[SOLVED] Linear PSU showing 0.3V constantly when switched on or off

[jim_griff]

Hi there,

Pics show my issue. Channel 2 on my linear power supply shows and outputs 0.3V whether the output is switched off or on. Changing the voltage setting or current limit doesn't change the output. It's a fixed 0.3V at all times. It's being switched on from cold after being off for a week.

It had this issue a couple of years ago but somehow resolved itself so I couldn't diagnose it at the time. Now it's doing it again. I'm sure it'll resolve itself again given enough time, but would love to have a permanent fix, not let it "fix" itself again.

Is there anywhere I should look first to track down the issue? I'm familiar with linear PSU design, but not with the control circuitry, so thought someone who's more familiar may be able to tell me exactly where to look first based upon the symptoms.

Cheers, Jim.

UPDATE 1:  Upon touching a single multimeter probe (the other probe isn't connected) to ANY part of channel 2 circuitry, channel 2 will suddenly enable. The relays click as it enables and the set voltage is output normally. Upon touching the single multimeter probe to any part of the circuit again, it switches the relays off and displays/outputs 0.3V fixed no matter what.

So, I've come to the conclusion there's some weird noise somewhere on channel 2 circuitry that's faking the control chip into switching off channel 2, even though channel 2 shouldn't be able to be switched off independently. Still, I have no idea when it comes to digital control circuitry and pull-up/pull-down type stuff, so I'm totally stumped. Might sell it on eBay for anyone who wants to fix it as I'm out of my depth.

UPDATE 2: SOLVED. It was a bad solder joint on the controller board. Fixed now. *crosses fingers and knocks on wood*

[tunk]

Don't have a clue, but I think I would have first checked the potentiometers.

[jim_griff]

Don't have a clue, but I think I would have first checked the potentiometers.

Aye thanks, checked all pots, cable connectors to no avail. Actualy recalibrated it all a few weeks ago when I got my 121GW meter.

It is a very strange issue, especially that it will disappear on its own if I wait long enough. 

[tunk]

Just an uneducated guess, bad solder joint(s)?

[jim_griff]

Just an uneducated guess, bad solder joint(s)?

I've literally just found a bad solder joint now, where the trace itself was uncovered and wasn't fully connected to the solder joint. Pot luck finding it! It was a small trace. Not sure exactly what it went to.

Quick bit of soldering later and it appears to be operating perfectly now.

[Neomys Sapiens]

Those solder joints to the right of that picture look outright evil. In the interest of reliability, I would touch up all the convex ones. Maybe even remove most of the solder and rework with proper quality stuff.