some help needed on bestec 300w 12 Z

[sam67]

Finally got the caps for this bestec from here on badcaps.net store..

Checked them all with esr meter before putting them in and all were good (by the way all the old caps that i removed tested good,i just changed them as a matter of precaution hoping they were the problem..Now while waiting for my caps (and other projects i have been playing with) i thought i would check a few of the components on the psu ,resisters diodes ,zenor diodes ,n channel fet and some schotty diodes as well as the optocouplers and transformers all tested good..but i did not test all the transistors and those blue ceramic caps . Thermistor looked good no signs of failure that i could see .

Soldered the new caps in and powered on psu and still same problem as before where when i press the power button on pc, all fans spin but psu does not boot the pc every-time .

I then power off and repress power button and the pc boots fine ..this happens every now and again..it might boot 5 times in a row and then not at all until i restart it again after one or 2 attempts ..

I tested all voltages with prime 95 and all are there but i noticed on the purple wire that is +5V SB the reading is fluctuating from 5v to 5.01 when idle but as soon as i press the power button it reads a steady 5v ..

I have the exact same hardware installed and not added anything new.

Any one have an idea what i could next check ?? what could cause this intermittent starting ?? i don't want to give up as it is annoying me and i now want to find out the cause..It is defo the psu and not the pc as i have another psu and all works fine .

It's as though some think is not triggering the mobo to start when it does not boot..I press power button and when it starts i know from the sound whether or not it will boot because the fans seem to slow for a fraction of a second when it boots normally and it does not do that when it does not boot , the fans just seem to spin without that slow down if you can understand what i am saying lol as it is hard to describe the sound i hear.

I appreciate any help as i don't want to chuck the psu when there has got to be a cause for this fault.

Thanks

sam67

[sam67]

Re: some help needed on bestec 300w 12 Z

Just took the psu apart to measure across big filter caps..There are 2 680uf 200v caps,i just hooked it up to a spare hdd and jumped the green and black wires to start up the hdd ..voltage across one cap was 162v and the other was 167v ..Don't know where to go from here

[everell]

Re: some help needed on bestec 300w 12 Z

I would suggest that you build (or buy) a power supply tester that has a LED indicator for each of the power supply voltages. I removed the ATX power connector from a bad motherboard, then added the LED and dropping resistor for each voltage. I installed a ON-OFF switch between PS-on and ground to turn the power supply on and off.

I used amber for the 5vsb, green for +12, +5, and +3.3, and red for PS good. This gives a good visual indication of what is happening. You might have a problem in the timing of the PS good signal which will cause start up problems.

I posted a thread earlier about this power supply tester with more photos and schematic drawing. Refer back to it if you decide to build one.

[sam67]

Re: some help needed on bestec 300w 12 Z

Dam i missed your post on power supply tester and i am waiting for a tester to come that i ordered (mainly for a quick go no go type of tester),does basically the same but would have been nice to make my own..still reckon that a dvm will still be the most accurate and thats whats doing my head in now because all voltages are there.

[sam67]

Re: some help needed on bestec 300w 12 Z

No one any ideas what i could check next to find this intermittent fault ,it is bugging me and i don't want this to get the better of me ..It's not that i need this psu as i have a spare anyway it's finding out what has caused this and once i can find the problem i can repair it.

[sam67]

Re: some help needed on bestec 300w 12 Z

Come on guys what could cause this psu to do this ? I can rule out the caps and transformers so what would you guys do now .Do i bin it and admit failure ,which i don't really wanna do as this is a good learning experience for me .. Do you guys come across a problem that you cannot solve and just move on or do you stick with it ? It's just a shame as i have recapped it and still no joy ..same problem as before.

[Khron]

Re: some help needed on bestec 300w 12 Z

It would be nice if you had access / could borrow an oscilloscope (and knew how to use it). I had to deal with a recapped / re-chipped Antec which wouldn't start up, just give a "blip" when powered on. I scoped the output voltages on power-up, and they didn't get up to the minimum required for the voltage supervisor IC to keep the psu turned on. Turned out that a reduction in the soft-start cap, near the main PWM IC cured the ailment, and that psu's still alive and kickin'

You WOULD have to trace out the circuit around the PWM IC though, and check the datasheet...

[sam67]

Re: some help needed on bestec 300w 12 Z

Hi thanks for the response ..i have been looking at fleabay at some scopes ..but i don't have a clue how to use one lol.. I don't mind buying one but not sure what i would need as i would like one to learn more .. My only trouble is i do not have anyone to learn from so i would have to rely on getting it set up right using the internet i suppose.I am a quick learner but it's not as easy when you are new to this stuff.

[everell]

Re: some help needed on bestec 300w 12 Z

I had an unusual problem with a Bestec ATX-250 12Z (circuitry very much the same as your 300 watt version). After reviewing my notes which I made more than a year ago, seems that the 12 volt line was loading the power supply. This symptom seems in line with your problem, the fan sometimes loads properly, sometimes doesn't. The fan is on the 12 volt line.

The reason I recommended the tester with LED indicator lights was so that you can observe for any irregularities with the PGO (power good output) signal.

On my 12Z, coming off the main switching transformer providing the 12 vac, is a parallel resistor combo R38 and R39, each being 431 ohms. This feeds a diode, which then feeds a resistor network leading to the PGI (power good input) of the supervisory chip. This network is where I found my problem.

There is a group of four 10K resistors labeled R48A, R48B, R48C, and R48D. R48A and R48B are in series, making 20K. R48C and R48D are in series making 20K. Then these two strings of 20K each are in parallel making the network the equivalent to 10K. This 10K network and resistor R41 20K form a voltage divider to provide the proper amount of voltage level for the power good to operate properly. All of these resistors are tiny surface mount resistors on the bottom of the pc board.

On my 12Z, one of the 10K resistors in the four resistor network was bad, making the effective resistance of the network lower enough to give unpredictable results for power good signal, which caused the problem. As I remember, I was able to see this problem using the power good LED indicator.

[everell]

Re: some help needed on bestec 300w 12 Z

Did you check the resistors listed in my last post? They are the resistors inside the red circle of the attached picture.

[sam67]

Re: some help needed on bestec 300w 12 Z

Did you check the resistors listed in my last post? They are the resistors inside the red circle of the attached picture.

Hi everell ,

yes i did check the resistors like you said and they were fine ..i have put it to one side for now as i am working on a monitor i got and also trying to understand how to operate my scope i just bought.

Thanks once again for your input as i do appreciate it . I will come back to the psu later as i would like to do a few tests with my scope once i get the hang of it lol.

Best

sam