GW Instek GPS-3303 PSU Curious Faults...

[danoki]

Hi,

I recently scored a couple of GW Instek GPS-3303 PSUs from the repairs bin to see if I could get one/or both working (been after a split rail PSU for home hobbying for years).

I did scour the google and this forum, but didn't turn up much for the particular faults I'm seeing...

NB: I do have the user manual (sadly no service manual is available) and I will have a go at the user manual tune up later.

The one I'm looking at right now seems generally OK. Opened it up and saw no obvious damage and it had no warm or burnt smells.

Curious fault 1:
CH2 reports a high voltage when the outputs are disabled (seems to ramp down a bit after a while like a cap is discharging when the PSU is turned on at the mains); for example when in independent mode, CH2 reports 70V+; but when you enable the outputs, all is fine and the voltages report correctly.

Curious fault 2:
When in "series" mode and the outputs enabled, CH2 seems to be at least 2V higher than CH1 (verified this with external DMMs). Of course I can run independent mode and wire the oututs together manually to get a split rail supply; but that's not the point is it? 

So why there are "livable" faults; anyone seen this sort of thing before or have any good ideas?

Thanks in advance.

danoki

[ArthurDent]

Where the power supply has 2 identical channels I would check for voltage differences at corresponding points between the 2. If you have the outputs turned off and there is a high voltage indicated on the meter, higher than the max output , I'd suspect a pass transistor being leaky. It would be like a high value resistor that allows voltage to pass up to the value of the raw supply across the main capacitor for that side.