COMPUSA 250 watt ATX power supply

[370forlife]

Re: COMPUSA 250 watt ATX power supply

What did you replace the primary switcher with? It is probably higher capacity.

[Khron]

Re: COMPUSA 250 watt ATX power supply

There's also some chance that the switcher might've been some chinese fake version. HERE you can find a few examples of what i mean.

[370forlife]

Re: COMPUSA 250 watt ATX power supply

In those two Yuelin YLP-013's I have they use to have imitation fairchild 13007's. The symbol with the fairchild "f" without the strike-through for the F so it really kind of looked like a J.

[everell]

Re: COMPUSA 250 watt ATX power supply

At the time it blew up, I had installed a new Fairchild 18N50. When I repaired it, I also installed a new Fairchild 18N50. I don't think the switcher is what caused the failure. More probable was the RULYCON capacitors OR a weak snubber diode (FR270 rated for 1000 volts at 2 amps).

[momaka]

Re: COMPUSA 250 watt ATX power supply

More probable was the RULYCON capacitors OR a weak snubber diode (FR270 rated for 1000 volts at 2 amps).

Yeah, Rulycon is complete crap (it's a cheap Rubycon imitation, after all ). Even GSC/Sacon/Evercon and Fuhjyyu are probably better than those (but not by much).
As for the snubber diode, I'm assuming that's one of the two diodes in front of the switcher. Is it the top one or the bottom one? I'm also curious what you used for the replacement.
----
Perhaps you should put more load on the PSU so it blows up again. It would point out the next weak spot. Then you can fix it and repeat the whole process again a few more times. By the time you're done, you should have a completely-rebuilt and [maybe] solid PSU .
Either that or just lots of blown and burned parts .

[stj]

Re: COMPUSA 250 watt ATX power supply

and video it!

[everell]

Re: COMPUSA 250 watt ATX power supply

The snubber is a three component network across the primary winding of the main transformer. It consists of a resistor and capacitor in parallel, and a diode in series with them. They are to take the backward kick (or snub) the signal thereby preventing a high reverse voltage from taking out the main switcher. The high speed switching diode in this network was a FR270 rated at 2 amps 1000 volts. I didn't have anything like it in my "junk box" from other junked power supplies, but I did have several FR160 diodes which are rated at 1 amp 600 volts. So I soldered two of them in series which should give me 1 amp at 1200 volts. It worked.

The RULYCONs are without doubt the worst capacitors I have seen. I replaced the Rulycons on the output voltage rails with Fuhiyyu capacitors I removed from my Antec power supplies. They had all been measured with my Blue ESR meter and checked out OK with very low ESR. They seem to be holding up well, and match the overall quality of the power supply!

I was only interested in seeing if this thing could handle a reasonable and moderate load - 133 watts total loading. It handled 71 watts on the first go around, but didn't make the 133 watts the first time. After repairs it did hold 133 watts. I don't want to push it any further - I simply have a 250 watt power supply that will handle about 50% of rated power. Can't expect more than that from these "gutless wonders" from China.

[everell]

Re: COMPUSA 250 watt ATX power supply

I decided that putting an 18 amp FET switcher might require more drive power than the original 7 amp FET switcher used. So I installed a buffer amplifier to provide better driving power. Works so far with my 133 watt load. This should improve the power supply's ability to handle some extra loading without blowing out the pwm chip. This buffer circuit might even be a good mod for one of my Bestecs.

[soniccj7]

Re: COMPUSA 250 watt ATX power supply

If my power supply goes bad, is it a good idea to go down to my local CompUSA store and buy a replacement? Here is a "used" CompUSA 250 watt ATX power supply which I have recently acquired.

Looks like lots of bulging capacitors. So I applied power to see what the 5vsb was doing. It came up a little over 5 volts. Good start, but thought I would change the bulging output capacitors before proceeding. All were Rulycon brand. Here is what I found:

2200 uF/10V 1.5 (esr)
2200 uF/10V 2.0
2200 uF/10V 1.7
1000 uF/16V 2.8
1000 uF/16V 3.5
470 uF/16V 8.6

1000 uF/10V 4.4 (5vsb filter)

Since ALL of these output capacitors were obviously bad, do you think the non-bulging smaller caps are bad (same brand)? I thought Fuhjyyu were the worst brand of capacitors, but I have never seen a psu where ALL of the capacitors were bad.

Main pwm chip is AP3843B and supervisor chip is marked Bestec B850. 5vsb switching transistors are K3067 and C945, common 2 transistor 5vsb circuit. Marking on 5vsb transformer is LP6100D. With no model number or OEM label, I wonder who made this psu....L&C, Bestec, or .........???

With output caps replaced, power came up fine as follows:
12V 12.03V
5V 5.25V
3.3V 3.38V
5vsb 5.02V

However, as I was monitoring and watching the 12 volt rail, the voltage varied from 12.02V to 12.73V, back and forth for a while. Finally seems to have settled at 12.03V. Maybe those little caps need replacement too!

The 12V and 5V output rails have PI filters, but the 3.3V looks somewhat lacking.

The QC label indicates this psu was manufactured July 2001. Overall, I would not have trusted it as a replacement. Would you?

On a google search I found this post with the pictures I needed to finish my ATX to bench power supply conversion. I removed all the wires to make it nice and clean inside the case, and removed the orange and black sense wires. These pictutres were a great reference.

Thanks for the pictures!!
ED

[b700029]

Re: COMPUSA 250 watt ATX power supply

Weird to see Bestec chip in a Leadman.