PSN ATX-300LL alias Bestec

[everell]

I have recently acquired a "new" dead power supply. Much too heavy to be a gutless wonder. On looking inside, about half of the caps were bloated, one had exploded. A quick spin the fan test revealed that the fan was stuck. This one definitely overheated. The layout looked very familiar. Then I noticed that on the pc board was written Bestec ATX-250-LL. This was apparently an early version before the Bestec ATX-250-12E. The 12E had some definite improvements over this model.

The switching transformer was much smaller than the ATX-250-12E and 12Z. I somewhat doubt that this power supply will do 300 watts. Maybe 250 watts. So far I have scrapped the old glue from the board. It had two large spots which were black, one of which was shorting the 5vsb rectifer to other parts. The capacitor which exploded was C1 the critical capacitor in the two transistor circuit. I took out the two transistor circuit and replaced it with a DM311 mod. I also replaced all of the capacitors. It is now a good working Bestec.

I want to investigate the turn-on circuit. The 12E model has a much more elaborate circuit. Perhaps I will make a few more changes. Then I will post some pictures of the final repair.

[Wester547]

Re: PSN ATX-300LL alias Bestec

...330uF primaries and a size 30 transformer with the heat sinks' metal base being that thin!? Depending on the secondary rectifiers and main switcher....that might as well be deemed a 200 watter.... Bestec have done much better but I suppose it (how much they overbuild) does depend upon who they build for. Was the fan a Jamicon sleeve bearing? I thought Jamicon fans were better than that, if so. Looks like that one has a 2004 (November) date code. In fact, I bet this would explode or shut down at 300W. It looks like they used a TO-220F (no metal tab) rectifier for the +12V rail, too...

[senz_90]

Re: PSN ATX-300LL alias Bestec

would like to see your DM311 mod and final repair. three pic817 act as 5Vsb, oh yes no 5VSB transformer. did this three things defective also? looks burned very bad.

I want to learn turn on circuit as you said, don't bother to upload please

[c_hegge]

Re: PSN ATX-300LL alias Bestec

As bad as the heat discoloration is, I must say I am surprised it is not worse. Last time I saw a PSU with a suck fan, most of the PCB was black

[ncovert]

Re: PSN ATX-300LL alias Bestec

You might also want to install the missing input filtering where the provisions can be found on the board. (I see a jumpered-over coil and no X caps.)

[everell]

Re: PSN ATX-300LL alias Bestec

...330uF primaries and a size 30 transformer with the heat sinks' metal base being that thin!? Depending on the secondary rectifiers and main switcher....that might as well be deemed a 200 watter.... Bestec have done much better but I suppose it (how much they overbuild) does depend upon who they build for. Was the fan a Jamicon sleeve bearing? I thought Jamicon fans were better than that, if so. Looks like that one has a 2004 (November) date code. In fact, I bet this would explode or shut down at 300W. It looks like they used a TO-220F (no metal tab) rectifier for the +12V rail, too...

The primary capacitors have been replaced with Nippon Chemicon 470 uF/299 volt capacitors. The main switching transformer is smaller than the 250 watt 12E/12z models. I took out the transformer and compared it with a transformer I had out of a 12Z model. The winding pinout was much different or I would have installed the bigger better transformer. The main switcher is a K2747 which is a FET rated for 900 volts at 7 amps. Not as hefty as the 12E/12Z models. So I would not expect more than 200 watts of performance out of it. The diode on the 12 volt rail is a TO-220, not sure on part number. The fan is Jamicon. I took it apart and cleaned it and oiled it. Spins nice and easy now. I haven't put power to it yet so don't know if it will work - and if it will have a wobble as some repaired fans have.

[kc8adu]

Re: PSN ATX-300LL alias Bestec

talk about stripped!
looks like a gutted atx250-12e

[everell]

Re: PSN ATX-300LL alias Bestec

You might also want to install the missing input filtering where the provisions can be found on the board. (I see a jumpered-over coil and no X caps.)

The Y caps were ordinary disc ceramic, so they were replaced with two Y caps. Two filter coils and two filter capacitors were added. Pictures show these additions, as well as the primary capacitors 470 uF/200 volts and the DM311 5vsb mod. The old capacitors were all Jamicon. The new capacitors are Nichicon PW series.

[momaka]

Re: PSN ATX-300LL alias Bestec

Bestec sure cut a lot of corners with this PSU.
I'd say it's good for 200W tops, and for an older 5V-based computer.
As usual, great job with the repair everell!

[senz_90]

Re: PSN ATX-300LL alias Bestec

what x and y caps value? 222 or 472? sometimes I have confused to attach new filter when I have cheapo PSU that none of filter on primary.

Btw the main transformer is so small, the others looks great, and if the burned mark on the board could be cleared, that would a bonus sight

[everell]

Re: PSN ATX-300LL alias Bestec

The disc ceramic y caps werre 472, so I replaced them with real y caps 472. I cleaned the board with alcohol, but the burn damage is burn damage.....alcohol will not remove it. The burn damage was not bad enough to stop the power supply from working. So it has been an interesting project. Both x capacitors I added were 0.1 uF that I had in my junk box.