GW Instek PSP-405 PSU: no output

[a_giannini]

Hi everybody here, I'm Armando, from Italy.
First of all, please apologise me for my english.

Some years ago I bought an Instek PSP-405 that I really used very few and recently started having problems.
At the beginning the PSU, after some minutes of normal work with a small load, switched off the display and the output voltage by itself.
Switching it phisically off and on again the PSU restarted to work normally until the next self switch off.

Having found anyone available to repair my PSU, I tried by myself but I'm not an electronic expert...
I tried to replace all the capacitors I found on the power, logic and connector PCB (on the front panel) and having still the same issue I then raplaced all the other components I could:
    Q1 e Q2 (2SC2625);
    IC1 (TOP201YAI);
    Q3 e Q4 (C945);
    IC3 (ST431);
    IC5 (L7805);
    Q5(IRFP250N);
    IC4 (TL494CN);
    Q7(C945);
    IC7 (LM393)
    IC8 (LM358N)
    IC10 (TL072CN).

On the logic PCB I also replaced U8 (74HC373N).

Now the PSU switches on correctly and in "Output Off" mode all the settings can be done via control panel.
But when I put the PSU in "Output On" mode:
- if the voltage set is <= 0,15 V the display shows the correct voltage but no voltage is present on the output connectors;
- if the voltage set is > 0,15 V the display always show 0,06 V and no voltage is present on the output connectors;
I can add the K1 relay on the power PCB reacts correctly respect the state of the Output button (checked with a multimeter) and I can feel the D16 e IC5 heat sink is a little warm.

I've not found the schematic diagrams of the PCBs, so I'm ready to give up
I hope this community can give me some advice, before to throw away an almost never used PSU...

Thanks in advance for any suggestion.
Best regards

Armando

[matt6ft9]

Replace these caps:
https://www.eevblog.com/forum/repair/a-gw-instek-psp-405-lab-bench-power-supply-repaired/

[a_giannini]

Hi, matt6ft9.

Thanks a lot for you reply.
I've read the post you point me to before start working on the PSU: really I've already replaced all the capacitors of the PCBs.
In any case I'll take a look again to those capacitors and I'll be back to you as soon as I can.

Best regards.
Armando

[a_giannini]

Hi, matt6ft9.

The 4 caps was all ok.
In any case I replace them all with new ones and the situation is practically still the same:
- if the voltage set is <= 0,13 V the display shows the correct voltage but no voltage is present on the output connectors;
- if the voltage set is > 0,13 V the display always show 0,03 V and no voltage is present on the output connectors;

Cheers,
Armando

[bitseeker]

Hi Armando,

I don't have detailed photos of that supply, so I don't have an exact answer for you. If you can post some photos, that would be helpful. Are there any test points on the power PCB that you can check to see that all the necessary voltages are being produced? If not, any connectors with labeled voltages?

I see from Matt's photo of his power supply that there's a fuse, F2, next to the opto-isolator, IC2. Is that fuse still OK? Verify it has continuity with the power supply off.

[bitseeker]

I tried to see if the schematics in the following thread might be for this power supply, but I'm not sure without being able to compare it to the actual device.

https://www.eevblog.com/forum/buysellwanted/i-need-gw-instek-psp-405-service-manual-and-winding-diagram-please-help/

[a_giannini]

Hi bitseeker,

thankyou for your post.
The fuse F2 is ok.
On the PCBs I couldn't find any test point or connectors with labeled voltages but checking the schematics you pointed me to I found one very close to the logic PCB: it is labelled DPS-4005 AQF2. 932. 768DL/4.
I hope the photos will be useful.

Armando

[bitseeker]

Are Q1 and Q2 getting power? If those transistors don't have power, then the switch mode supply that drives the main power rails for them must not be starting. You already changed the capacitors, which is the most common failure. Also check the power resistors, such as R11 and the one in the middle of the board that neither has gone high or open. Verify that the SMPS controller is providing a switching signal (I'm not sure if you replaced it). If that's OK, test the opto-isolator as it provides the feedback loop. Switch mode power supplies aren't my forte, but these are the common issues.

Since the display works, the supply for the digital part seems OK.

[a_giannini]

Hi bitseeker,

Thankyou for your suggestion.
I know I'm so late with my post, but I've been very busy in the past weeks...

The power resistors are OK.
Q1, Q2 and the optoisolators appeared to be ok, in any case I replaced them all.
TL494CN has already been replaced: do you suspect it can now be at fault again?

I can add that taking another deep look at the power board I found a control point, in parallel with the electrolytic capacitor C9 220 uF (image in attachment).
It is labelled 16 V but I read only 14,6 V.

Thanks again, have a nice day.
Armando

[Ordinaryman1971]

You said you've changed all the caps on the power board? Every single one?
Check the sensing resistor, the cement one by the edge of the board, see if it has any cracks or maybe it's broken. You've probably done it already, just wanted to mention it.

[bitseeker]

I can add that taking another deep look at the power board I found a control point, in parallel with the electrolytic capacitor C9 220 uF (image in attachment).
It is labelled 16 V but I read only 14,6 V.

That's a useful find. Trace out what else is connected to that part of the circuit and loading it down (short circuit or nearly so).

[a_giannini]

Hi guys, here I am again.

To Ordinaryman1971: yes, I have replaced all the electrolytic capacitors.
The sensing resistor is ok: it is a 400 ohm 10 W, I removed and tested off the PCB and it gives a reading of 398 ohm.

To bitseeker: for how I searched I couldn't find any short circuit on the PCB.
Maybe I'm wrong, but I'm starting to think the feedback from the digital PCB is some way faulty.

Cheers,
Armando

[bitseeker]

Do you see a PWM signal from the switching controller IC? Get the datasheet for it and verify that (1) it's alive and emitting a signal, then (2) see if the pulse width it generates varies at all. The duty cycle will change to increase or decrease the output of the power supply.

[a_giannini]

Hi everybody.
I'm sorry to be so late...

>
Do you see a PWM signal from the switching controller IC?
Get the datasheet for it and verify that
(1) it's alive and emitting a signal, then
(2) see if the pulse width it generates varies at all. The duty cycle will change to increase or decrease the output of the power supply.
>

The switching controller seems to be ok, I can see a signal but the pulse does not change when I change the output.

Have a nice day.
Armando

[bitseeker]

OK! Sounds like you're getting closer.

If the pulse width doesn't change, check the reference voltage and the control voltage(s). If any of them is not correct (or control voltage doesn't change when you change the output), then trace back to the source of the problem.