Sorting out old PC power supplies?

[roadrash]

I have just been trying to sort out a couple of oldish PC power supplies that had stopped working. I found lots of domed caps and replaced them all. Still not got them working yest but I did notice a burn mark around one of the AC inputs to the PCB. There were 2 small capacitors near the burn mark as well and I removed them to check. They were both covered in some sort of black heat proof plastic. When I removed the plastic from one of them the blue colored cap inside fell into 2 parts (see photos attached). So I think this might be one of the problems. I cannot tell what these caps are but they look like ceramics and when I measure them the good ones show a value of around 160pf. I cant understand the markings on the caps so can someone tell what type these are and the value so I can order some.


thanks
Jon

[stj]

MOV's probably

[roadrash]

Originally Posted by stj

MOV's probably

After googling I guess this is a type of mica cap (Muscovite mica).
What value should it be? Is it 150pf?

[stj]

lol
Metal Oxide Varister.
a surge suppressor device.
was there a pair of these in parallel with the 2 smoothing caps?

[roadrash]

Originally Posted by stj

lol
Metal Oxide Varister.
a surge suppressor device.
was there a pair of these in parallel with the 2 smoothing caps?

Yes I think you are right. Hahaha a mica cap????
So what value is the one in my photo as I think I will need to replace it before the PSU will work again??

[stj]

i suspect it was 150v, although that may be too low for a 240v input.
maybe that's why it blew up.

total rectified voltage is going to be about 340v so divided by 2 it's 170v
so using parts rated between 200 and 250v is probably wise

btw, the psu will run without them - so if it's dead it probably blew the fuse or killed an input filter

[momaka]

Originally Posted by stj

btw, the psu will run without them - so if it's dead it probably blew the fuse or killed an input filter

^ This.

The MOVs are for surge / high-voltage protection on the input. If they are blown, probably the voltage on the input got way higher than it should have been (which BTW can also happen if some fool puts the 115/230V switch on the 115V position and the AC voltage is 230/240V... which it is on your side of the world.)

So just check the fuse, NTC, and bridge rectiifer. If they check out normally (or at least, you will probably need to replace the fuse, as I suspect that will be blown), the PSU should be able to turn On.

That's about all I can say here. Pictures of the individual PSUs would help if this still doesn't get them going.

[roadrash]

Originally Posted by stj

i suspect it was 150v, although that may be too low for a 240v input.
maybe that's why it blew up.

total rectified voltage is going to be about 340v so divided by 2 it's 170v
so using parts rated between 200 and 250v is probably wise

btw, the psu will run without them - so if it's dead it probably blew the fuse or killed an input filter

Thanks for that information STJ. I couldn't work out what the markings on the capacitor meant. One of those photos is of a good one that is exactly the same as the blown one. So if you can make sense of the makings in white on each side of the good one can tell what value it is?

[roadrash]

Originally Posted by momaka

^ This.

The MOVs are for surge / high-voltage protection on the input. If they are blown, probably the voltage on the input got way higher than it should have been (which BTW can also happen if some fool puts the 115/230V switch on the 115V position and the AC voltage is 230/240V... which it is on your side of the world.)

So just check the fuse, NTC, and bridge rectiifer. If they check out normally (or at least, you will probably need to replace the fuse, as I suspect that will be blown), the PSU should be able to turn On.

That's about all I can say here. Pictures of the individual PSUs would help if this still doesn't get them going.

Thanks but afraid it wasn't over voltage (by me anyway) as the switch is set to 230v. Is it possible that this cap was just too old and failed like electrolytic's do? There is no fuse in the PSU but there would have been one in the plug on the power lead. I haven't checked the Rectifier yet but all other diodes are ok. I will remove it shortly to check as its quite likely to have got damaged to.

[stj]

power spikes slowly kill surge supressors.
i had one explode in front of me once - the heatshrink is there for a reason!!

[momaka]

Originally Posted by roadrash

Is it possible that this cap was just too old and failed like electrolytic's do?

It's not a cap, it's an MOV - completely different component.

As stj mention, sometimes they can also fail with time as they absorb high voltage spikes on the line.

If each of these is placed in parallel with the two 200V caps, then the new MOVs should have a rating of 180-220V. 180V may be a bit too low, though.

But again, these are not necessary, at least for determining if the PSU is working or not. First do that, because there is no point in replacing the MOVs if a voltage spike has also managed to blow something else downstream. In some cases, PSUs from damage like that may not always be economically repairable. So that's why you should find out if the PSU is working first (and if not, what else may be the problem, besides the blown MOVs.)

[R_J]

Here is a mov datasheet, most are similar. 150 is the A/C rating which is likely around 200vdc with a clamping voltage around 400v

[roadrash]

Thanks everyone for that very very useful info. As you can tell this is the first time iv'e encountered one of these MOVS blown up. Its a wonder you can see when they blow when they hide them inside a black plastic covering.
Well iv'e now got 2 of these type of PSU's not working that I have to sort. Iv'e just recapped 4 others that are working fine now.
Only one of the last two had a blown MOV and iv'e checked its bridge rectifier and its ok.
Basically both these PSU's are dead and so far iv'e checked all diodes, bridge rectifiers and replaced all Electrolytic caps some of which were domed.
What should I check next following this? Iv'e uploaded some pictures of these PSUs.
Thanks

[roadrash]

Since my earlier post I found a fuse funnily enough well hidden covered in black plastic like the MOV's were.
I replaced the fuse amd tested both PSU's again. One is still dead and the other went pop and has blown a couple of surface mount resistors of the board.
So I think this one is not worth bothering with now.
The other one should still work once the fault it traced.

[stj]

delta supplies are quality and always worth fixing.

btw, if the mov was not in the sleeving it would have made a fucking mess all over that side of the board!

[roadrash]

Originally Posted by stj

delta supplies are quality and always worth fixing.

btw, if the mov was not in the sleeving it would have made a fucking mess all over that side of the board!

Oh I wondered why it was covered in that heat shrink plastic. Shame about that one having a short somewhere. I suppose I can fix it if I can get the 2 surface mount parts out of an old scrap pcb and find the short that caused it.
Can you point me where to look next on the other delta supply that is ok but still dead. Like I said ,its got all new caps and ive checked all diodes, checked bridge rectifier and replaced fuse so far. Nothing happens when powered up its just dead.

[stj]

let me try to find a schem - what is the details.
like part number or whatage, and the number on the control chip.

[PeteS in CA]

Originally Posted by stj

power spikes slowly kill surge supressors.
i had one explode in front of me once - the heatshrink is there for a reason!!

I think I recognize the logo for Uppermost Electronic Industries on one of the parts. Definitely an MOV. Among surge suppressor diodes, MOVs, and gas discharge tubes, MOVs are faster than the tubes, absorb more energy than the diodes, and are inexpensive. The one major fault they have is that every time they absorb a hit a little bit of damage is done. Over time this lowers the voltage at which they start to conduct and eventually that voltage is low enough that they clamp at normal line voltage and blow up.

Power strips that have MOV surge suppressors should either be replaced entirely or have the MOVs replaced every 5 or 10 years, depending on your area's weather.

[PeteS in CA]

2007 vintage, probably 300W or 350W. Passive PFC. Looks like it's forward converter topology. For lower end off-the-shelf PSUs Delta probably would have used a 384x PWN and a Power Integrations regulator for the 5VSB.

The color coding looks like standard ATX, except I don't see a white -5V wire. I don't know if that had become common back in 2007. Look at the location on the ATX connector where the -5V wire normally would be. If it's -5V or there is no wire to the location, well and good. However in the late 90s and early 2000s I know (former Delta employee, here) Apple used the -5V location for an extra 3.3V socket. So if there is a wire and it's orange, you have an Apple power supply.

The point at which I suggest starting is whether or not you have 5VSB, the purple wire. If no 5VSB then that's where you start looking.

If you have 5VSB and grounding the PS-ON (green) wire doesn't turn on the main outputs then the PWM circuit is your starting point. Check the Vcc voltage on the 384x. If it's too low then there's probably a ~47uF cap that might be bad, or the resistor that charges it might be bad. Those provide the voltage to start up the 384x. One the PSU starts up the 384x Vcc usually is supplied by a bootstrap winding.

[roadrash]

Originally Posted by stj

let me try to find a schem - what is the details.
like part number or whatage, and the number on the control chip.

The label says its a "Enlight GPS-350ABC
here is photo of that label.