PC PSU repair

[fractal5]

I have a PSU which turns on only sporadically. The PSU also makes a weak humming noise coming from the electronics. If I lightly tap the PSU it might turn off. Drawing power from it seems to make it work less often.

I know poking around inside the PSU is probably not such a good idea if I don't know what I'm doing. But I thought this would be a nice opportunity to make myself learn more about PSUs. I don't care about the most cheap way to get a new one (I already have a spare), I'd like to repair this one, even if I have to buy a component that costs a large fraction of the price of the PSU.

Is it possible to make an educated guess about what is wrong in my case? What, statistically speaking, is most likely to have failed given these symptoms? The PSU is quite new, and I couldn't find anything immediately bad inside it, like a broken solder joint, loose components, swollen capacitors.

Edit: Pictures are posted, see further down.

 

[DerStrom8]

Usually the first things to go bad in a PSU are the electrolytic filter capacitors. They're the big ones right after the transformer. Check to see if they are bulging or leaking--those are sure signs that they have gone bad. If you do not see any bulging or leaking capacitors at all, that does not necessarily mean your capacitors are ok. You'll probably need to check them with an ESR meter.

That's where I would start, anyway. Is there any chance you could post some photos of the inside of the PSU in question?

Regards,
Matt

 

[fractal5]

Usually the first things to go bad in a PSU are the electrolytic filter capacitors. They're the big ones right after the transformer. Check to see if they are bulging or leaking--those are sure signs that they have gone bad. If you do not see any bulging or leaking capacitors at all, that does not necessarily mean your capacitors are ok. You'll probably need to check them with an ESR meter.

That's where I would start, anyway. Is there any chance you could post some photos of the inside of the PSU in question?

Regards,
Matt

I will post some photos, thanks.

 

[Reloadron]

This is a drawing (schematic diagram) of an older 200 watt ATX form factor PSU.

While we seldom see a 200 watt version today the principal or theory of operation has not really changed much between a unit like this and a 1 KW ATX PSU. The link is good because it does a good job of explaining the theory of operation. The best way to fix or repair something is likely by starting with a solid understanding of how it works. While looking at for a burned resistor all you know is that you see a burned up resistor. Let's say I see a burned 2.2 K resistor and conclude that is my problem so I replace it. I turn on my PSU and in a heartbeat I now have another burned up 2.2 K resistor. All I am seeing is a symptom and the symptom has a cause. Unless I know the root cause looking at symptoms all day long will result in a large pile of toasted burned up 2.2 K ohm resistors.

The front end (power in) of an ATX power supply does deal with some high voltages. In the linked to schematic capacitors like C5 and C6 build up a pretty beefy charge. Note how the input voltage select switch is used and how on 120 VAC operation a voltage doubler is formed. OK, so I rectify 240 VAC to 240 (approx) DC. The caps will charge to the peak value of 240 VAC which is 1.414 X 240 = 340 VDC which will absolutely ring your bell if you get across those capacitors. Those are what DerStorm8 is referring to but this way you see them up close and personal in the drawing.

All power supplies like this one and later rely on the 5 V SB (5 Volt Stand By) power to start. Read the link and get a handle on how that works. Q1 and Q2 are the big deal as they form the inverter oscillator that drives transformer T3 in the schematic. Their size and ratings in conjunction with T3 size determine what the PSU power is capable of.

Based on your description my guess would be a cold solder joint or poor connection on the circuit board. Purely a guess. Without a schematic or drawing of a specific model these animals are difficult to trouble shoot. They are overall made inexpensive with low quality control standards. Anyway, maybe the link will help get you going.

Ron

 

[tvtech]

Hi Ron. You try so hard to help

SMPS....I hate them. Unfortunately, they are here to stay. And getting smaller, more efficient and cheaper as time goes by...

I deal with them everyday of my life as a CRT tech....no modules to swop and change and get lucky with CRT....you either fix the blown PSU yourself or reject the entire set/TV. The PSU and the TV are one on one common PCB...

I often think about the Guys who are in the business of fixing LCD, PLASMA, LED and other newer TV's....they have the luxury of having boards to swap out to isolate problems.....no real repairs..except the obvious of "caps"........Replace the Electrolytics blah, blah, blah. Monkey see monkey do. Almost every answer lately about SMPS problems is "change the Caps". Even my Gran knows that by now.

So no real board level repairs anymore. Changing boards is easier.

So, that is why knowledge on actual SMPS repairs is going downhill fast. Nobody cares anymore to try and fix them or understand them...Hey!!! just buy another one. Certainly cheap enough anyway.

A sad state of affairs indeed.

I was going to post more...but why bother??
Except one little thing: A SMPS is only as good as it's Main Smoothing Cap.....that can be rated up to 450VDC....which is available easily...that means that it can "theoretically" handle a Mains Surge of around 280VAC MAXIMUM.....not continious...otherwise it pops...

Money for jam for us here....I fixed five sets here today...all from a Guest House up the road..all urgent...all Main Smoothing Caps popped.

Some blew the Bridge Rectifier as well and the Mains Fuse.....but not one blown Chopper Stage...... SMPS is a money maker here

Regards,
tvtech

EDIT: Sorry for the rant...had to get it off my chest

 

[Reloadron]

Hey tvtech, agree 100%. Personally unless it can be a quick simple fix I remove and replace with known good PSU. I can buy a PSU to replace most ATX form factor junk for $29 USD so why screw with it. Hell even now retired with time on my hands I would rather clean a gun or spend a day at the range than screw with a PSU. However, that said, the moral of my initial post is simple. If you expect to actually do fault diagnosis on something it is sort of essential to understand how things work. I figure if the OP wants to try and fix something I would trash, then cool with me.

Ron

 

[tvtech]

Thanks Ron ^^^

Stay well and sleep well.

All the best,
tvtech

 

[fractal5]

Here are some pictures.

 

[Reloadron]

Looks pretty much like how all of them look. Looks like a ATX form factor SMPS (Switch Mode Power Supply). So what is the plan now? Nice pictures though and did you read or use the link I provided that explains the operation of these things?

Ron

 

[fractal5]

Looks pretty much like how all of them look. Looks like a ATX form factor SMPS (Switch Mode Power Supply). So what is the plan now? Nice pictures though and did you read or use the link I provided that explains the operation of these things?

Ron

I will desolder the capacitors and test them.

If that doesn't come up with anything I will start to learn how the PSU works and what values I should expect to see where, and start poking around with an oscilloscope.

I looked at the link, thanks!

 

[DerStrom8]

Some of the solder joints look awfully messy in the image of the bottom of the board. Did something happen to it, or is the photo deceiving?

 

[fractal5]

Some of the solder joints look awfully messy in the image of the bottom of the board. Did something happen to it, or is the photo deceiving?

I'm not sure which solder joints you're referring to. Do you mean in the lower bottom right corner in the picture? This is where the bundle of yellow wires are soldered in. It does look like a mess, but I'm not sure if it matters. Did you imply that there is some kind of short there?

So far I've only managed to desolder a single capacitor. It is very hard to work with this PCB, the capacitors are glued down, very tightly packed and hard to get a hold of. At the same time this huge bundle of wires is also annoying to have to deal with.

 

[wnr]

could check the electronics caps in output voltage.it will helps you