Help with dead AT PC Power Supply - Page 1
Forum section: electronics / repair

author: naujoks
author_rank: Regular Contributor
timestamp: October 06, 2024, 01:40:42 pm
content: 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?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 07, 2024, 01:24:40 pm
content: 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.

author: naujoks
author_rank: Regular Contributor
timestamp: October 07, 2024, 01:33:59 pm
content: Picture of the back attached.Is the PWM generating the +5/+12V?

author: naujoks
author_rank: Regular Contributor
timestamp: October 07, 2024, 01:36:02 pm
content: PS: I've taken the thyristor out for testing, that's why there's three empty holes on the back.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 07, 2024, 01:54:19 pm
content: Quote from: naujoks on October 07, 2024, 01:36:02 pmThyristor?

author: tunk
author_rank: Super Contributor
timestamp: October 07, 2024, 02:09:44 pm
content: 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.

author: naujoks
author_rank: Regular Contributor
timestamp: October 07, 2024, 02:13:33 pm
content: Yes, a TYN412.

author: naujoks
author_rank: Regular Contributor
timestamp: October 07, 2024, 02:15:40 pm
content: Quote from: tunk on October 07, 2024, 02:09:44 pmYes, I checked all caps with an ESR meter.There was only one which was out of spec, 680uF instead of 1000uF, which I replaced.

author: naujoks
author_rank: Regular Contributor
timestamp: October 07, 2024, 02:57:19 pm
content: 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?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 07, 2024, 05:25:32 pm
content: Please take another photo to get a better view of these areas.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 05:40:51 am
content: As requested. What are we looking for?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 09:34:15 am
content: 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.NEThttps://drive.google.com/file/d/1oZoXP_6CauAqUHhR-VeVlyOd5lAP2KV1/view?usp=sharing

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 11:52:06 am
content: 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?

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 02:05:24 pm
content: So I turned on the PSU as normal and measured at the yellow points.I'm getting 91mV. Yes, millivolts.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 03:42:29 pm
content: Quote from: naujoks on October 08, 2024, 11:52:06 amJust 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.QuoteYes. We do it rail by rail to check PS functionality.QuoteYes 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.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 03:55:02 pm
content: I'm getting 370V at the capacitor.I will proceed to test the other lines.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 03:59:32 pm
content: Always remember that the high-tension capacitor needs time to discharge. Especially in non-working PS units!Monitor it with a voltmeter.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 04:02:08 pm
content: Check also C8 for short by multimeter.PSThere is some high voltage in your mains as much as 260 VAC!

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 04:08:41 pm
content: Yes, thank you. The capacitor discharges quite quickly by itself.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 04:36:40 pm
content: I measured all the rails, and I'm getting the correct power output on all of them.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 05:11:05 pm
content: Check the following points in yellow. (Without mains connection.)

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 05:30:31 pm
content: The diode is ok.The resitstor measures 91kOhmThe capacitor measures 0.016V in diode mode.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 05:44:59 pm
content: The controller power line is shorted. Unsolder C8 capacitor and jumper wire next to it that goes to 91k resistor and measure again.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 05:51:41 pm
content: I measures the same without the capacitor and jumper wire.0.016V

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 06:03:15 pm
content: 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.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 06:13:52 pm
content: Ok, I used IPA on the PCB and touched it, but nothing is getting warm.The bench PSU draws only 0.031A.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 06:21:56 pm
content: Increase the voltage step by step to get 1W consumption. Maximum limit is 12V.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 06:27:47 pm
content: Ok, the biggest IC on the board is getting warm.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 06:35:47 pm
content: This is PWM controller itself. Probably internal zener diode is broken.Try cleaning the black stuff off the chip top to clarify its name.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 06:58:13 pm
content: Ahhh I was hoping the black stuff is some kind of plastic shrink warp, but the material is almost like the body of an IC.I tried to scrape it off, but the model number of the IC came off as well. I thought I could see "A23", but no guarantee, and it isn't much to go by

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 07:05:25 pm
content: Don't worry.Play with the light direction. Wet the surface. Take some photos but only with a tripod.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 07:24:20 pm
content: I tried to be carefuly, but it didn't work. The number came off with the plastic. It's gone. I'm so sorry!

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 08, 2024, 07:47:17 pm
content: Ring the chip legs. Find Vcc, Gnd, Drive and CS numbers.Try to clean with hot iron.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 07:49:25 pm
content: Yes, I can access the legs, it wasn't difficult to get the material off the legs.

author: naujoks
author_rank: Regular Contributor
timestamp: October 08, 2024, 09:32:19 pm
content: It turns out that vcc, drive and gnd are all shorted out.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 08:20:24 am
content: Drill out the center of the chip with the crystal.Check all the pins for short.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 09:00:37 am
content: Ok, done. The short is gone.Here are the chip connections (see picture):Chip pin 1 - contact 2Chip pin 2 - contact 1Chip pin 3 & 4 not connectedChip pin 5 - contact 3Chip pin 6 - contact 4Chip pin 7 - contact 5Chip pin 8 - contact 6 & 7

author: pienari
author_rank: Regular Contributor
timestamp: October 09, 2024, 09:02:09 am
content: Is that dip8 just pwm controller?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 10:22:14 am
content: Please see PWM controller pinout.Double check Current Sense pin.Clean pins 4, 5, 6 of optocoupler and ring the connection as well.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 10:48:49 am
content: I drawn lines in to the picture where the pins of the PWM connect to.Is the opto coupler the chip next to the PWM? Are those the right pin numbers?Where are the pins supposed to connect to?

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 11:16:00 am
content: Please see the corrected picture

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 11:26:13 am
content: Measure C10 capacitorUse the drawing below to understand the possible connection.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 12:36:50 pm
content: Pin 4 of the opto coupler is connected to pin 6/7 on the board (pin 8 of the PWM).Pin 4 and 6 do not connect to the PWM or the board.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 01:02:14 pm
content: Ring optocoupler pins 4,5,6 in diode mode, assuming that this is NPN transistor.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 03:28:58 pm
content: I didn't know which pins to treat as B, C and E, so I just took some measurements:Red probe on 6: 0.61V between 6-5 and 6-4. OL when black on 6.Red on 5: OL between 4 and 6. Black on 5: 0.61V between 5 and 6, OL between 5 and 4Red on 4: OL between 5 and 6. Black on 6: OL between 4 and 5, 0.61 between 6.I hope that's useful.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 04:22:42 pm
content: Good.Please measure resistance between pins 8 and 4 of PWM.Double check that pin 8 PWM is connected to pin 4 of optocoupler.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 07:55:19 pm
content: The resistance between pin 4 and 8 on the PWM is 12k Ohm.There really is no continuity between PWM pin 8 and optocoupler pin 4.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 09, 2024, 08:29:05 pm
content: Quote from: naujoks on October 09, 2024, 12:36:50 pmQuote from: naujoks on October 09, 2024, 07:55:19 pmWhat pin of opto coupler is connected to pin 8 of PWM?Please send a photo with С10 marking.

author: naujoks
author_rank: Regular Contributor
timestamp: October 09, 2024, 09:43:10 pm
content: Pin 8 of the PWM is connected to pin 5 of the optocoupler.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 10, 2024, 08:32:46 am
content: Quote from: naujoks on October 09, 2024, 09:43:10 pmMuch better!Please measure the capacitance of C10.

author: naujoks
author_rank: Regular Contributor
timestamp: October 10, 2024, 08:47:16 am
content: The capacitance is 1000pF.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 10, 2024, 02:45:59 pm
content: I think that Discontinuous Conduction Mode (DCM) is used here.The reasons are as follows:- Compact dimensions dictate the choice of a smaller transformer.- High oscillator frequency of 144 kHz (Rt = 12 k, Ct = 1000 pF) which means controller using divide by 2.-The construction utilizes additional filters and snubbers to reduce EMI.So we are going to replace the broken controller with UC3844 that has 50% duty cycle.Of course I may be wrong. Please correct me.

author: naujoks
author_rank: Regular Contributor
timestamp: October 10, 2024, 02:58:28 pm
content: Ok! I don't know anything about these things, so I will order one, put it in and then we will see what happens.It will take a few days to arrive.Thank you so much for all your help so far!

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 11, 2024, 02:31:15 pm
content: Hi!Do not remove the faulty chip. Clean the pins and solder a new one on top as on the picture.Before connecting repaired AT PS to the mains perform the following tests:1. Connect a bench PSU to C8 capacitor (see the picture below). Measure voltage at the Reference point, slowly increase bench PSU output until 5V appear at the Reference point. The voltage of bench PSU should be around 16V.Reduce the voltage until 5V disappear. The power supply readings are about 10 volts.2. Turn on the controller raising the voltage up to 16V.Connect oscilloscope to the Gate point and observe meander.The frequency should be around 70kHz. Check it.3. Connect another bench PSU with 5V to AT PS output as you did earlier (see the picture below). Slightly decrease and increase the voltage while observing with oscilloscope a signal changes at the Gate point. Keep in mind that when the voltage increases 5V, the OVP circuit (thyristor) will cause a short at this rail. Disconnect 5V PSU and the short will go away.If all tests are successful, proceed to full-scale testing. Be careful. In case of MOSFET breakdown, pieces of its case, as well as controller, can injure you.

author: naujoks
author_rank: Regular Contributor
timestamp: October 12, 2024, 07:44:05 pm
content: The PWM arrived today and everything is working!All voltages are spot on. I'm so happy!You put in so much work into this, thank you sooooo much! Please check your PM, I'd like to express my gratitude.