[PLUG] computer will not start (2X)

[Denis Heidtmann]

On Fri, Jul 18, 2014 at 2:32 PM, King Beowulf <kingbeowulf at gmail.com> wrote:
> On 07/18/2014 12:37 PM, Denis Heidtmann wrote:
>> Narrowed down to a problem with the standby 5V.  The MB has an LED
>> which indicates that the standby 5 volt supply is on.  This LED
>> flashes when the problem occurs, indicating overcurrent, shutdown,
>> repeat.  The two times this has occurred it went away spontaneously
>> (Nothing I was doing can be said to be the cause for the problem going
>> away.)  I watched the LED while unplugging back cables (keyboard,
>> mouse, USB, etc) one at a time.  Nothing changed the status.  Then
>> while doing nothing it suddenly changed--the LED became steady on.
>> Then the thing started fine.
>>
>> The computer is often completely powered off--no AC to the power cord.
>> Other times it is off with AC on.  On the 2nd occurrence it was in the
>> latter state when I attempted to start it.
>>
>> The PS is a 380 watt Antec.
>>
>> I am open to suggestions on how to diagnose this problem.  My first
>> though is to measure the standby 5V to see if it is marginal.  I have
>> to see where is the safest place to put a probe.
>>
>> -Denis
>
> What's the history of that Power Supply (PS)? Age? Stuck straight into
> the wall? UL 1449 surge power strip? UPS? Any good Lightning Storms or
> power failures over the years?

These questions point to the possibility that the issue is with the
PS.  Agreed that it could be the problem.  How to diagnose in the
absence of a spare PS is one of my questions.

>
> How many doodads are plugged into your system that uses 5V? All USB
> devices do. CPU fan is 12V but some case fans run on 5V. DVD/CD/hard
> drives use both 5V and 12V.  PS wattage is not as important as the
> current (in amps) the 5V and 12V rails can support. The biggest current
> draw is when you are firing it all up. Perhaps the PS just can't push
> rated current anymore. You can monitor the voltage off of an unused PS
> connector when you power up and run.
...
> Ed


The problem does not occur with the machine running.  It is the
Standby 5V that I need to check.  It is not at the unused 4-pin
connectors.  The USB appears to have 5 V on it when the machine is
off.  I will check that the next time this issue arises.

What I gleaned from my web searches is there are basically two
possibilities:  The PS is flakey in that it produces insufficient
current to meet the standby needs, or some component demands more than
it should.  What things use standby 5V?  I have a mouse, a keyboard,
an external serial hard drive with a separate power source and powered
off, an internal serial hard drive, an internal dvd, and a (gasp!)
floppy, on-board sound, and a flat-panel monitor.  My guess is that
the keyboard, the monitor, and possibly the mouse at least sense the
standby 5V.  All of these external things got unplugged during my
test.  I suppose a broken front panel switch could be the problem, or
a scraped wire thereto.

-Denis