Bestec ATX-250 12Z problems and solutions

[everell]

I have acquired a Bestec ATX-250 12Z and thought I would start a new thread just for this model Bestec - problems and solutions.

Don't know the full story on why this one was pulled out of service. It powered up with the following voltage readings:
5vsb 4.99 volts
12v 12.02 volts
5v 4.99 volts
3.3v 3.38 volts

The first capacitor of the 5vsb pi filter (C36) was bulged. Also, in two areas, the glue on parts was black and brittle. Other areas of the power supply the glue was a brownish yellow. This would indicate to me that certain areas of the power supply were hotter than others - probably when the computer was powered down, the 5vsb still working, and the fan off. The 5vsb diodes were located in these two areas. A picture is worth a thousand words, so take a look at the attached picture.

[everell]

Re: Bestec ATX-250 12Z problems and solutions

Obviously the bulging capacitor needed replacement. Notice from the picture that it is wedged between a large diode (in 5vsb circuit) and the heat sink. The black burnt glue is to the right of this capacitor. If I simply replace the capacitor, the heat problem has not been solved. Also, what about the other black area? Two more 5vsb diodes are covered by it.

So first, I scraped the black mess away and replaced the two diodes with 4935 high speed 1 amp diodes. Note in the attached picture that the two diodes are not installed all the way to the pc board, but I left them about 1/4 inch up to improve air circulation around them and to prevent any burning of the pc board.

Next, the capacitor has been removed and replaced. The old (bulging) capacitor is a Jamicon 2200 uF/10 volt with esr 2.2. The new capacitor is Nichicon PW 2200 uF/10 volt with esr .03. But where did it go?

[everell]

Re: Bestec ATX-250 12Z problems and solutions

To get the new capacitor out of the heat zone, I drilled two new holes and mounted it on the other side of the heat sink. See attached picture. The output capacitor of the pi filter is also a Jamicon 2200 uF/10 volt and measured esr .02. Nothing wrong with it. It was not in the heat zone.

I think fixing this power supply's problems requires more than just replacing bad parts. To do so means later down the road the same problems are repeated. This power supply has a pwm chip in the 5vsb, and a scr for crowbar circuit if 5vsb ever tries to go overvolt. I think it will be a very good and reliable power supply....after the heat problems are resolved.

[sam67]

Re: Bestec ATX-250 12Z problems and solutions

Hi Everell, i have the same power supply but mine is the 300w 12z ..everything inside my psu is cramped but no visable caps bulging..Have not took any out to check with esr meter.I think i messed my mobo up back probing the connection testing voltages as i think maybe i put one of the probes into a live rail thinking it was in the ground pin..Thats all i can assume as my mobo won't show any screen on my monitor apart from no signal..When i recheck all my voltage rails they are all good .Fans spin front lights up but no beep or live screen..Looks like a new mobo now..It's an old one anyway but it's a lesson i have learned lol.


p.s forgot to say this pc i have had for 5 years with this psu and never had a problem untill i messed about probing the connector on mobo.

[ratdude747]

Re: Bestec ATX-250 12Z problems and solutions

i thought the 12z was a good one. i shall check mine again...

[everell]

Re: Bestec ATX-250 12Z problems and solutions

The 12Z is a good one......but most power supplies could use an improvement here and there. Replacing a bulging capacitor is common for all of them. The bad "killer" bestec is the ATX-250 12E. To make the 12E a reliable safe power supply requires extensive modification to the 5vsb. On the 12Z, the 5vsb design is great! No problem.

[ratdude747]

Re: Bestec ATX-250 12Z problems and solutions

good, no bad caps on it.

ironically, my ac-bel 550w puts out more heat... but then again, it is over 2x the power

[everell]

Re: Bestec ATX-250 12Z problems and solutions

Heat is the enemy of aluminum electrolytic capacitors. What ideas and improvements can you make?

[everell]

Re: Bestec ATX-250 12Z problems and solutions

Unsoldering a bulging capacitor C36 is easy. Soldering a new replacement capacitor is also easy. The hard part in relocating capacitor C36 is drilling two new holes without causing lots of new problems. So here are some pictures of where to put the two new holes (which I recently did to another 12Z power supply).

The first picture, locating where the new capacitor goes. The second picture shows the two new holes drilled. The third picture shows the new capacitor soldered in place. Piece of cake!

Almost forgot to mention.....I was using a dremmel tool and a VERY small diameter drill to make these two holes.

[Microbit]

Re: Bestec ATX-250 12Z problems and solutions

Hello Everell.

I have a ATX250-12E that stopped working. Searching by google found your post about the possible repairs to do.
That made me register on BadCaps forum. I'll try to fix it.
Thanks for sharing your knowledge.

Sorry because I used Google to translate and post.

Evandro

[kaboom]

Re: Bestec ATX-250 12Z problems and solutions

Make sure you remove all that glue. A potential fault path exists between primary and secondary via that stuff. On one of these that I recapped, there was discolored glue all around the transformers. Scraping pieces of it off revealed even darker 'holes,' bubbles kind of, that looks to have been the start of it breaking down. Don't want any leakage around the switching FET, either. So while recapping it, I scraped all the glue off the entire board and cleaned it up.

Replaced all caps with Panasonic FC, except the two big 'doubler' caps in the primary- they were fine.

Make sure you check under the big transformer for glue bridging the isolation barrier.

Nice job with C36

-Paul

[everell]

Re: Bestec ATX-250 12Z problems and solutions

I was so pleased with the second fan circuit for the 5vsb which I made on the Bestec ATX-250 12E that I decided to see if I could make one for the 12Z model. I was going to put the additional parts on a small add on board. When comparing the 12E circuit with what would be needed on the 12Z, I discovered that the pc board for the 12Z already had the needed components laid out on the pc board. All I had to do was to add the following parts:

R7 10K resistor surface mount on bottom of board.
D50 general purpose diode surface mount on bottom of board.
D6 general purpose diode can be mounted on top of board OR can be surface mount like D50
Q16 pnp transistor install on top of pc board. I used A733 transistor, but any of the general purpose pnp transistors will work. Just make sure the pinout is the same.

After adding just these four parts, install the fan where resistor R102 should be installed.

When you plug the power supply into the wall outlet, the 5vsb fan comes on. When you turn the computer on, the 5vsb fan stops and the usual fan comes on. When you turn the computer off, the regular fan goes off and the 5vsb fan comes on.

The fan must be hooked up with proper polarity. If it doesn't work on first try, reverse the two wires and all should be well.

Two pictures showing where parts are to be installed, and after parts are installed.

[goodpsusearch]

Re: Bestec ATX-250 12Z problems and solutions

Good idea but it will collect dust if the fan works 24h per day....

[everell]

Re: Bestec ATX-250 12Z problems and solutions

Here is a picture of the Bestec ATX-250 12Z sporting its new 5vsb fan

[everell]

Re: Bestec ATX-250 12Z problems and solutions

So what improvement is left on the Bestec ATX-250 12Z?

The important previous steps have been replacing ALL of the electrolytic capacitors. While doing this, remove all of the glue from the pc board - it causes leakage and shorts. Relocating capacitor C36 is a worthwhile improvement. Adding a 5vsb fan is nice. The last thing I can think of is to adjust the 5 and 12 volt rails.

I have noticed that several of the Bestec power supplies I have worked on have +5 and +12 volts on the low side. These two voltages are controlled by a "431" regulator chip.

Looking at the attached schematic, R57 connects the 12 volt rail to the 431 regulator gate. R61 connects the 5 volt rail to the 431 regulator gate. The parallel combination R59 and R60 connects the regulator gate to ground. Resistor R60 is used to set the regulation point for this circuit. Its value can vary between 27K to 51K. Changing the value of this resistor will increase or decrease BOTH the 5 volt and 12 volt rails at the same time.

WARNING: Resetting the operating point requires changing R60 which is a surface mount resistor on the bottom of the pc board. Do not attempt changing this resistor unless you are up to learning the techniques of soldering and changing surface mount components. It can be very tricky! Also, do not attempt resetting the voltage regulation resistor R60 before replacing the electrolytic capacitors. A bad capacitor could cause you to use the wrong resistance value.

Photo attached shows the four resistors which are used in the regulator circuit. All four are surface mount resistors on the bottom of the pc board.

[everell]

Re: Bestec ATX-250 12Z problems and solutions

A while back someone mentioned adding a LED indicator to the psu like the one on a Bestec ATX-250 12Z. So I decided to look at the circuit, and here is the schematic. It was a more interesting circuit than I expected because it tells you three things about the computer.

First, when the computer is plugged into a wall outlet, the LED indicates that the 5vsb is working. The measurement from the LED indicator is actually taken from the PS-ON circuit which gets its power from the 5vsb.

Second, when you push the power on button on the computer, this signals the south bridge chip to take the PS-ON in the power supply from high state (5vsb) to low state (ground). This is like shorting the green wire to a black wire on the power supply plug. But you have to remember that the south bridge chip is what actually does this. So the LED indicator light goes off. This means that the south bridge chip is working.

Third, the LED indicator also monitors the PGO (power good out) signal from the power supply supervisor chip. When PGO is good, the signal is high (5 volts) and the LED comes on. When PGO is bad, the signal is low (0 volts) and the LED is off.

Power up sequence for the LED indicator is: when plugged into wall outlet, LED should come on. When you push the power on switch, the LED should blink off then on. This indicates that the south bridge chip turned the power supply on and that the power good signal came on quickly.

[everell]

Re: Bestec ATX-250 12Z problems and solutions

For those who want to add a LED indicator circuit similar to this, here is a schematic drawing simplified. I wired this one on a small perfboard and installed it in a Bestec ATX-250 12E. Works great.

[hkqq2010]

Re: Bestec ATX-250 12Z problems and solutions

For those who want to add a LED indicator circuit similar to this, here is a schematic drawing simplified. I wired this one on a small perfboard and installed it in a Bestec ATX-250 12E. Works great.

I had a bestect atx-300, it always burn the fuse. After isolated the 5Vsb circuit, it stop burn out the fuse. I will try to fix it.

Thanks.

[Luposian]

Re: Bestec ATX-250 12Z problems and solutions

I have a Bestec 250 12Z. Seems to work better, now that I opened it up, hosed it out, and let it dry. It was shutting down frequently, in my hot (90F+) workshop, but now doesn't. I think the dust was acting as an insulator and causing the heat to "stick" and cause the PSU to shut down.

Does anyone have a complete schematic for the Bestec 250w 12Z, or know where to get one? I'm curious to know what makes this PSU tick. What is the difference, component-wise, between the Bestec 250 12Z and the Bestec 300 12Z? What gives it the extra 50w of power?

[Wester547]

Re: Bestec ATX-250 12Z problems and solutions

^ A number of things. The 250 12Z uses a temperature controlled, medium speed sleeve bearing fan from Jamicon, the 300 12Z uses a high speed, temperature controlled sleeve bearing fan from Jamicon. The 250 12Z uses 470uF input capacitors, the 300 12Z uses 680uF input capacitors. The 250 12Z has a 4A bridge, the 300 12Z has a 6A bridge (both without a heatsink but close to the fan). The 250 12Z uses a TO-247 switcher with an ON resistance of 1700 ohms, the 300 12Z uses a TO-247 or TO-3P switcher with an ON resistance of around 1300-1400 ohms (@25*C). The 250 12Z uses a TO-247 30A part for the +5V rail, a TO-220 20A part for the +3.3V rail, and a 20A part for the +12V rail. The 300 12Z uses two 30A TO-247 parts for the +5V rail, a single 30A TO-247 for the +3.3V rail, and two 20A parts in parallel for the +12V rail, so it is quite a step up. I think the 250 12Z is rated at 25A for the +5V rail, 18A for the +3.3V rail, 14A for the +12V rail, 165W combined for +5V&3.3V, 218W for +5V and +12V combined, etc.... the 300W 12Z is rated at 30A for the +5V rail, 28A for the +3.3V rail, 19A for the +12V rail, 180W for 5V&3.3V combined, 288W for +5V and +12V combined, etc... the 300 12Z probably has a larger main transformer too and better heatsinks, and thicker inductors.

I think the 300 12Z would run quite a bit cooler than the 250 12Z and would be capable of quite a bit more. Note that PSUs have many small revisions and some internals used may vary from PSU to PSU so the differences between them I listed may not be entirely accurate but they are the trends I've seen from Bestec and I think it gives off a general notion. Neither of them have the best brand of capacitors - CapXon, OST, Teapo, Jamicon, etc, and some of them have conductive glue, but other than that and the lack of OPP they are good PSUs, IMO.