling the smaller 30V,3A version at a discount for just $113.  So I'd give up half the amperage, but replace a very rough unit with shiny new unit at a decent price. 

I thought it might be the coarse voltage pot, but I ran it back and forth, and it didn't get better.  The lower end of the range seems a little above zero, which suggests the pot could be the problem (too much R, too little V).   Arguing against the pot hypothesis is the fact that the problem partly fixes itself slowly -
I turned it on again, and it is at 16 volts at the highest setting, and it will eventually crawl to 30 volts by itself.

I wonder if self-generated heat could be fixing it.