Thurlby 30V 2A Bench Power Supply

[Northy]

Hi all,

As part of building up my home lab I've dug out an old power supply I acquired a long time ago.
It used to work back in the day, but got shelved one day when it stopped working.

There's no output voltage even though both voltage pots and the current limit pot are turned fully clockwise (they are in the picture, just the lines on the knobs don't line up correctly.
After it's been switched on a couple of minutes the current limit 'dots' on the right hand display flash.
Any ideas where to start to fix it?

Thanks,

G

[bd139]

Open it up and look for visual clues first. The things are fairly simple but this looks like an early one so that guarantee might not hold. Find the service manual off the internet and check the reference voltage and the supply rails for the front panel board. Then check the electrolytic capacitors aren’t dead.

Common problems on these are electrolytic capacitors, short circuit diode across output, blown up pass transistors and the CD4052 analogue multiplexer causing problems.

If it’s a very early one it might have a massive wire wound pot for coarse voltage control so check that if it has one as they drop dead.

If you can get a picture of the front panel board so we can ID what version of the supply it is.

[MosherIV]

The wire link on the red terminal to green terminal looks wrong.
It should be black to green if anything.
The flashing dots means it is current limiting.

Mine had problems with the pots. Give the pots a good clean with contact cleaner (NOT  WD40 ! I did that, worked for a while then died again and now really problematic due to the residue  )

[bd139]

Will work fine if it's strapped to earth like that. The supplies are floating. All that does is reference it as a negative supply.

[Northy]

Sorry, I should have included inside pictures.

[bd139]

Ok yeah that's the really early one. Probably a short electrolytic cap on the front panel board or the main voltage pot is knackered. Again start with checking internal rails.

Service manual for this model is on post #2 in this thread: https://www.eevblog.com/forum/repair/old-thurby-pl320-tgp-(sinclair)-dual-channel-lab-power-supply/

Places to test:

IC1 output = ~5.7  +/- 0.2 volts
Z1 negative = 5.6 +/- 0.4 volts
D7/D8 cathodes = usually about 40 volts DC or something.

If any of those change when it drops into current limit, replace all the electrolytic caps on the front panel board.

[Northy]

Brilliant, thank you!
What actual model is mine? I can't actually remember how I managed to acquire it 
Are they usually repairable?
What if it's the big mad looking pot?

G

[bd139]

Yours is early Sinclair / thurlby PL320 - see updated post above.

Sometimes repairable. The later models are much much easier to repair.

Big pot is just a wire wound one which has decent temperature characteristics.

[Northy]

Thanks again for the information.

Sorry if this is a daft question, but what's the best way to get the front board out?
Then should I just leave it dangling on it's wires while testing?

Thanks,

G

[bd139]

Can’t remember if all the pots are soldered to the board or not. Undo the front four screws and see what’s still stuck and undo it. The knobs have caps on and nuts underneath if you need to undo them.

This is one of the early models so it’s a battle of wire length vs patience as to where to prop it all up while debugging. Do what you can but avoid shorts for obvious reasons.