How to fix it?

[sanok]

Hi all,
I'm new here.
I've got a Seasonic PC power supply model SR-250FS ( very old 1999)
It blows fuses right after switching on. There is a short somewhere, but I can't find it.
The green thermistor (close to input ) has a sign of melting on it, when tested for continuity it lets the current flow,( not sure if that suppose to be like that).
I've replaced it with a similar one from a working PC power supply and it blew to pieces.
Have removed and tested rectifier, 2 big electrolitic caps, 2 black rectangular caps, the FET transistor on a heatsink, all OK.
I've tested the 2 rectangular caps for continuity to see if they are shorted, they're not, but multimeter has only 9V battery.
Is it possible that they might leak current at higher voltages?
What else should I look for?
thanks for any help

 

[spwalek]

I would check all your semiconductors for shorts with a diode/continuity checker. It sounds like you've got a hard short somewhere. Also check the electrolytics on the low voltage side for shorts. I also wouldn't spend a ton of time.. these are relatively inexpensive to replace.

Good luck

 

[Mooly]

These type of supplies are highly dangerous to work on as the primary side is at half mains potential at all times and can remain charged too.
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The number one cause of all SMPS failures is dried out caps, particularly any coupling drive to the switching transistor. Any rail caps are highly suspect too. Then there is the damage to other parts caused by the catastrophic failure of the transistor going short.

Fixing these type of problems used to be the day job With the greatest respect it sounds as though you haven't the experience to work safely on these.

 

[KatieandDad]

As Mooly has said - It's not what failed originally but what failed as a result.

PC Power Supplies are virtually pennies these days - I wouldn't try to repair it.

Beware - some PC PSU failures can cause Motherboard failures.

 

[wintermute]

As stated, PC Power supplies can be VERY dangerous. Don't poke around in it if you are not 100% sure of what you are doing. I've actually seen one supply (out of an old compaq server) that rectified the mains directly. It was the first (and last) switchmode power supply I tried to fix. When I realised what was going on I put it back together and didn't touch it again

Tony.

 

[KatieandDad]

Wish you were in the UK. I've got a spare 250W PSU that is going in the bin as I move house.

If you are moving up then consider a bigger PSU.

Modern PCs are becoming Power GREEDY. The Video Cards are demanding enormous power out of the PSU, 850W is not uncommon these days.

Dont be tempted to buy cheap as NOT all POWER SUPPLIES are EQUAL.

 

[KatieandDad]

In all cases look at what you are demanding from the PSU.

850W would be divided across the available ouputs.

In a High-End PC you might want 12V at 40A (or even more) CHECK to see that the PSU can provide the current that you need at the voltage that you need.

 

[AndrewT]

Why are we seeing a PC power supply problem in the Power Amplifier PSU section?
Why are we seeing discussion on a direct on line PSU when it is banned on the grounds of safety?

We are required to only discuss mains power supplies that use an isolating transformer between the mains and the circuit.

 

[sanok]

OK thanks for all the replys.
I was hoping to fix it just for satisfaction and maybe transform it into lab power supply.
But will throw it away as I don't have experience to go any further.
cheers

 

[TonyTecson]

OK thanks for all the replys.
I was hoping to fix it just for satisfaction and maybe transform it into lab power supply.
But will throw it away as I don't have experience to go any further.
cheers

It blows fuses right after switching on.

whatever you do, make sure that you unplug the psu from the wall....
a fuse blowing indicates a short on the primary side....
the way these things are designed, the rectify they mains directly, using an ntc thermistor, the green thing, to limit current, so that any shorted diodes or mosfets may cause it to fail...

this mains rectifier charges the 2 big caps to about 300volts dc, and the rectifier itself can be a full wave bridge at 230 voltas ac and a doubler at 115 volts ac input...

there are vdr's shunting the big caps, these may have shorted as well...so be thorough in checking those parts..

i have repaired hundreds of these psu's some years ago...