[SOLVED] AXIO MET AX-3010DS bench power supply repair

Hello everyone,

It's been a while since I visited this forum, yet my bench PSU is now toasted but I do not know for what reason (last time it worked like a charm, but baby came and I did not turned it on at all in the past 6m).

It is a switching bench power supply able to provide 30v and 10a (never went above 5~6a). Indeed, it is a cheap PSU, but I'd rather repair it than replace it.

The PSU won't even start normally, its LCD backlight and digits are dimply flickering and a light click can be heard (although there are no relays inside). After I turn it off, the flickering is more rapid until it dies. The power on switch breaks both live and null.

I inspected thoroughly the PCB and found no swollen caps and no visible damage, but only a resistor which seemed very heated (couldn't distinguish the colors) but after desiring and measuring it I could tell it's 226k +/-1%, and the "browned" colors could be red-red-blue-orange-brown, indeed. This resistor sits next to the TNY277PN ic, at the enable / under voltage pin (will try to draw a schematic and upload it later on).

However, since the resistor seems functional, I'm not sure if I am wrong or not by suspecting it. It has a voltage drop off 265v while the device is on. I have not measured the 5v rail, but I suppose this is the problem; I would say the TNY277PN and the small-to-medium inductor next to it (plus passives, optocouplers) are producing the 5v required by the control circuitry. Will measure it later on today if the little one allows me .

If anyone had any advice or different assumption, I am open to follow it.

Thank you all!

Please pardon my mistakes, did not re-read it thoroughly.. Corrections:
* [...] its LCD backlight and digits are simply flickering [...]
* [...] but after desoldering and measuring it I could [...]

Hello ro0ter,

After I turn it off, the flickering is more rapid until it dies. The power on switch breaks both live and null.

Click to expand...

This seems to indicate a short somewhere. It almost sounds like the unit is hiccuping: The supply fires up, senses the short, then shuts down, repeating the scenario. The main filter capacitor/s discharging is what's keeping it going when switching the unit off.
Check the the main rectifier diodes for shorts or any that have gone open circuit Also check the switching transistor/MOSFET for any shorts or open circuits between the electrodes.

How old is the unit?
A schematic would be awesome if you could get one, or reproduce it, though that can be a tad tedious.
Regards,
Relayer

Blame the baby!
Assume it did the deed ...

Looking online I can see schematics where the enable pin has a far higher value resistor. That however may not be the cause of the problem just a result of the fault and it will need changing eventually.
A schematic with voltage measurements would help greatly to diagnose this.

Indeed, a schematic would greatly help.... Tired of searching for all sorts of smps psu 3010ds etc... The resistor in the TNY277PN datasheet is indeed in the megaohm range, I know that as well.. Wondering if I`ll fry something if changing its value from kilo to mega. Still, I`ll have to draw some schematic and measure some values before I do that.

One more thing I remembered just about now: some very tini-tiny capacitor (orange-brown, ceramic 3~4mm in diameter, couldn`t see anything on it) was crushed at the far end of the rectified voltage... not sure if actually crushed or if it blew up, only noticed it after dismantling everything. I`ll upload some pictures with what I have so far, maybe some of you know these things better than me.

@Mattylad - I have seen no short, what-so-ever and I have tested most diodes in the circuit. Will also test the mosfet, see if it is the culprit.

@Relayer - The unit is barely 2.5 years old, warranty over, I suppose your question is relevant with respect to electrolytic capacitors?

Well, well, indeed it was a short. Another ceramic disk capacitor (103 = 10nf)... Hidden near the inductor coil exit (low side), between some resistor and a diode... It was boiling hot and almost black, not orange anymore.

What was very strange is that although it has a 500 ohm resistance, the capacitance meter showed it stil behaved as a capacitor... This is freaky...

The resistor at the inductor's high side, near the TNY277PN is also boiling hot... Maybe due to the short.

However, changing this cap with a 104 (.1uf) capacitor (this is what I had at hand) seemed to solve the problem, yet there still is inductor audible sound. The screen turns on, the voltage control works, all is good, but it still smells like hot after 10~20 seconds since switching on and when I turn the voltage trim pot there is additional, variable audible sound which seems to depend on the chosen voltage..

The other broken very small ceramic disk capacitor (really small in diameter, maybe 102? Couldn't read anything on it) is on the controlled current/voltage rails, I suppose it has nothing to do with this side, yet not 100% sure.

Will keep you all updated with my progress.

- - - Updated - - -

As I promised, here some images:

I have also measured voltage.. This voltage rail is 12v and not 5v as I initially though. Actually it has 11.2v and it oscillates a lot with about 0.3v (the AC component is however 0). There is still a lot of audible switching noise and the resistor before the TNY277PN is still very hot. The ceramic disk capacitor I have replaced is also very hot, and considering the voltage rail (12v) - I will replace the 226k resistor with a 3.6m resistor as in the recommended design for the TNY277PN.

To be continued

Update:

Two working units went through my hands on Friday. They have the same overheating problem, yet these two were working without problems. There seems to be a thermal design flaw - both working units have 226k r21 (next to the tny277pn), 10ohm r23 and 10n c10 (in the feedback circuitry, before the oc) running very hot. R23 actually reaches 120°c.... It seems that c10 failed in my unit and shorted the rail/feedback, but it did not damaged any other components. I suppose the unit would have been fully functional but I would have had to use the multimeter every time.

I have changed the c10 with a 0.1u instead of 10n and that's about it... I am considering to change the two resistors with higher wattage versions.

Question: can a ceramic capacitor be affected by heat? All electrolytic capacitors nearby are healthy (not sure to what extent, but none is shorted out leaked). C10 however was in the close proximity of r23 which is almost glowing hot...

I desoldered all the parts on the low side of the inductor following the tnyxxx, verified them and resoldered them back. The unit seems to work stable now, I had it running under supervision for 30 min and all went well. The audible noise with respect to selected voltage/current is normal, according to the other 2 working units I have inspected.

Thank you all for your patience and help!

If any of you should find the schematic of this cheap TME unit, please do share, I am really interested in performing some calculations to see whether they really have a bad design or if these units are somehow built to fail over time.

Not usually (not in the same detrimental way), electrolytics fail often due to heat because of the electrolytic within them.
Ceramics have a ceramic layer between the plates, so are not as subject to heat as electrolytics.

Just some feedback after some months of usage..

The fix is still valid after 4+ months. Everything is working very stable. I did not open the bench PSU to check its ceramic disk capacitor color as it is still working.

Thank you all for your help and input!