Bestec ATX100-5 no +5VSB

[GnatGoSplat]

I was using this on a Mini ITX Hackintosh for years, and then one day I plugged it in after a few months of non-use and some magic smoke came out. I opened it and couldn't find anything burned. Checked for shorted semiconductors and bad caps, and found none (recapped it a few years back).
No +5VSB, but fuse is good and there is primary input voltage.

I identify what I believe to be the primary coil pins of the 5VSB transformer, one pin connected to + at the rectifier/input caps, and other pin connected to Collector of a TO-220 heatsinked 2SC4020 NPN transistor whose emitter is connected to - at rectifier/input caps through a 4.3ohm series resistor, so I presume this to be the 5VSB switcher. However, there is no continuity across these 2 pins on the transformer. One of the pins does have continuity to a little nub that looks like a pin cut off (center tap, I would guess, as it has 2 tiny copper wires going to it), but the other (that goes to the switcher) has no continuity to the nub or any other pin.

Does this also sound like a blown 5VSB transformer to you experts?

If so, can I get a 5VSB transformer any place, or is this the end of the line for this PSU?

[everell]

Re: Bestec ATX100-5 no +5VSB

I have never seen a Bestec of this model. Could you post some pictures please.

The 5vsb transformer might be the same as the Bestec ATX-250 12E or 12Z. So tell us the numbers on the top of the transformer (or a picture of the top of the transformer).

From what you have said, it sounds like the 5vsb is a two transistor circuit with a blown transformer. If you really want it fixed........sure, why not. Most people on this forum throw away anything with Bestec on the label. Oh well................

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

It's a MicroATX sized PSU, OEM for HP. Has an HP p/n on it 5184-2191. A coworker gave it to me years ago because he knew I liked to tinker. Original problem I think was the fan was too noisy, so I dismantled and lubed the fan. Found a few bloated caps while I had it open and replaced them with stuff from my junk bin.

Numbers on the transformer looks like T-ATX-100T2I 00 44 CICB, but I've also included pics of it.

Yes, it looks like a two transistor 5VSB to me. I know Bestec is crap, but I have a hard time trashing anything because I hate waste. That, and I actually do need a small mATX sized PSU!

[retiredcaps]

Re: Bestec ATX100-5 no +5VSB

I have never seen a Bestec of this model.

I saw a video of this about 1 year ago on the CubeComputerChannel

http://www.youtube.com/watch?v=Prmmd1tHV0Y&feature=plcp

[ben7]

Re: Bestec ATX100-5 no +5VSB

It's a MicroATX sized PSU, OEM for HP. Has an HP p/n on it 5184-2191. A coworker gave it to me years ago because he knew I liked to tinker. Original problem I think was the fan was too noisy, so I dismantled and lubed the fan. Found a few bloated caps while I had it open and replaced them with stuff from my junk bin.

Numbers on the transformer looks like T-ATX-100T2I 00 44 CICB, but I've also included pics of it.

Yes, it looks like a two transistor 5VSB to me. I know Bestec is crap, but I have a hard time trashing anything because I hate waste. That, and I actually do need a small mATX sized PSU!

Looks like the pin came out of the transformer, better solder that back on and re-attach the wires to the pin!

Check the transistors in the 5vsb primary circuit.

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

Looks like the pin came out of the transformer, better solder that back on and re-attach the wires to the pin!

Check the transistors in the 5vsb primary circuit.

Do you mean 3rd from left? If so, that's supposed to just be a nub (no hole for it on the PCB). I think it's supposed to be an unused center tap for the 1st and 2nd pins from left. I've attached a closer pic so you can see what I mean.

[ben7]

Re: Bestec ATX100-5 no +5VSB

Do you mean 3rd from left? If so, that's supposed to just be a nub (no hole for it on the PCB). I think it's supposed to be an unused center tap for the 1st and 2nd pins from left. I've attached a closer pic so you can see what I mean.

Ah yeah nevermind...

Check the transistors in the 5vsb circuit, especially the one on the heatsink.

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

Thanks, transistor looks fine using multimeter's diode check, though PSU is probably destined for the junk bin if I can't get a transformer for it.

[ben7]

Re: Bestec ATX100-5 no +5VSB

Thanks, transistor looks fine using multimeter's diode check, though PSU is probably destined for the junk bin if I can't get a transformer for it.

Don't assume all the pins will have some resistance between each other!
There are two windings on the primary side, one for the switching input, and one for feedback.

Can you check the diode that rectifies the 5vsb? That might be bad.

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

Nah, I didn't use the resistance range, I check transistors in-circuit with the Diode Check function. 5VSB rectifier diode looks okay as well in Diode Check mode.

[ben7]

Re: Bestec ATX100-5 no +5VSB

Nah, I didn't use the resistance range, I check transistors in-circuit with the Diode Check function. 5VSB rectifier diode looks okay as well in Diode Check mode.

Why do you think the transformer is burnt out then? Its extremely rare for them to burn out.

[everell]

Re: Bestec ATX100-5 no +5VSB

I had a Powerman psu which blew out the 5vsb transformer....so, yes it does happen. I put a want ad for a replacement transformer on this forum, and nobody responded having one. So my solution was to build an add-on board with the complete 5vsb circuit using a transformer from a badly burnt Fortron psu. You can see how I did it at this thread:

https://www.badcaps.net/forum/showth...light=powerman

I will agree that your transformer is probably bad. You can try looking for one at Radio Shack, or ebay, but doubt you will find one. You can try advertising to find a complete psu like yours and buy it for parts. This model looks like such an older one I doubt you will have much luck. If you do manage to find a replacement transformer, that two transistor circuit is very tricky to repair. Most I have attempted to repair simply blew up again. Then you end up with two bad transformers. That is why I like to modify the 5vsb circuit with one of the pwm chips.

[ben7]

Re: Bestec ATX100-5 no +5VSB

I had a Powerman psu which blew out the 5vsb transformer....so, yes it does happen. I put a want ad for a replacement transformer on this forum, and nobody responded having one. So my solution was to build an add-on board with the complete 5vsb circuit using a transformer from a badly burnt Fortron psu. You can see how I did it at this thread:

https://www.badcaps.net/forum/showth...light=powerman

I will agree that your transformer is probably bad. You can try looking for one at Radio Shack, or ebay, but doubt you will find one. You can try advertising to find a complete psu like yours and buy it for parts. This model looks like such an older one I doubt you will have much luck. If you do manage to find a replacement transformer, that two transistor circuit is very tricky to repair. Most I have attempted to repair simply blew up again. Then you end up with two bad transformers. That is why I like to modify the 5vsb circuit with one of the pwm chips.

One thing is certain, never mention Radioshack! they don't sell electronics components anymore! only cellphones and tvs and crap...


Yeah I agree it will probably be best to make a new 5vsb smps instead of trying to fix it.

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

Why do you think the transformer is burnt out then? Its extremely rare for them to burn out.

It's open-circuit between the two primary input pins. Actually, it's open-circuit between one pin and the center-tap nub, so I think it's that specific winding.

I was curious, so I decided to perform a transformer autopsy. I had to unwind the main secondary coil to get to the primary winding, but it happened to be the winding directly below that. There was a darkened burned looking spot and both ends of the coil were severed at that spot. It appears to be where the wires were cris-crossed and touching. Only thing I can think of is the enamel wore through at that spot causing that coil to short and it generated enough heat to burn through both wires, but that's just a guess. I've attached a photo, but I took the pic at night so camera flash washed it out pretty badly.

[ben7]

Re: Bestec ATX100-5 no +5VSB

It's open-circuit between the two primary input pins. Actually, it's open-circuit between one pin and the center-tap nub, so I think it's that specific winding.

I was curious, so I decided to perform a transformer autopsy. I had to unwind the main secondary coil to get to the primary winding, but it happened to be the winding directly below that. There was a darkened burned looking spot and both ends of the coil were severed at that spot. It appears to be where the wires were cris-crossed and touching. Only thing I can think of is the enamel wore through at that spot causing that coil to short and it generated enough heat to burn through both wires, but that's just a guess. I've attached a photo, but I took the pic at night so camera flash washed it out pretty badly.

Smoking transformers are no fun, I've burnt out a few myself xD

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

I soldered the little wires back together and re-wrapped the transformer. I put it back in, expecting the PSU to either fry or work. It did neither. No magic smoke, no voltage from 5VSB, nothing, just DOA. Checked primary coil resistance again and it looks fine. Looks like the switcher isn't switching, although I'm not sure if you can tell that with just a multimeter.

How can I isolate the ground on an AC-powered earth-grounded oscilloscope so I can see what kind of signal is going into the base pin of the transistor?

[b700029]

Re: Bestec ATX100-5 no +5VSB

Isolation transformer.

[ben7]

Re: Bestec ATX100-5 no +5VSB

I soldered the little wires back together and re-wrapped the transformer. I put it back in, expecting the PSU to either fry or work. It did neither. No magic smoke, no voltage from 5VSB, nothing, just DOA. Checked primary coil resistance again and it looks fine. Looks like the switcher isn't switching, although I'm not sure if you can tell that with just a multimeter.

How can I isolate the ground on an AC-powered earth-grounded oscilloscope so I can see what kind of signal is going into the base pin of the transistor?

Are you sure you put the transformer back together the right way?

Check the power transistor and the transistor that drives it, one or both are likely bad. If they measure good, one or both could be open circuit.

[GnatGoSplat]

Re: Bestec ATX100-5 no +5VSB

Isolation transformer.

Is that safe? Tektronix has a PDF that says isolation transformers are a big no-no.

Are you sure you put the transformer back together the right way?

Check the power transistor and the transistor that drives it, one or both are likely bad. If they measure good, one or both could be open circuit.

I think I did, I took lots of pics during the unraveling, although I did break the ferrite core trying to get that apart. I've read they will still work even when broken though.

I'll pull the transistors to test out of circuit later on when I feel like messing with it again.

[b700029]

Re: Bestec ATX100-5 no +5VSB

Isolate the DUT using the isolation transformer, not the scope.

More information here:
http://www.eevblog.com/forum/beginne...ge-to-a-scope/