Bestec ATX250-12E up and running!

[370forlife]

Re: Bestec ATX250-12E up and running!

Not true. I've seen them fail and the PSU is fine after replacement. Especially true if the fuse is good. Most likely they blew because of they were underrated.

I don't think there is anything else wrong, as the PSU carried on working after the "pop", giving me time to shut the PC down normally.

Especially since it was still working even after it was blown, Fox's probably shorted when it blew, a lot of cheap psu's just jumper wire through these.

[everell]

Re: Bestec ATX250-12E up and running!

I have begun modifications on a third Bestec ATX-250 12E. This time I am replacing the 5vsb circuit with Infineon ICE3BR4765J pwm chip. This add on board requires one less wire than my previous DM311 circuit.

As a review of the DM311 circuit, it required no calibration of the regulator resistors, and came up on 5.00 volts with no load, and 4.95 volts with 2.5 ohm load and other computer voltages loaded and running. Very stable and everything as expected.

With the ICE3BR4765J, it came up at 4.70 volts with no load. I had to "calibrate" the voltage regulator by bridging a 20K resistor across resistor R78. It now comes up at 4.98 volts, and after running a couple of minutes, stabilizes at 5.00 volts. With a 2.5 ohm load, voltage goes UP to 5.25 volts, and stabilizes at 5.27 volts after a couple of minutes. I was expecting the voltage to drop slightly under heavy load, but just the opposite happened!

So under load, 5.27 volts 2.5 ohms 2.1 amps 11 watts

If anyone is interested in a drawing on this one, I can post one later.

[everell]

Re: Bestec ATX250-12E up and running!

Here is a drawing of the ICE circuit and a couple of pictures of the add-on board.

[Toasty]

Re: Bestec ATX250-12E up and running!

Excellent!

Less wire and fewer components. Every bean counters dream!

[edlx]

Re: Bestec ATX250-12E up and running!

Wow, impressive! When I first started searching this problem PS, I had steeled myself for the usual river of vague garbage posts and commercial ads.. You guys have nailed this to the wall.. Very good job! Spot on!

[everell]

Re: Bestec ATX250-12E up and running!

Again I am working on a Bestec ATX-250 12E. After fixing the 5vsb circuit, I find that the 5 volt rail is measuring only 4.96 volts and the 12 volt rail is only 11.93 volts. 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!

Rather than doing a lot of guesswork to figure out the best value for a replacement R60 resistor, I decided to install a variable resistor. I used a Bourns 50K trimpot part number 3266W-1-503LF. Digikey part number 3266W-503LF-ND. Before installing the trimpot, use an ohmmeter to center the trimpot by measuring from the middle pin to either side. Adjust the trimpot for about 25K. Then install the trimpot using the center pin and one of the other pins. Just bent the unused pin out of the way. Once the trimpot is installed, power up and adjust the output voltages as desired. I set mine at 5.10 volts and 12.24 volts.

The first picture shows the four resistors which are used in the regulator circuit. All four are surface mount resistors on the bottom of the pc board. The next picture shows R60 removed and two holes drilled for the added variable resistor.

This technique can be used on other Bestec models too, such as the Bestec ATX-250 12Z, the Bestec ATX-300 12E, and the Bestec ATX-300 12Z.

[everell]

Re: Bestec ATX250-12E up and running!

I decided to add a LED indicator to this psu like the one on a Bestec ATX-250 12Z. 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 ATX250-12E up and running!

My Bestec went kaboom. Shorted out the main FET switching transistor and took out the fuse. I wasn't sure if this failure damaged the pwm chip or not.

Why this failure? Could be that the main switching power FET just wimped out with age. Or it could have been that the pwm chip was not driving the FET all the way into saturation, and with time caused this failure. Or, there is the possibility that when this Bestec originally went overvoltage on the 5vsb, it could have done some damage to the pwm chip.

The 5vsb circuit has been fixed with a DM311 mod. I replaced the pwm chip (UC3842B) and the main switching FET transistor (9N90). To make sure that the pwm chip is causing the power transistor to go all the way into saturation, and to provide some isolation, I added a buffer driver. Removed two resistors and a diode. Added two resistors and two transistors. Take a look at the schematic of before and after, and let me know what you think. (It does work!)

[everell]

Re: Bestec ATX250-12E up and running!

I also moved capacitor C36 from between the two heat sinks (a hot zone) to the output side of the heatsink. It wasn't bulging yet, but since most Bestecs have a bulging capacitor at this location, I decided to move C36 while I had the circuit board out of the case. Removing C36 is easy, and soldering C36 in a new location is also easy. The hard part is to determine where to drill the two new holes for mounting the capacitor. So here is a picture showing the best location with holes drilled using a dremmil tool and a very small drill bit.

[pronet]

Re: Bestec ATX250-12E up and running!

I took the hand-drawn schematic of this power supply and put it in a cad program and redrew it.. I have it in .pdf format if anyone is interested. Just email me and ask for the schematic and I will send it to you. Thanks to the person who posted up the hand-drawn version of this, you made my day with the information. Been in the computer business 30 years, and this is one of the schematics that I have needed for a long time.

John
Techno dude