[UK-CONTEST] Computer PSU Problem - Off topic

[Dave Sergeant]

On 21 Feb 2005 at 16:14, G3SJJ wrote:

> Any thoughts on this?
> 
> Switch on Mains, nothing happens, no PSU fan. Plug fuse OK, 230v at
> other end of mains cable. No dc voltages from PSU. My thoughts are
> that the Power Supply is duff. I could get a replacement and install
> it but this is a neighbour's computer and I don't want to start taking
> it apart unless I am certain I can affect a proper repair.
> 
> The only sign of life is a green LED on the mother board next to the
> DC input socket, which extinguishes when you remove mains plug.

First thought is that the ATX power on line from the computer is not 
being generated. ATX supplies have an on/off command which comes from 
the motherboard and turns on most of the psu lines when the ON button 
(usually on the front of the case) is operated. There will only be a 
single standby 5V coming from the psu in this case. You can check by 
shorting the relavent pin to ground with the psu disconnected.

The most common faults in computer (or any switch mode supply), not 
mentioned at all in Ian's In Practice article, are the high value 
resistors (100k-470k) and associated electrolytic capacitor in the 
power supply start up circuit. In fact 99% of faults in power 
supplies are caused by duff electrolytics which rapidly exceed their 
10,000 hour of so lifetime when left powered up 24/7. Usually this 
results in the complaint it was fine till they had a power cut (then 
refused to start up when power was restored). Capacitors used in 
computers tend to be better quality than those used in TV/VCRs but 
they will still benefit from being switched off when not used. (there 
is similarly nothing to be gained from leaving your TV in standby 
24/7 other than wasted electricity and shorter MTBF - and 24/7 
intereference to our amateur bands!).

73 Dave G3YMC

http://www.davesergeant.com