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Computer power supply trouble

 
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Pundit Moshai



Joined: 12 Dec 2007
Posts: 2

PostPosted: Mon May 12, 2008 8:25 am    Post subject: Computer power supply trouble Reply with quote

My Computer ATX 400W SMPS power supply is giving low voltage output --i get +4VDC and +10VDC instead of the normal +5V and +12V DC--can anyone please give me any suggestions on how to repair?--the circuit uses SG6105 20 pin chip as PWM IC---the equivalent is the HS8108 chip---thanks!
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon May 12, 2008 10:41 am    Post subject: Reply with quote

I think you should first check for bad electrolytic caps in the power supply. Usually, but not always, the bad ones can be identified by a bulging top and possibly leakage. Small capacitors (6-7 mm diameter and smaller) that have been subjected to heat from other components often get bad without bulging.

Also, check for open diodes in the primary bridge rectifier. If one of them is open, the bridge will operate as a half-wave rectifier, causing too low voltage and much mains frequency ripple on the primary DC bus. The voltage there should normally be 1,4 x the mains voltage if the power supply has no PFC (power factor correction, mandatory in Europe since some years and becoming more and more common worldwide). For units with passive PFC (a big coil with laminated core in series with the mains input), that voltage will be somewhat lower, while units with active PFC always have a higher DC bus voltage, typically 350-380 V.
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9manuals



Joined: 04 Jun 2008
Posts: 10
Location: http://www.9manuals.com

PostPosted: Thu Jun 05, 2008 12:11 am    Post subject: Replace it Reply with quote

Nobody repairs ATX power supply, just by one for about 20$ and replace it
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Thu Jun 05, 2008 2:26 am    Post subject: Re: Replace it Reply with quote

9manuals wrote:
Nobody repairs ATX power supply, just by one for about 20$ and replace it


No Ashton, people in third world countries do & have always been fixing things as much as possible---strictly out of necessity & economics.
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