How to fix it? - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 15th May 2012, 02:25 AM   #1
sanok is offline sanok  Australia
diyAudio Member
 
Join Date: Apr 2012
Default How to fix it?

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
Attached Images
File Type: jpg 100_3340.JPG (840.1 KB, 122 views)
File Type: jpg 100_3341.JPG (940.8 KB, 115 views)
  Reply With Quote
Old 15th May 2012, 12:02 PM   #2
spwalek is offline spwalek  United States
diyAudio Member
 
Join Date: Mar 2012
Location: West Seneca NY
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
  Reply With Quote
Old 15th May 2012, 12:36 PM   #3
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
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.
----------------------------------------------------------------------------------------------------------

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.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 15th May 2012, 12:47 PM   #4
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
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.

Last edited by KatieandDad; 15th May 2012 at 12:55 PM.
  Reply With Quote
Old 15th May 2012, 01:20 PM   #5
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Blog Entries: 22
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.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 15th May 2012, 01:22 PM   #6
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
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.
  Reply With Quote
Old 15th May 2012, 01:36 PM   #7
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
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.
  Reply With Quote
Old 15th May 2012, 04:52 PM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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.
__________________
regards Andrew T.
  Reply With Quote
Old 16th May 2012, 12:27 AM   #9
sanok is offline sanok  Australia
diyAudio Member
 
Join Date: Apr 2012
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
  Reply With Quote
Old 21st May 2012, 06:44 AM   #10
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
Quote:
Originally Posted by sanok View Post
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
Quote:
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...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fix my RIAA thuur Analogue Source 7 24th November 2008 02:48 PM
How can I fix this? Tenson Everything Else 12 28th August 2008 07:54 PM
Perpetuum-Ebner PE66: To fix or not to fix... Grendel the Cat Everything Else 0 6th April 2007 05:37 AM


New To Site? Need Help?

All times are GMT. The time now is 06:49 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2