Failing PC PSU

Discussion in 'General Electronics Chat' started by ISB123, Jul 10, 2015.

  1. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    527
    Hello,I have a problem that I can't solve by my self so I kinda need your help:oops:.
    Problem is in 400W PSU that I had for 10 years it shuts down every time there is a full load on GPU which renders the PC useless for anything but internet browsing so its probably overload protection kicking in.
    The problem started one day when I went to turn on the PC but nothing happened so I immediately knew that problem must be in the PSU,I took it apart and found 16V 2x2000uF leaked caps on secondary side,I didn't have any 2000uF caps so I replaced them with 35V 4000uF caps it solved the problem and PC booted up and performed very stable,no voltage fluctuation.So everything worked fine for 2 weeks and then problems started again,PC would fail to boot after windows loading logo and would shut down if I loaded my GPU hard.So I took it apart again and replaced the caps with proper ones but problem still persists.I noticed that voltage on -12 rail is actually -6V all other voltages are fine.I'm not sure why is the PC even booting since it shouldn't be sending power good signal since -12V is low.So the problem is probably in 12V rail which falls below required voltage.

    isolating-atx-smps-ka5h0165r-sg6105-schematic-diagram (1).png

    http://www.sg.com.tw/semigp/data/6105/6105-datasheet.pdf
     
    Last edited: Jul 10, 2015
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,006
    3,763
    @ISB123
    The inductor could have overheated one time. Once that happens (especially in a 10-year-old power supply), the iron powder toroidal core becomes slightly oxidized. On e that happens, the weight percent of elemetal (magnetic) iron has decreased. Now it will saturate at a much lower current than it was designed for and the voltage in the core will surge to higher levels to achieve the current you need (or attempt to draw).

    I recommend a new power supply. For $30 or $40 you can get a very durable PSU at much higher power, much lighter weight and runs much cooler.

    The new once run at higher frequencies and, therefore, have smaller inductors and the modern MOSFETS run at lower losses - well worth the money.
     
    ISB123 likes this.
  3. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    Thanks,I'm going to buy a new PSU in 1-2 weeks,just wanted to fix this one until I buy a new one.I have all the replacement parts for this PSU,going to try to replace the inductor.
     
  4. nigelwright7557

    Senior Member

    May 10, 2008
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    I had a power supply do the same thing.
    The fan had got blocked with dust.
    The power supply was over heating and shutting down.
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    That's a common fault with the capacitors - the HF ripple can upset the hard drives. One of my PSUs did that recently just as I was trying to do a backup - every file that got accessed got trashed! Windows also got messed up, luckily I have a drive for booting with only the OS on it, as I had to format it and start over.

    You can't use just any old electrolytics - they have to be very low ESR type.
     
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  6. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    They were actually brand new low ESR type capacitors of same value.Yeah I noticed HDD acting strange while I performed windows boot repair the HDD kept clicking while the process was running.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    A clicking hard drive was the first indication I got that something was wrong - usually when a HDD starts clicking the BIOS won't recognise it, but that symptom wasn't present.

    Various HDD diagnostics passed the HDD fit and healthy. then it started to dawn on me that something else was causing the problem.
     
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