Finding the fault in the SMPS in our PC

[Wrichik Basu]

The SMPS in our PC went out a few months back. I confirmed that the device was not working by shorting the green and anyone black wires on the 24-pin motherboard connector. This was doubly confirmed by the mechanic who came to install a new SMPS.

I was checking online for procedures to detect which component is malfunctioning. Many sites said that the fuse goes out in most cases. I checked the fuse for continuity, and it was alright.

This site has listed out procedures for checking the SMPS. I am stuck at step 7 itself. In addition, the site has not said what to do if I fail at any step.

Can you help me out? I thought of checking each component, but that will be too tedious. After the fuse, what components should I check (in order of highest to lowest priority)?

By the way, the mains in my country are at 220V RMS. My multimeter is capable of measuring frequency and capacitance, in addition to the normal voltages/currents. It is actually an AC/DC Clamp Meter.

 

[Guineafowl]

1. Characterise the fault properly. You say it ‘went out’, but what exactly? Are any voltages present on the output?

2. Basics: do a good visual inspection of the whole board - burns, popped components, bulged capacitors.

2a. Try to find the schematic, or at least a general overview of ATX supply topology. As you do your inspection, identify the main areas and key components.

3. Checking each component is tedious, so focus in on the hard workers - identify power transistors, output rectifiers.

4. It IS worth testing every electrolytic capacitor with an in-circuit type ESR meter.

5. Switch on and carefully measure voltages, starting at the input, then moving towards the output. Many switching supplies have the transformer output voltages labelled.
^^ A very broad overview, but troubleshooting circuits is hard enough without doing it remotely. Go through the steps and tell us what you find. Some pics might help.

 

[Rive]

Could you please tell us the exact type of that PSU?
Does it worth that much for you to try to repair it without proper qualifications? Most likely at some point you will have to start measure in the circuit while it is powered, you know... I would suggest to dispose it properly instead.

 

[Wrichik Basu]

Characterise the fault properly. You say it ‘went out’, but what exactly?

It just stopped running one day. As I said, even on shorting the fan didn't start.

Are any voltages present on the output?

I haven't checked that. I'll do it soon.

Basics: do a good visual inspection of the whole board - burns, popped components, bulged capacitors.

A good visual inspection didn't reveal any problem.

Try to find the schematic, or at least a general overview of ATX supply topology. As you do your inspection, identify the main areas and key components.

This is difficult. I can give you the link to a similar type of SMPS:
https://www.amazon.com/dp/B07MDKHKD3/?tag=pfamazon01-20
Finding a schematic is next to impossible. In India, most companies don't reveal the schematic of their devices unless they are under any legal obligation. I can say that our SMPS 24-pin connector doesn't have the -5 VDC white wire.

Switch on and carefully measure voltages, starting at the input, then moving towards the output. Many switching supplies have the transformer output voltages labelled.

will do that and let you know what I find.

It IS worth testing every electrolytic capacitor with an in-circuit type ESR meter.

Will this be the same as using the capacitance measuring function on my multimeter?

 

[Wrichik Basu]

Could you please tell us the exact type of that PSU?

Please have a look at the link I posted in #4.

Does it worth that much for you to try to repair it without proper qualifications?

It's not actually about worth, it's about my interest in fixing things. I have a passion for fixing old machines that don't work. I think Jim hardy had once said that (not verbatim) a machine doesn't have life, but the person who makes the machine, has. Fixing an old machine is honouring that person's work.

 

[Guineafowl]

Will this be the same as using the capacitance measuring function on my multimeter?

No. Look up ESR - it’s worth knowing about if you plan on fixing electronics.

Don’t give ME the schematic - look at it in comparison to your board, and use it to learn what does what. It all helps.

Given what you’ve said, and assuming there are no voltages, my next move would be to check the power components with the circuit unplugged. It’s wise to discharge the large filter cap on the input. I use a 180ohm power resistor.

Pics of the board, please.

 

[Rive]

Fixing an old machine is honouring that person's work.

I do agree with that in general (I have my part in restoring some decent old hardware), but the item in question is not an 'old machine' but a cheap, at most few year 'old' low cost trash which is expected to break down within a limited time anyway.
Should you fix it up and replace all the caps and aging components it'll remain a typical expendable electronics to illustrate all the dangers of using cheap components in computer.
You should be happy that it went down without bringing the PC with itself and dispose it.

I still own some famous killer PSUs from the time when ATX was a new thing (and it was common to have a PC wasted by PSU breakdown till the point not even a memory stick survived) but to keep those in historically consistent shape I does not dare to change even a capacitor.
Well, I don't plan to switch them on either... That's the only honour these kind of components have.

Ps.: to honour the work of others and practice repairing I suggest to get a decent, old transistor based audio amplifier instead. Far more fun with far less danger.

 

[Guineafowl]

I do agree with that in general (I have my part in restoring some decent old hardware), but the item in question is not an 'old machine' but a cheap, at most few year 'old' low cost trash which is expected to break down within a limited time anyway.
Should you fix it up and replace all the caps and aging components it'll remain a typical expendable electronics to illustrate all the dangers of using cheap components in computer.
You should be happy that it went down without bringing the PC with itself and dispose it.

I still own some famous killer PSUs from the time when ATX was a new thing (and it was common to have a PC wasted by PSU breakdown till the point not even a memory stick survived) but to keep those in historically consistent shape I does not dare to change even a capacitor.
Well, I don't plan to switch them on either... That's the only honour these kind of components have.

Ps.: to honour the work of others and practice repairing I suggest to get a decent, old transistor based audio amplifier instead. Far more fun with far less danger.

I have to agree with @Rive on this - fixing up worthless stuff can be educational, but perhaps poking around an ATX supply is not the best way to learn. I do have sympathy, however, as this sort of thing is how I ‘learnt’.

I recommend “How to diagnose and fix everything electronic” by Michael Geier. It’s available as an ebook. It’s slightly dated now, but covers switch-mode power supplies.

From your postings above, I’d guess you aren’t quite ready to fix mains-powered circuits. Walk before you run. It’s hard for us to guide you through diagnostics when we have to keep digressing into explaining concepts. Makes for a messy and confusing thread.Read the book, digest it, apply it to the (unpowered) circuit and get back to us with what you think.

 

[Wrichik Basu]

Should you fix it up and replace all the caps and aging components it'll remain a typical expendable electronics to illustrate all the dangers of using cheap components in computer.

Even if i fix it, I am not going to put it back into the PC. I can, however, use it for my experiments.

 

[rbelli1]

5. Switch on and carefully measure voltages, starting at the input, then moving towards the output. Many switching supplies have the transformer output voltages labelled.

One thing to keep in mind is that one or more of the heat sinks in that type of supply are often at voltage. On the input side this is ~300VDC with a sizable set of capacitors behind it. Be careful.

BoB

 

[Averagesupernova]

One thing to keep in mind is that one or more of the heat sinks in that type of supply are often at voltage. On the input side this is ~300VDC with a sizable set of capacitors behind it. Be careful.

BoB

Yep. A beginners mistake is clipping the scope probe ground on a heat sink.

 

[essenmein]

One thing to keep in mind is that one or more of the heat sinks in that type of supply are often at voltage. On the input side this is ~300VDC with a sizable set of capacitors behind it. Be careful.

BoB

To add to that, the "ground" ie return for the HV dc side of the PSU is typically from a bridge rectified mains source, is NOT the same as Earth ground, it gets full halve wave negative of the mains wrt to Earth ground. If you want to probe any thing on the HV DC side you must use either differential probes, or better, use a 1:1 isolating transformer to supply the mains to PSU you want to probe, then you can ground the internal negative, ie the portion of the circuit where the DC link capacitor negative connects to.

 

[dlgoff]

I think Jim hardy had once said that (not verbatim) a machine doesn't have life, but the person who makes the machine, has. Fixing an old machine is honouring that person's work.

See my signature.

 

[Wrichik Basu]

See my signature.

Thanks, I knew it was in someone's signature, but forgot that it was you.

 

[Tom.G]

A schematic of a 200W PS is available here. The general circuit is the same across a wide range of power ratings. The higher power ones will have larger value capacitors, higher power transistors and rectifiers, and the details in the lower right of the schematic may vary by manufacturer.

http://archive.espec.ws/files/at200w.rar
Cheers,
Tom

(Have Fun!)