Bestec ATX-250 12E with 5vsb mod

[everell]

My good friend LDSishere made a DM311 5vsb mod circuit board for me. I installed parts and installed the new board on a recently acquired Bestec ATX-250 12E. This is the mother board killer power supply until retrofitted with a 5vsb modification such as this. I began by removing capacitors and removing glue, some of which had turned black and was obviously causing leakage problems. After replacing the old Jamicon capacitors with mostly Nichicon PW series, I applied power. Nothing. Very frustrating after all that work. Troubleshooting, I determined that the 5vsb transformer feedback winding was not delivering any voltage to the pwm chip. Bestec "12E" transformers are scarce and hard to find. So................

Pictures show the new pc board DM311 5vsb mod.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

I removed the 5vsb transformer and noticed that some of that black glue was coating the feedback windings. I carefully scraped the black glue away.......finding two of the wires were corroded under the black glue, and had corroded apart leaving open windings. Once the black glue was gone, I soldered tiny thin wires to the windings and reconnected them to the transformer pins. Installed the repaired transformer, and everything now works OK.

I wanted to start this thread to show the effects of corrosion under the black glue on this power supply. I am sure other people will see some of the same type of problem on other power supplies. This demonstrates that a repair can be done! See pictures of the repair viewed under a microscope.

[senz_90]

Re: Bestec ATX-250 12E with 5vsb mod

btw what is number for pwm ic? as usual you are well modder and fast troubleshooter power supply !! congrats and thanks for picture.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

The PWM chip used in this modification is the DM311. It is the same chip and circuit used in the first Bestec "12E" I modified a few years ago.

[LDSisHere]

Re: Bestec ATX-250 12E with 5vsb mod

I am glad to see the board worked, did you have to mod it any? Is the modded board on the other heat sink for the added fan?

Good find and repair on the transformer, I bet that was a hair puller for a while.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

I did not have to make any changes or mods to your board. Once the transformer was repaired, the 5vsb came right up.

The other board is for two functions. The resistor on top is 100 ohms in series with the fan to serve as a load when the 5vsb is on and working, but the computer is off. Keeps the 5vsb circuitry cool when the computer is off. The other components are an indicator circuit, tied to the ps-on and pg-out lines. It has a LED which comes on when 5vsb is on but computer is off. When you turn the computer on, the power good output keeps the LED on.

[momaka]

Re: Bestec ATX-250 12E with 5vsb mod

Nice mod yet again everell!

Yeah, that tan glue really is corrosive. I've seen it corrode terminals on resistors quite badly. It's even more frustrating when they goop-up the whole PSU in that stuff. One of the Bestec PSUs I fixed for a friend (ATX-250 12Z) was a really bad case. Almost made me want to give up.

[senz_90]

Re: Bestec ATX-250 12E with 5vsb mod

did 5vsb circuitry run hot when computer is off so you make this fan mod? i have never make attention for this temperature and if you don't mind to answer i am curious.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

Just found this article in another forum. Looks just like the glue I have been finding in older Bestec power supplies.

[kc8adu]

Re: Bestec ATX-250 12E with 5vsb mod

just cleaned a bunch of that shit out of the vco units on a kenwood ts-440 hf radio.
the stuff is an intentional built in failure.i tossed several bestecs that had so much glue in there it wasnt worth my time.a cap change is just barely worth it.looked like they dumped a gallon in each.thats an exaggeration of course.

[momaka]

Re: Bestec ATX-250 12E with 5vsb mod

looked like they dumped a gallon in each.thats an exaggeration of course.

More likely the glue gun in the Bestec factory just doesn't have an OFF/STOP button, so it's constantly jizzing glue . Worker simply just directs where the glue should "goop" the most.

[budm]

Re: Bestec ATX-250 12E with 5vsb mod

Those darn bad brown glues, but the expert thinks otherwise:

https://www.badcaps.net/forum/showth...982#post413982

[c_hegge]

Re: Bestec ATX-250 12E with 5vsb mod

Nice repair!

btw, I modified the second post with Jpeg images.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

just cleaned a bunch of that shit out of the vco units on a kenwood ts-440 hf radio.
the stuff is an intentional built in failure.i tossed several bestecs that had so much glue in there it wasnt worth my time.a cap change is just barely worth it.looked like they dumped a gallon in each.thats an exaggeration of course.

Don't toss any more of those bestecs with lots of glue. They are now "vintage" antique hard to find classics. No kidding! I was looking on ebay to see what kind of cheap replacement I could find for an old western digital 6 gig hard drive for an old pentium 2 computer. Hard drives with under 19 gig capacity are listed as "vintage" and priced $30 to $100 (and higher). Even pieces of the hard drive were being sold for ridiculous prices.

Besides, I can clean the glue off an old bestec, recap it, and it will outperform those old Leadman and PowMax pos. I still have a CompUSA made by Leadman which I have not been able to fix. Uses UC3843 pwm chip in current forward design. The switching transformer has a strange primary windings design.

[everell]

Re: Bestec ATX-250 12E with 5vsb mod

Nice repair!

btw, I modified the second post with Jpeg images.

Thanks. Looks much better. I was using a microscope at work tied into an old Windows 95 program which made the pictures in .bmp format only. I tried converting to .jpg format, but my Linux machine could not interpret it.

[budm]

Re: Bestec ATX-250 12E with 5vsb mod

Hi Everell, you can contact CRC.ORG about those <20G HD, I do my volunteer works there every weekend and see so many of those drives being recycle. Just send them E-mail telling them Bud told you to contact them and what you are looking for.