Ot - computer won't boot

[Gene Heskett]

On Friday 17 March 2017 19:54:56 Phil wrote:

> Thank you for reading this and I know that this a vague  question but
> you may be able to give me a lead. For the past  couple of days my
> desktop computer has been very slow to begin the boot process, perhaps
> 5 minutes or so. Now it won't boot all. The disk activity led blinks
> at about a 3 second rate, the fans run, and speed up once the system
> warms, and I can measure 5 and 12 volts at the disk drive. There is no
> video from the on-board video chip (which is normal until the system
> boots up) and the system won't boot from the cd drive either. I've
> reseated various connecters and the ram board. So, the power supply
> and disk drive are propably ok which leaves the mother board, ram and
> cpu but how I determine which one is at fault? I'm at least a 1000km
> from civilisation so I cannot speak to service personnel.
>
> Sent from Samsung tablet.
> Regards,
> Phil

This isn't much to go on, but as a C.E.T., I'd be looking at the tops of 
the electrolytic caps on the mainboard, particularly those right near 
the cpu socket, looking for bulged or even open cracks in the safety 
scratches in the aluminum on top of them. I'd also open the psu and look 
at the caps in it, again looking for bulged or vented tops. This would 
be a sign of the capacitor in a switching circuit, and these all are, of 
excessive ESR, Equivalent Series Resistance.  PSU's of more recent 
history than 10 years, seem to have a built in voltage fade with time, 
and the 5 volt line, if below 4.75 to 4.8 volts when first turned on, 
will hold the cpu in reset until the voltage gets above 4.8 as these 
caps warm up, but eventually the ESR causes so much heating they will 
vent and its game over.

What you do next depends on the soldering tools you have, and a src of 
spare capacitors to install. Lacking those, you may well be out that 
1000km drive back to civilization to just upgrade to fresh stuff. Even 
with a hot air rework station, its hard to change the motherboard caps 
because the pattern on the board is big and fat so it can serve as 
additional cooling heat sinks for the caps, so you really have to turn 
up the heat and potentially cook the board just to get the old ones out.

Thats my best shot at it, based on 65 years of chasing electrons to make 
them do usefull work.  Best of luck guy.

Cheers, Gene Heskett
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