Electrical Engineering - No Power On PC Diagnosis help

Author

No Power On PC Diagnosis help

ehsjr

2007-08-13, 3:25 am

3.5 year old Dell. Won't power on, supply fan doesn't spin.
Pulled supply & it powers up outside the PC. (10 ohms between
any red & black wire; green wire shorted to black for testing
outside the PC). Pulled everything from the mobo (motherboard)
one thing at a time, still won't power up. Tested PC power on
switch with ohmmeter - tests good. No shorts at power connector
pins (ohmmeter), no visible burns, no swelled lytics. Was working
a week ago.

Anyone have any thoughts on what else I can check?

Thanks,
Ed

Palindrome

2007-08-13, 3:25 am

ehsjr wrote:
> 3.5 year old Dell. Won't power on, supply fan doesn't spin.
> Pulled supply & it powers up outside the PC. (10 ohms between
> any red & black wire; green wire shorted to black for testing
> outside the PC). Pulled everything from the mobo (motherboard)
> one thing at a time, still won't power up. Tested PC power on
> switch with ohmmeter - tests good. No shorts at power connector
> pins (ohmmeter), no visible burns, no swelled lytics. Was working
> a week ago.
>
> Anyone have any thoughts on what else I can check?
>

Note: below refers to standard psus- you don't say the model of your
Dell - which may not have standard pinout/colour coding

Here's what I would do:
1) Ask at a pc hardware group with the model number - ideally a Dell one
- in case the pinout or colour code is different..
2) Disconnect all the output leads from the power supply
3) connect a dummy load (some psus wont work (reliably) without a
minimum load) - I use an old 20MB (yess 20 meg) disk drive).
4) Check the mains power lead on a different appliance.
5) plug in mains power lead.
6) Check for standby 5v power between pin 9 (purple) on ATX connector
and ground (any black) - no power, psu is faulty.
7) Start up main power supply by shorting pin 14 (green) and ground.
8) CHeck for volts at the disk drive connectors/other connections -

If all the above is ok - the mobo is probably dead.

If not ok - the power supply is dead.

--
Sue

ehsjr

2007-08-22, 3:25 am

ehsjr wrote:
> 3.5 year old Dell. Won't power on, supply fan doesn't spin.
> Pulled supply & it powers up outside the PC. (10 ohms between
> any red & black wire; green wire shorted to black for testing
> outside the PC). Pulled everything from the mobo (motherboard)
> one thing at a time, still won't power up. Tested PC power on
> switch with ohmmeter - tests good. No shorts at power connector
> pins (ohmmeter), no visible burns, no swelled lytics. Was working
> a week ago.
>
> Anyone have any thoughts on what else I can check?
>
> Thanks,
> Ed

It is now fixed. Bad motherboard. What a pain - the new
mobo required a new cpu, new cooling fan/sink and new memory
and it wasn't just a "drop in" replacement. Windows had to be
re-installed, as the new mobo and the existing Windows on the
hard drive would not work together. So I bought a new HD,
installed Windows on it and copied the data over from the old
HD. I spent a day reinstalling Windows, downloading and installing
all the Windows updates, then reinstalling all the programs,
downloading drivers & installing, etc. etc. A "nice" extra
pain in the XXX was that the FDD header on the new mobo
did not match the floppy drive ribbon cable.

Anyway, its a much faster mobo/cpu, and now the C: drive
is 320 gig SATA (vs the old 80(?) gb ide) so its a nice
upgrade.

Thanks to those who replied.

Ed

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