Author Topic: Help with dead AT PC Power Supply  (Read 2847 times)

0 Members and 1 Guest are viewing this topic.

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Help with dead AT PC Power Supply
« on: October 06, 2024, 01:40:42 pm »
Here I have a dead AT power supply from 1994. I'd like to fix it, because the slim line form factor makes it quite rare and it lives in a nice pizza box PC case.

I'm not getting any power output at all.

I identified a faulty double diode S30SC4M, which had a dead short on the right hand side, it was therefore shorting the +5V rail. That short disappeared with a replacement diode, but still didn't give me any power output.
There are no blown out or burnt parts. I checked the electrolytic caps. The MOSFET that lives to the right of the S30SC4M is also fine, so is another double diode that is in above the S30. Then we have two power regulators for -5/-12V (fine).
There is a number of mystery components in shrink warp on the very top in the middle of the picture. I don't know what's on that board. I haven't discovered voltage regulators for +5/12V, could they be in there? The components seem to be too small for that, also they're not on a heatsink.
The primary fuse is ok.
There are two RIFA caps, which for the moment are not cracked (can I take them out of the circuit without replacements, for now).
What would be the procedure to test for faults from here?
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #1 on: October 07, 2024, 01:24:40 pm »
Hi!

You have to find PWM controller, find a datasheet and check the supply voltage.
But seems the controller is on separate black PCB.

Could you provide PCB bottom as well.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #2 on: October 07, 2024, 01:33:59 pm »
Picture of the back attached.
Is the PWM generating the +5/+12V?
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #3 on: October 07, 2024, 01:36:02 pm »
PS: I've taken the thyristor out for testing, that's why there's three empty holes on the back.
 

Offline wasedadoc

  • Super Contributor
  • ***
  • Posts: 1686
  • Country: gb
Re: Help with dead AT PC Power Supply
« Reply #4 on: October 07, 2024, 01:54:19 pm »
PS: I've taken the thyristor out for testing, that's why there's three empty holes on the back.
Thyristor?
 

Online tunk

  • Super Contributor
  • ***
  • Posts: 1099
  • Country: no
Re: Help with dead AT PC Power Supply
« Reply #5 on: October 07, 2024, 02:09:44 pm »
How did you check the capacitors?
Caps that visually look ok may still be bad.
Beware that PSUs have high voltages that may be fatal.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #6 on: October 07, 2024, 02:13:33 pm »
Yes, a TYN412.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #7 on: October 07, 2024, 02:15:40 pm »
How did you check the capacitors?
Caps that visually look ok may still be bad.
Beware that PSUs have high voltages that may be fatal.
Yes, I  checked all caps with an ESR meter.
There was only one which was out of spec, 680uF instead of 1000uF, which I replaced.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #8 on: October 07, 2024, 02:57:19 pm »
Is the fault more likely to be on the secondary side?
It appears there's only one component under the black shroud on the secondary side, and it has three contacts. Would that be the PWM?
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #9 on: October 07, 2024, 05:25:32 pm »
Please take another photo to get a better view of these areas.
« Last Edit: October 07, 2024, 06:54:24 pm by Harry_22 »
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #10 on: October 08, 2024, 05:40:51 am »
As requested. What are we looking for?
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #11 on: October 08, 2024, 09:34:15 am »
Here we are!
Please see the PCB drawing. Only the DIP8 chip (bottom left) remains unknown.
The thyristor protects against overvoltage. It simply short +5V rail.

Apply the voltages marked in red from an external power source one by one and check that there is no short circuit. Then measure the power on the controller (marked in yellow).

The source file can be viewed in Paint.NET
https://drive.google.com/file/d/1oZoXP_6CauAqUHhR-VeVlyOd5lAP2KV1/view?usp=sharing
« Last Edit: October 08, 2024, 11:34:33 am by Harry_22 »
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #12 on: October 08, 2024, 11:52:06 am »
My external PSU can only do positive voltages.  And did you really mean -7 and -15V, or -5 and -12V?
Wouldn't any shorts also show up without any external power supplied?
Do you want me to measure the voltage at the yellow points when the PSU is powered normally, i.e. no external power supplied?
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #13 on: October 08, 2024, 02:05:24 pm »
So I turned on the PSU as normal and measured at the yellow points.
I'm getting 91mV. Yes, millivolts.
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #14 on: October 08, 2024, 03:42:29 pm »
My external PSU can only do positive voltages.  And did you really mean -7 and -15V, or -5 and -12V?
Just swap the positive and negative wires. Yes I mean -7 and -15V to get -5 and -12V after installed voltage stabilizers. Both are placed on PCB.
Please check -5 and -12V at output connector.

Quote
Wouldn't any shorts also show up without any external power supplied?
Yes. We do it rail by rail to check PS functionality.

Quote
Do you want me to measure the voltage at the yellow points when the PSU is powered normally, i.e. no external power supplied?
Yes you already did that. You got basically zero voltage but expected around 13-15VDC.
Let's move on.
Please carefully check the voltage between the big capacitor pins. Expected 320 VDC.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #15 on: October 08, 2024, 03:55:02 pm »
I'm getting 370V at the capacitor.
I will proceed to test the other lines.
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #16 on: October 08, 2024, 03:59:32 pm »
Always remember that the high-tension capacitor needs time to discharge. Especially in non-working PS units!
Monitor it with a voltmeter.
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #17 on: October 08, 2024, 04:02:08 pm »
Check also C8 for short by multimeter.

PS
There is some high voltage in your mains as much as 260 VAC!
« Last Edit: October 08, 2024, 04:07:20 pm by Harry_22 »
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #18 on: October 08, 2024, 04:08:41 pm »
Yes, thank you. The capacitor discharges quite quickly by itself.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #19 on: October 08, 2024, 04:36:40 pm »
I measured all the rails, and I'm getting the correct power output on all of them.
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #20 on: October 08, 2024, 05:11:05 pm »
Check the following points in yellow. (Without mains connection.)
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #21 on: October 08, 2024, 05:30:31 pm »
The diode is ok.
The resitstor measures 91kOhm
The capacitor measures 0.016V in diode mode.
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #22 on: October 08, 2024, 05:44:59 pm »
The controller power line is shorted. Unsolder C8 capacitor and jumper wire next to it that goes to 91k resistor and measure again.
 

Offline naujoksTopic starter

  • Regular Contributor
  • *
  • Posts: 64
  • Country: de
Re: Help with dead AT PC Power Supply
« Reply #23 on: October 08, 2024, 05:51:41 pm »
I measures the same without the capacitor and jumper wire.
0.016V
 

Offline Harry_22

  • Frequent Contributor
  • **
  • Posts: 603
  • Country: 00
Re: Help with dead AT PC Power Supply
« Reply #24 on: October 08, 2024, 06:03:15 pm »
Perfect!
Take external DC power supply. Set up 1V and 1A current limit and check the current between power lines by short them.
Connect external PS between Vcc and Gnd of controller.
Try to find a hot point on this Black PCB. If possible use the thermal imager.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf