Micronet 24-Port switch with Seasonic PSU - power cycles constantly

[Agent24]

I recently won (on trademe) some network equipment (and other things, but that's another story) from an office cleanout

Included was a nice 24-port switch - a Micronet SP624E v3.....

...Of course I wouldn't be posting this if it worked, plug it in and it just cycles power repeatedly and PSU goes 'tick-tick-tick' in time with the lights all flashing.

It's a Seasonic SSF-0252-1 which has two outputs, 3.3v and 5v @ 5A and 1A respectively

I tried disconnecting the PSU, and it still ticks and the voltages go up and down between about 1.2v and 2.2v

I read this can happen because there is no load, the output is going too high and the controller is shutting it down because of the overvolt protection. But then it still does it when plugged into the main board (which seems like it wants to work as all the lights come on) so I suspect the PSU may be faulty.

I think the insides are probably a generic design which is rebranded for different companies because the PSU looks identical to the one mentioned here: https://www.badcaps.net/forum/showthread.php?t=5816 and it's the same thing (24 port switch)

Except in my case the PSU has Rubycons which aren't leaking

Anyone seen one of these PSU before? know anything about them (do they need at least some kind of load to work properly?)

Also, the main board is full of Canicon and Suncap low ESR, one of which was leaking. I replaced it with another piece of junk (starts with T) from a scrap motherboard, but the problem still remained (yeah I know, not a great idea)

And that comes to the other question, if I get this working, I'd like it to keep working. I want to replace all the caps because they're obviously junk - what would be good? (remember I'm in NZ)

[Agent24]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

Here's some photos (forgot to add them)

[dood]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

The canicon and suncap are crap... replace them before you start blaming the PSU. Hopefully, if it _is_ still the PSU, you can find one somewhere to replace it.

It's nice that they actually put a clear designator of a model on the power board. I hate when they leave it all blank, impossible to find replacements. I google search of that model led to a few places you could get one. But again, I'd start with the other craps first.

[Agent24]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

The canicon and suncap are crap... replace them before you start blaming the PSU. Hopefully, if it _is_ still the PSU, you can find one somewhere to replace it.

It's nice that they actually put a clear designator of a model on the power board. I hate when they leave it all blank, impossible to find replacements. I google search of that model led to a few places you could get one. But again, I'd start with the other craps first.

Well I want to replace the caps anyway, obviously...

But I don't know about getting a new PSU.. I don't know how you searched but all I can find are big distributors who'd want to you buy at least 10,000 of the things

Plus it would probably turn out for the same price I could buy another 3 switches 2nd hand...

I'm thinking of going down to DSE now and getting a couple of suitable resistors for a dummy load of say 50% the PSU's rating (in the event it's causing an overvoltage condition with no load). That should prove if the PSU is OK or not.

my other theory that it's the PSU is that if the main board is making such a light load (eg: something has gone open circuit) then I think it wouldn't even get any LED activity, because no power could come through, right?

[Agent24]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

Well, I hooked up the resistors - the voltage I was measuring would go from about 0.5 to 1.0, up and down (well so much as a slow DMM can show it) and the PSU was still 'ticking' but a lot slower

Unless my load wasn't high enough, I reckon the PSU is at fault. Any ideas?

[Gianni]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

What kind of IC controller is there for power switching?
I can't see it from pics.

I have repaired some PSU that had a faulty caps on the IC controller (UC3842): the start up resistor was able to bring the voltage up to UVLO (16V) threshold but since the ELCAP on pin 7 was open, the PSU stopped after few seconds because the supply voltage dropped down to lower threshold UVLO ( 10V).

Gianni

[Agent24]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

The controller is integrated into the switching transistor which is a Fairchild KA1H0265R (IC1)

IC2 is a KA78R05 Low Dropout Voltage Regulator, IC101 a KA431A Programmable Shunt Regulator.

Rectification appears to be done with D101 - Schottky SBLF2040CT, and a single large diode next to it, not sure of the number.

[Gianni]

Re: Micronet 24-Port switch with Seasonic PSU - power cycles constantly

As you can see from datasheet, this device has the UVLO.
You should check with a scope what happen on pin 3: if the supply voltage is going up and down between UVLO threshold maybe the supply cap is faulty, I think it is C4, the small ELCAP close to HS if I'm not wrong.
Check also D2, it should be the rectifier for aux voltage feeding IC1.

Another reason could be an overload due to faulty caps o something wrong with output diodes: the IC shut down because the internal overload threshold triggers so the on-off cycles go on.
I think you can check output diodes without problem with a DMM.

When I have to repair a power supply I feed with an external adjustable power supply the control IC (just above the UVLO start threshold) and than I feed the PSU input voltage with another adjustable power supply with current limit set to a low level (200~500mA) to see if I obtain the output voltages incresing the input voltage slowly, from 0V to nominal value.
In this way, with an adjustable power supply with current limitation I can work safely and if there is a short circuit on the output I can avoid any problem just limiting the input current. Obviously you don't have to apply load during this test. If the output are present, than I bring the input voltage at nominal level (if input is 110Vac then I apply 155Vdc, if it is 230Vac I apply 310Vdc). At this point I disconnect the external supply from controller IC and if the PSU shut down I try to apply a minimum load because some PSU need it to stay on.

I don't know if you can proceed in this way.
You can also read the Fairchild AN to understand how they ICs work , the file is too big to attach here.

Ciao
Gianni