Bestec atx-300 12z power supply

[deni2710]

Hi all,

I am extremely new at this - please forgive my ignorance.

My question - is there a fuse in the Bestec atx-300-12Z power supply.
If yes where do I find it.

many thanks,

Ian.

[b700029]

Re: Bestec atx-300 12z power supply

[t.j.]

Re: Bestec atx-300 12z power supply

yes there is. follow the lines in from plug on back! fuse should be right in that area!

[deni2710]

Re: Bestec atx-300 12z power supply

Thank you SO much.

Much appreciated.

Have a good day,

Ian :-)

[ben7]

Re: Bestec atx-300 12z power supply

Thank you SO much.

Much appreciated.

Have a good day,

Ian :-)

If you find it blown, there is probably something else shorted.

[Jidis]

Re: Bestec atx-300 12z power supply

I know this is a long shot here without you guys being able to see it in person, but I was somewhat defeated by an ATX-300-12Z (Rev:BD) last night and feel like I have unfinished business.

Someone had a machine which was giving the flashing PSU LED and not booting, and gave it to me. I read about some of the cap issues and sure enough C36 is slightly bulged and reads high ESR. I replace that, along with the two giant main filter caps (just because I had an almost new pair beside me), screw the board back in, and plug it into the wall....BANG!...poof ...stinky smell.

Damn if I can tell what exactly it was that blew up, but the three caps went in with correct polarity. I also rechecked all my soldered points and they were perfect. I did notice when I pulled the board back out that Bestec had done a rather sloppy job of trimming leads on the through hole parts, and some had almost a quarter inch sticking through the bottom in random directions, so it's possible something got squashed into a short on reassembly. It also had a few spots of the darkened brown glue which may have gone conductive (wasn't aware of that before reading here), but if that were the cause, I suppose it would have blown up or malfunctioned the last time it was working, which was only a week or two back.

Strange part is, I happened to have a Bestec PSU board I had scavenged for heatsinks or something here which had almost the same layout and about 60 or 70% of the same parts, so I began randomly swapping similar things and must have replaced about twenty components with no change. No light, no power, and it doesn't seem to properly discharge itself after I unplug it. Not sure how or what they normally do there, but it seems to dump current back onto the AC line after I switch the power strip off, and can actually "re-light" the light on the power strip, which I've never seen any healthy supply do.

I've checked the main fuse of course, swapped several disc caps, the main bridge rectifier, both main yellow transformers, a few diodes, a couple of the main (heatsinked) transistors, one or two of the 8-pin chips and a bunch of other mess. I've checked many of the other diodes and stuff with the basic diode check on my DMM, but I'm not sure how much that can tell me.

Sorry for all the text, but I figure anybody that knows these units might want to know. If there's any specific part or area I might replace or check for something, I'd be interested.

Thanks!

[everell]

Re: Bestec atx-300 12z power supply

If you have changed out both yellow transformers, the large one is probably the same, but the smaller one is not the same between the 12Z model and the 12E model. This lack of compatability can cause a big bang. Check the pc board you are taking your spare parts from for a model number written on the pc board and let us know what you have.

[Jidis]

Re: Bestec atx-300 12z power supply

Thanks Everell, and sorry it took me a bit to reply.

No, all the part swapping stuff was actually post-bang. When it blew up, I had only replaced C36 and the two big 200V caps. I'm not sure what that Bestec board is I had laying around, but I don't think it's even from an ATX-300-12 anything. Only number I see is "0314". It has smaller 470uF filter caps and the layout is different in a bunch of places. I don't think any of my scrap supplies had a 24-pin connector either (they're mostly P4 era junk). I was checking the actual numbers on the replaced components as I went along, and I'm pretty sure they were all the same. The smaller transformer is a T-ATX250HPT2E on both boards.

That explosion still baffles me. It sounded like a mid-sized cap blowing up, and I'd expected to have at least seen some scorch marks or something pointing to where it was, but there was nothing. I still think it was shorted leads on the bottom side.

If there are any pointers to tracing this fault, or places I can check, I'd be interested. With all that this board has been through at this point, I'm not sure I'll trust it for anything important, but I'd feel I'd accomplished something if I could revive it somehow.

Thanks again!

George

PS- I guess there is the remote possibility that this "donor" board was in my parts box for a reason, and that I've replaced something with the exact same failed component from something else, but I have a feeling it's just one of the parts I didn't get to yet.

[everell]

Re: Bestec atx-300 12z power supply

Need to get out the ohmmeter and check the fuse. Should read zero ohms. Next check the current limiter. Looks like a green disk ceramic capacitor, also measures zero ohms. Next measure the bridge diode. Should not be zero between any of the four legs. For starters let me know what you find.

[Jidis]

Re: Bestec atx-300 12z power supply

Hey again Everell,

The fuse was one of the first things I checked and is still OK. If that current limiter is what I'm looking at (labelled as "NTH8" on my board, sitting in front of the 4-pin rectifier), that's also OK. The bridge rectifier was one of the parts I tried replacing, and seems OK (no continuity between the pins).

Thanks!

[everell]

Re: Bestec atx-300 12z power supply

Before powering up one of these old power supplies, I highly recommend that you get a pair of safety glasses if you don't already use them. Fixing an old piece of electronic junk is not worth losing your eyesight, and these things sometimes have an exploding component.

I believe you mentioned that you have already powered up this supply, and it is acting dead. Next step is to apply power and measure the dc voltage across those two large capacitors you replaced. Each should have about 170 volts DC. They are "stacked", that is wired in series, so you should be able to measure about 330 volts across the two of them. If not, this is where troubleshooting begins.

There should be a purple wire that extends from the power supply to the 20 pin mother board connector. With power applied, measure (at the mother board connector) between the purple wire and any of the black wires (ground). You should get 5 volts dc. This is the 5 volt standby circuit, often called 5vsb. It MUST be working or nothing else will work.

So check for 170 volts across each large capacitor, and 330 volts across the two of them. Also check that 5vsb purple wire for 5 volts. Let us know what you find.

[Jidis]

Re: Bestec atx-300 12z power supply

Thanks Everell,

Looks like that standby line may be an issue here. I get exactly what you told me across each cap (and in series), but pin 9 bounces around in the point something millivolt range.

Let me know what to check next - and no hurry.

Thanks!

George

PS- I'm with you on the goggles. Mains voltage and big caps scare the crap out of me. If I don't have safety glasses nearby, I usually put on the ones I use for looking at small stuff.

[everell]

Re: Bestec atx-300 12z power supply

Without applying power, it is now time to check out a lot of components. I am posting a drawing I made of the 5vsb used by the ATX-250 12Z which should be very close if not identical to your power supply. First, most of these power supplies used the A6351 8 pin chip. Some of the later ones used the VIPer 22A. You will need to determine which chip your power supply uses. So let us know which chip you have, study the drawing, and check out as many parts as you can with the ohmmeter, and let us know what you find. It would be helpful if you would post a picture of the 5vsb area of your power supply.

[Jidis]

Re: Bestec atx-300 12z power supply

I had a bad feeling that's the chip you were talking about. It's one of the ones I didn't swap, and on my board, it's totally buried by components and heatsinks (I've got those 'L' shaped ones that cover that whole area).

I'll try to get on it later tonight, but I may have to do some desoldering again. FWIW, the donor board does in fact have an A6351 if I need one.

George

[Jidis]

Re: Bestec atx-300 12z power supply

Hey again Everell,

Finally got a chance to dig that heatsink off again. It is indeed an A6351A (on both boards actually). I thought I was onto something the other night as I thought I found one of the surface mount resistors in that area to be open, but I may have had my meter on too low a setting or something. I'm hoping whatever component is at the root of the failure isn't popping certain things in its path when it powers up. I'm getting weird readings on R125 now, which was the one I believe was open, and it was replaced the other night (supply stayed dead though).

If you get a minute, let me know what you think I should do. I extracted that 6351 chip from the donor board, but like I mentioned, I'm not sure why I have that board or if it contains any failed parts itself.

Thanks,

George

[everell]

Re: Bestec atx-300 12z power supply

Measure the 1.2 ohm resistor coming off pin 1 of the A6351 chip - should be about 1 ohm (labeled R124 on my drawing). Measure the 180k resistor coming off pin 3 of the A6351 chip (labeled R130 on my drawing). Also coming off pin 3 of the A6351 chip is a 10 uF/50 volt capacitor - measure its esr. If these components all look good, double check that you don't have any solder bridges in the 5vsb area of the pc board. You can power up the board with the main switcher and heat sink still out of the board. Pin 3 of the A6351 should measure about 16 volts.

Be careful. Plenty of dangerous voltages present. Wear a pair of leather gloves when making these measurements. Be sure to use a light bulb in series with the 110 volt AC line to handle any shorts.

[Jidis]

Re: Bestec atx-300 12z power supply

Didn't make it past the first checkpoint Everell. R124 read as wide open and when I pulled it, there was a nice little black hole hiding on the backside. May finally have an answer to my big bang mystery.

I'm off to track down a replacement. I'll let you know what it does (with safety goggles, of course ).

Thanks!

George

[Jidis]

Re: Bestec atx-300 12z power supply

This thing's piling up the casualties.

Much quieter bang this time. The R124 replacement is still intact, but it blew the whole pin 1 corner off the A6351. I just put the one from that donor board on there, but will probably poke around a bit more before applying power to make sure nothing obvious is wrong (probably should have done that before the last run).

The replacement chip is dark enough on one side that you can hardly read the label, but I don't know if it's just "tanned" from nearby heat or if the label started out like that. I'm hoping it's still a working chip.

Take Care

George

[everell]

Re: Bestec atx-300 12z power supply

Before power up, I have a suspicion that R125 between pins 1 and 4 is also bad. This is the current sensor resistor - doesn't sound like it was doing its job unless the chip was bad when you started. While you have the bad chip out, measure between pins 1 and 4 should be 560 ohms. And of course, verify that you didn't blow the R124 resistor again. In my drawing, the 250 watt version, the value was 1 ohm. Looks like yours was 0.82 ohm. Not that it makes that much difference.

[Jidis]

Re: Bestec atx-300 12z power supply

If Everell's around,

Sorry I've been juggling this with a couple audio electronics projects and had to hold it a few days. Just came back to it.

I had already installed the donor chip when you posted last time, but the little surface mount resistor (R125) was in fact popped again at some point. R124 from the donor board was still OK. I tracked down another 560 ohm resistor and replaced R125, but even with that, and the swapped donor A6351 chip, it still doesn't power up and none of the voltage pins read anything. Bear in mind, this is with that big 9N90 thing and its heatsink still off the board, which you said was OK to do.

As I mentioned, there is the remote possibility that the A6351 on the donor board was no good to begin with, if that's a part which commonly fails on Bestecs. I don't remember what that supply came from, or why the PCB was in my parts box. On the plus side, nothing blew up this time and the replacement parts still seem OK. I do still get the feeling it's not discharging like a proper supply after power down though, as it's still dumping enough current through my power strip to keep the LED on it lit.

If there's something else to check, let me know, but if the supply sounds like too much hassle for someone on my level to resurrect, I'll understand. It's already been replaced anyhow.

Take Care,

George

PS- There's a chip cap right alongside R125 (possibly in parallel) which I couldn't get a solid reading on (in circuit). Can't remember if I tried swapping that part, but it's definitely not shorted.