Bestec atx-300 12z power supply

[everell]

Re: Bestec atx-300 12z power supply

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.

Please explain.....are you saying that the LED mounted to the power supply case comes on and stays on for a while after power down? That would indicate that the 5vsb is working. When that LED is on, measure the purple wire and see if it measures 5 volts. You do not need the heat sink and main switching circuit soldered in place to do this test. In fact, until you get the 5vsb working, best to leave it out so nothing else blows up!

For that LED to stay on for a while after power is removed is normal because there is very little load on it.

[ben7]

Re: Bestec atx-300 12z power supply

That is NOT normal!!!

Power shouldn't be feeding back through the bridge rectifier! Something major is wrong! Try un-plugging the PSU from the outlet (or power strip), and then measure the voltage across the plug prongs. If there is a large voltage on the prongs, say more than 50V, do not touch the prongs with your fingers or else you might get a surprise shock! :P

[Jidis]

Re: Bestec atx-300 12z power supply

Thanks Everell and Ben.

I probably should have been more clear on that. It's as Ben figured. It's actually feeding power back down the AC line and lighting the LED on the power strip I have the supply plugged into, even if I unplug that from the wall. Definitely not right, and I've never seen anything here do that. I've never bothered to check, but I'd guess there has to be something happening across the prongs on the mains while that's going on. The caps can also supply a nice little zap when I discharge them. After I've plugged it in to test it, I'm really careful not to touch anything on it in the state it's in until all the discharge stuff is taken care of.

Everell,

The supply PCB has actually been out of the case for most of the time I've been messing with it. I soldered my own LED into the holes on the board just so I could tell if it ever came back to life. That light and the original Bestec one have not been on once since I blew it up the first time (when the resistor exploded).

Take Care

[everell]

Re: Bestec atx-300 12z power supply

OK - so your 5vsb is not working. On the first bang you blew the R124 resistor between the FET source inside the chip and common. Note that common is the "ground" for the input side of the power supply but is not a true ground. Measuring from common to ground should show high resistance (or open). This means that something caused the circuit to draw high current - opening R124. I think it must have damaged and opened the circuit board trace also. When you replaced R124, instead of the current going thru R124 to "common", it went inside the chip and created lots of damage to the chip while finding a path to "common" thus blowing a chunk off the chip. That would explain why the replacement resistor R124 did not blow open again. An open ground or common trace on the pc board might also be the source of trouble causing the backfeed.

Post a picture of the bottom side of the pc board for us. With all of us looking, perhaps we can spot the problem area.

[Jidis]

Re: Bestec atx-300 12z power supply

Everell,

Brace yourself, this thing's gnarly looking up close. I took one inside the other day, but it wasn't clear enough. Keep in mind, some of this stuff is from all the desoldering/resoldering and my failure to clean the board, but that was all after the big explosion anyhow. IIRC, there were already a few "browned" areas when I started.

Take Care

<edit> And let me know if you want the other end of that board. from what I can tell, there's nothing scorched or damaged looking over there anyhow.

[everell]

Re: Bestec atx-300 12z power supply

Get a bottle of rubbing alcohol and a small brush. Give that board a good scrubbing. If you see anything that appears to be shorting across traces, use an ice pick or similiar tool to scrape it away. Solder bridges are sometimes hard to see with the naked eye. You would be surprised at how many problems go away just by cleaning a board with alcohol.

[Jidis]

Re: Bestec atx-300 12z power supply

I'm pretty sure that (a short) is what happened to blow it up the first time, but the soldering was all OK. All I had done was to replace a couple caps. There were a bunch of leads extending through the bottom of the board which I later clipped off, but I'm thinking one of those may have bent over or shorted against the case somehow.

I'll clean it up as you say, but I think I probed around at most of the messy looking areas already to make sure they were making (or not making) contact.

Take Care,

George

PS- I was surprised to see not only those untrimmed through-hole leads, but also the sloppiness of some of the surface mount parts. I always figured that sort of "robot made" stuff was all or nothing.

[everell]

Re: Bestec atx-300 12z power supply

I always figured that sort of "robot made" stuff was all or nothing.

Its "ether" theory.......ether it works or it doesn't!

Remove R130 (180K 5 watt) resistor from the top side of the board. Measure it and make sure it is still 180K. Solder it back in place and then clean that board with alcohol.....completely clean. On bottom of board make sure that the capacitor next to R125 (561) is not shorting on either side. Carefully clean between these two components. On bottom of board measure J21......must measure zero ohms or you have lost a ground.

[Jidis]

Re: Bestec atx-300 12z power supply

Hi Everell,

R130 is good (checked). There does appear to be continuity between pins 1&2(GND?) of the A6351 chip, so I'm going to try to look into that cap/R125 combo and make sure nothing is bridged. Without removing them I can't tell exactly what the traces are supposed to be doing, but I'm going to try to avoid it.

Thanks!

George

PS- Sorry it takes me a bit to check sometimes. I'm still bouncing between two or three electronics projects, but I'm doing OK considering.