GWI PSP-1020 repair

[vintagepc]

Howdy all,

Hoping someone here has some insight into the GW Instek PSP series, specifically a 1020. There seems to be little in the way of service documentation for these, so I'm hoping that this issue's been seen before to save me time reverse engineering the circuitry.

Symptom: Powers up OK, but on switching on the output, current readout goes to max and zero volts at the terminal, akin to a short on the output. No sign of a short using a DMM; resistance across the output rail before the relay is ~4.2K, roughly corresponding to a 4.7k (bleeder?) parked in that location. Here's where I'm at so far in troubleshooting:

-> Nothing looks obviously cooked and no telltale magic smoke smell.
-> problem is on main board; still present if front panel binding post leads removed.
-> Output MOSFET is not shorted, problem still exists even with it removed. The 100uF cap across the output also checks out ok.
-> The head scratcher is the current reading, which suggests the load must be after the sense resistors, which I believe are a pair of 0.30 ohm resistors in parallel on the underside of the PCB. But there is very little here, and they are after the output MOSFET; so removing it would then also make it go away.
-> No thermal camera, or I'd make use of that to see what's getting hot in short order, assuming it is a short.

This would suggest something is wrong with the current sense circuitry, instead of an internal output short. Does that make sense, before I dive into poking around on the digital control board?

[vintagepc]

Update: Bit of a red herring in my original description. The current reading seems to have a constant offset of ~230mA. (240, dropping to 220 as voltage is increased) The unit is a "dumpster find" for having failed its last calibration, so I'm guessing this is not a cal problem or an actual short. Voltage output is fine, so long as I have the current limit > 230 mA

[vintagepc]

Update - I've ID'ed the current sense op-amp so I can start poking around in that area to determine what's up.

But, before I do that, I've noticed that the +/-15v rails in the system are reading at +/-12v.

While this is probably well within what the supply needs to deliver its output voltage range, (20v) , it is a little odd since the rails are marked 15v in a few places and it's probably worth chasing this down prior to any further repair.

I don't see any adjustments in the SMPS though. If someone with this same supply could confirm for me what their rails read the it'll confirm I need to go hunting for a failed component.

Thanks!

[vintagepc]

Did more digging today; getting closer.

The current sense op-amp has ~0.8mV difference between its inputs. This difference does not exist at the current sense resistor it's measuring, it has to be coming from either internally in the op amp (op-amp is damaged) or the feedback loop. Will dig deeper tomorrow to see if I can isolate the origin of the voltage leaking onto the input.

[vintagepc]

Swapped the op-amp and the 4 noted caps from another PSP thread. Now the current offset is ~450mA, so it's almost definitely a cal issue.

Shame there's no published calibration procedure for these supplies. I've found docs on other GWI models which suggests it might be doable over serial as those have explicit commands, but there is nothing in the documentation for this model and it has no internal adjustments.

I did find two hidden button pads behind the front panel but they did not appear to have any immediate effect when pressed. One is SW15, the other was (IIRC) "SHIHT"