Vintage 130W IBM5160 Schrack Elektronik EG07168-E PSU Repair - Page 1
Forum section: electronics / repair

author: hozone
author_rank: Regular Contributor
timestamp: September 08, 2023, 03:56:39 pm
content: Hello,I'm trying to repair an old PSU of an IBM 5160.It's a Schrack Elektronik EG07168-E, 130W supply.The problem is that on the 12V and 5V line I've low voltage output.Of course I can not find any schematic for this or similar models.I'm not an expert in switching supply, expecitally old design one. So I'm here asking your help.Those kind of supply needs some load to work, but even with load I read low voltage.With PSU disconnected I can read+9.7V almost (on the +12V line)+4.7V almost (on the +5V line)With all connected (and PC powered on - note: there's no problem on motherboard and devices cause even with an external "working" load, like 2 or 3 HDD same low voltage behaviour)+10.5V almost (on the +12V line)+4.4V almost (on the +5V line)With just one "modern" PATA HDD connected+10.5V almost (on the +12V line)+4.4V almost (on the +5V line)What I've done so far:Resoldered all the joint.Replaced the 3 main switching transistors (BUT11A) they are working but I have 3 new one so I replace them: ok.Checked output capacitors: ok. (at present I've connected spare one)Checked the others "big" capacitor: ok.Checked the two "big" diode: ok.Checked the only 7815 rugulator: 15.10V out. Injected 15V here, request is 150mA, and makes no difference if is 15V bang on down to 10V.I can't find a PWM switching regulator chip, just some comparators UPC339G and one HEF4013P (for protection circuit).BUT I've notice that If i warm up (with a hot air soldering station) the secondary PCB mounted, the only with some SMD component, main voltage output goes down. It has something that seems Carbon Printed Resistors. Seems to be this board to control the output.Tracing the board is somehow difficult cause it goes into "unknown" or custom model coils.I don't know if it's something related to the feedback flayback regulator or other. Do the comparators works as flyback regulator for the switching?I attach here some picture of the PSU I've found on web. Mine is like this. My pictures between those one are the one of the SMD board.Any suggestion appreciated.

author: Tim@psutest.com
author_rank: Newbie
timestamp: September 08, 2023, 05:02:08 pm
content: Check out this link:http://asknotes.com/2021/09/16/ibm-5150-psu-repair/

author: fmashockie
author_rank: Frequent Contributor
timestamp: September 09, 2023, 03:30:50 pm
content: You really need to provide more info to get more help on this issue. You have to make an effort to trace out this circuit a bit. Especially if it is not clear to you from looking at it how it works. For example, you have two quad op-amps here. One of those could be used as a PWM for switching. From the photos you shared, the board is one sided, which means it should not be that difficult to trace some of it out.If you are saying the 7815 is outputing 15V, then I would suspect your issue downstream of that. Check all those electrolytic caps on the secondary side first.

author: hozone
author_rank: Regular Contributor
timestamp: September 10, 2023, 08:31:32 am
content: Thanks,@Tim@psutest.comthe asknotes.com post unluckily does not help me, it's a different damage.@fmashockieAll caps tested. No problem at all.At visual inspection nothing seems wrong.I have the hope it was something "simpler", as I said tracing is a bit of a mess. Anyway, find attached the main output trace, and also what I think is the trace of the right zone (the one with MC3403) of the SMD board. With picture. This is the zone that warmed up change the voltage output. Feedback from +5 and +12 seems to go in the middle C339G.

author: xavier60
author_rank: Super Contributor
timestamp: September 10, 2023, 12:22:49 pm
content: The MC3403 being opamps likely for error amp, the blue diode could be the reference Zener.The 339s can be used for various functions like, oscillator, PWM comparator, steering logic, rail monitor. The 4013 would be the flip-flop for the steering logic.Faulty Zeners are usually temperature sensitive.

author: xavier60
author_rank: Super Contributor
timestamp: September 10, 2023, 01:32:08 pm
content: With that large, gapped inductor, could this be a resonant converter?The output inductor doesn't seem large enough to suit a half-bridge PWM type converter.Id like to see the coil drawn between the main transformer and diodes. Transductor maybe?

author: fmashockie
author_rank: Frequent Contributor
timestamp: September 10, 2023, 02:33:57 pm
content: Nice job tracing out the circuit a bit!I've definitely seen this kind of issue due to an intermittent/bad zener diode part of the feedback circuit for other SMPS I've worked on.That being said, if the feedback circuit was the issue, I'm having trouble seeing how that wouldn't affect the 15V rail. OP says the 15V is measuring fine correct?

author: hozone
author_rank: Regular Contributor
timestamp: September 11, 2023, 05:36:20 am
content: @fmashockieThe only diode i can see in the SMD additional board is the blue one... and maybe the other thing on top right? Never seen a component like that.On the main PCB there are a lot of diodes.The 15V rail comes from a 7815, and it's fine at 15.1V almost. I've also removed the 7815 and inject external 15V from my bench supply. All works fine (well with lower output error but works). Down to 10V, then it stop working.@xavier60I'm not an expert in SMPS, expecially old one design. Both the diodes for the 12V and 5V rail comes from the large yellow transformer. Then on both the lines there are two smaller inductor, I think for filtering signal.I think the 15V rail supply the power to this board (and the other component next to this board... R,C,D one UPC339C and one HEF4013). My suspicion goes to this board cause if I warm this up, voltage goes down. But maybe that's something "normal".Maybe Ive to trace the feedback too, but this is starting to be an hellDo you think measuring the PWM on the base pin of BUT11A could help?

author: xavier60
author_rank: Super Contributor
timestamp: September 11, 2023, 06:03:21 am
content: Looking at the PWM might tell us something. It will be safest to probe it at one of the transformer outputs. You could also check the 5V and 12V outputs for excessive ripple.That BUT11A at the corner of the PCB near the large black reservoir capacitor. If it has a direct connection to the large, gapped inductor, means the PSU has a PFC stage which would be unusual for such an old PSU.Can you direct heat directly at the blue diode on the small board while to see what effect it has on the output?Or heat it directly while off with a soldering iron, then switch it on while the diode is still warm.

author: xavier60
author_rank: Super Contributor
timestamp: September 12, 2023, 02:29:46 am
content: If it does have a PFC stage, a fault in the PFC could result in low output voltages.

author: hozone
author_rank: Regular Contributor
timestamp: October 22, 2023, 05:55:49 pm
content: Finally I had time to make a couple of tests. I'm so sorry for my delay.I'm using a portable DSO150 oscilloscope to prevent my main scope from damage, don't want to risk with high voltage.I've tested both three BUT11A. The two next to the "big" transformer seems to have a 21.5k base square both. If i warm the "smd" circuit the freq rise up to 21.7k slowly and the voltage goes does. But the 21.5k and square shape makes me think there's nothing wrong there.Then I've tested the first BUT11A, the one on the high voltage side. It has an "ECG" like shape. See attached picture (probe at 10x) connected from Base to Emitter.

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2023, 08:17:20 pm
content: Show the waveform at one of the output stud diodes. If it's full duty cycle, it could mean that the PFC stage isn't working properly.

author: aeg
author_rank: Regular Contributor
timestamp: October 22, 2023, 10:26:18 pm
content: I don't know the situation in whatever market this was made for, but in the US, PC power supplies certainly did not have PFC in 1983.

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2023, 10:58:23 pm
content: Quote from: aeg on October 22, 2023, 10:26:18 pmThere needs to be a purpose for the large, gapped inductor on the right and the 3rd BUT11A. Buck regulator maybe.

author: xavier60
author_rank: Super Contributor
timestamp: October 23, 2023, 12:54:57 am
content: A Buck regulator feeding the half-bridge stage wouldn't need an output capacitor. The power path needs to be traced out.

author: p.larner
author_rank: Frequent Contributor
timestamp: October 23, 2023, 01:36:50 am
content: what scope is that?,it looks handy for low freq stuff.

author: hozone
author_rank: Regular Contributor
timestamp: October 23, 2023, 05:57:35 am
content: Thanks all. Now I'm going to office, this evening I'll do the tests.DSO150 has 200kHz bandwith, not that much, but with high voltage and possibly mains connection my main bench scope may risk.

author: hozone
author_rank: Regular Contributor
timestamp: October 23, 2023, 05:38:20 pm
content: Attached the scope output from one diode, both looks the same. Probe at 10x. Ive take the signal from the output of the diode (anode) and the catode, but I think I've made a mistake, which should be the ground for this measurement?Also the circuit traced from output to the big yellow transformer. The two big diodes (the one I've take measurement) are the one on top. I think there's a sensing wires on the inductors you find in picture.It's difficult to trace it out more due to the lack of my knowldege on this task, and due to the fact that I read 0.15 to 0.20 ohm on trace there, but that may be winding of the transformer.

author: xavier60
author_rank: Super Contributor
timestamp: October 23, 2023, 08:03:39 pm
content: That output stage is unusual with the 5V being choke filtered and the 12V being capacitor filtered. The reason could be to achieve independent 5V and 12V regulation. You could check the waveforms at the 5V and 12V diode cathodes for now with respect to ground.I would like to know how the large, gapped inductor is being used just to get a better idea of how the PSU operates.

author: hozone
author_rank: Regular Contributor
timestamp: October 23, 2023, 09:01:59 pm
content: Thanks!I'll check waveforms tomorrom morning before office.As for inductors, do you mean number 1, 2, 3, 4 or other?3 and 4 are the output filtering inductors.1 and 2 are a bit difficult to trace, It seems to me (looking at the traces) they are "sensing" output and input like in the image... but without disconnecting wires it's difficult, cause it's a mess of sub ohm values reading in that zone.

author: xavier60
author_rank: Super Contributor
timestamp: October 23, 2023, 09:49:58 pm
content: This one. Also, what is your mains voltage?

author: hozone
author_rank: Regular Contributor
timestamp: October 24, 2023, 06:18:47 am
content: Attached the two wave at the output of the diodes, not much ripples and voltage almost the same read from the multimeter.Forget to mention that there are a couple of trace, from +12 and +5 that goes in the SMD circuit, ?probably for feedback?The 300V input positive goes in the first BUT11A (the one with the "ECG" form), then goes to this inductor you are asking me, then to the yellow inductor.

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2023, 06:56:03 am
content: The single BUT11A and the large inductor is a Buck regulator feeding a push-pull stage consisting of the BUT11A pair and the yellow transformer with center tapped primary. The blue capacitor near the large inductor which I thought was the coupling cap for a half-bridge stage now appears to be a snubber for the BUT11A pair.Except maybe for the messy Buck drive waveform, I think that there are no faults in the power paths.You could scope the high voltage waveforms for curiosity if can be done safely.That Zener diode on the SMD board is now suspect.

author: hozone
author_rank: Regular Contributor
timestamp: October 24, 2023, 09:38:30 am
content: Thanks!Quote from: xavier60 on October 24, 2023, 06:56:03 amWhere do you want me to test it?Quote from: xavier60 on October 24, 2023, 06:56:03 amYou mean the blue one (number 2 in image)? number 1 I don't even know what It can be.Back from office I'll test.

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2023, 09:56:37 am
content: Id like to see the waveform at the Emitter of the single BUT11A and the Collector of one of the BUT11A pair.Both with respect to live side ground, the negative of the large black 385V capacitor.This is the live side of the PSU, referenced to the mains supply. What safety precautions with you be taking?The blue diode in the photos in post #3. You could try to read its part number or carefully measure the voltage across it.I sometimes tack in some wires from an old IDE cable and run them to a DMM. Avoids probe slip accidents.Reference Zeners are almost always 5.6V.

author: fmashockie
author_rank: Frequent Contributor
timestamp: October 24, 2023, 02:19:53 pm
content: I've had feedback circuit issues related to the zener diode on SMPSs where the zener still appears to test fine (when the circuit is not live such as with a DMM on diode test function), but it ended up being the issue. Especially if thermal stress appears to make the problem worse. I would be suspecting that blue zener diode as well.

author: hozone
author_rank: Regular Contributor
timestamp: October 24, 2023, 05:56:55 pm
content: THANK YOU! To all the people who help me during this repair, expecially @xavier60.The guilty is the 1N823, thermally compensated zener diode. (https://4donline.ihs.com/images/VipMasterIC/IC/AMPD/AMPDS00002/AMPDS00002-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0)I've put a 1N4735A (6.2V zener), I get now 12.35V and 5.3V almost with the actual load. (A pretty old pata 40Gb HDD).Find attached a few picture. (I may have to replace the blue cap - should be 1n5K400 from another picture I have - that I've accidentally melt a bit).Now I have to find a seller with decent shipping price. This diode is 4Eur almost and 20Eur shipping :-(I must admit that the 1N4735 that I've mounted works well, even if I heat up with the heat gun it goes up to 5.35V and 12.45V, which I think could be in the IBM 5160 motherboard limits.Now I'm going to reassembly that PSU and check it connected with the PC.What's your opinion about leaving the 1n4735 there?

author: CJay
author_rank: Super Contributor
timestamp: October 24, 2023, 06:27:47 pm
content: Ooof, I *think* that's the PSU from an IBM XT-286.We used to repaiur lots of them and found that they fried themselves quite nicely and comprehensively.Our fix was a rewound driver transformer (it gets destroyed) and a pile of other replacement parts as a kit but the details of that are *long* lost to time which is entirely unhelpful.

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2023, 10:34:07 pm
content: Leaving the 1N4735A sounds reasonable to me.If you want to alter the voltage, you could try fitting other 1N4735A's from your stock which could give you slightly different voltages.

author: hozone
author_rank: Regular Contributor
timestamp: October 25, 2023, 05:19:31 am
content: Thank you,today I'm going to reassembly the PSU, and tomorrow I hope I've time to mount this on the IBM and let you know.

author: hozone
author_rank: Regular Contributor
timestamp: October 26, 2023, 05:03:07 pm
content: Aseembled and working again!THANKS!

author: fmashockie
author_rank: Frequent Contributor
timestamp: October 27, 2023, 02:12:25 pm
content: Awesome! Glad you were able to get it running again!