Re: Help! My PC won't switch on.

Condescending is irrelevant. Many so fear anything more complicated
than 'shotgunning' as to bitterly attack the multimeter. "It's too
complex" is a common refrain among many who consider themselves
"computer literate" or who are A+ Certified. Up front, if a meter is
too challenging, then the human also cannot use an iPod that is far
more complex. Fear of something that only looks complex is widespread
among the so-called 'computer literate'. It was not condescending.
It addressed a common problem.

A defective power supply can measure good on a power supply tester.
To see some defects, the power supply must be under full load. Well,
we don't have a load large enough. Only a computer can come close.
Therefore all computer peripherals are working (multitasking)
simultaneously so that load is as large as possible. Only then will
some troublesome power system defects be detected. To provide
sufficient load, a power supply tester would be as hot as three or
four 100 watt light bulbs. Obviously a power supply tester is a near
zero load - insufficient for testing. And finally, the power supply
tester does not provide numbers. Those numbers include other useful
information. Power supply testers are all but useless. But they
don't 'look' complex.

I did not say what the battery voltage is because battery can vary
for each motherboard. For example, the CR2023 coin cell is 3 volts.
That means battery voltage would be well below 2.8 volts (often well
below 2.6 volts) to cause computer problems. Of course, this voltage
is best measured with battery connected to a load - not removed from
motherboard. If battery is below 2.9 volts, then make plans to
replace that battery in the next six months.

If battery is 4.5, then battery would be well below 3 volts before
causing a problem.

All wires must be normally connected to PC when making any
measurement. What happens on each voltage as power turns on will
provide significant information. (Only one of each color wire need be
monitored). For example, if some voltages rise (and then fall) but one
voltage does not rise, then the one failed circuit is identified.
Move on to identify which of those isolated suspects has failed - a
shorted load or a failed power supply.

Numerous types of failures can be observed in only two minutes.
When those numbers for 'before' power on and 'during' power on are
described, then where to look next can track down the suspect AND
teach much about how a computer really works. Any failed number does
not automatically mean power supply failure. Failed numbers can
identify other failures. Numbers contain more information than GO-
NOGO.

Almost all failures have no visual indication. Some failures can be
seen by the meter before those failures cause operational problems.
If we end up replacing a power supply, that supply is not known good
until confirmed under load by numbers.

No power supply (that meets industry standards) will harm
motherboard and peripherals. This was industry standard long before
Intel also demanded same. A greater risk to a computer system is a
human changing wires - trying to fix by shotgunning or forgetting to
disconnect AC power cord.

Industry standard - no failed motherboard or other peripheral can
damage a properly constructed power supply. Short together all power
supply outputs, power on the computer, and the power supply must not
be damaged. Some industry specs even define how large that shorting
wire must be - the standard is that old and well established.

No reason to worry about a defective power supply or motherboard
causing more damage. Don't disconnect anything until numbers identify
suspects. (As purple wire demonstrates on meter, always remove power
plug from wall before disconnecting anything - otherwise damage can
result.)

On Oct 2, 8:20 pm, "Mortimer" <m...@xxxxxxxxxxx> wrote:
Thanks for the link to your very informative earlier posting. Now I know
what to look for in terms of voltages on various pins, I'll be able to
diagnose "failure to start when the button is pressed" errors a bit better.
I'll print out the wire colours and threshold voltages, and keep it with me.
Would you advise measuring voltages rather then plugging the PSU into a PSU
tester for diagnosing whether the PSU is OK?

I think you're being slightly condescending in your comments about "If using
the meter is too challenging, then repairing the computer is just as
challenging - take it to the shop": maybe some people (such as myself) are
fully conversant with using a multimeter but aren't sure (until they've seen
your checklist) what to look for.

You don't say above what the nominal voltage of a BIOS battery is. I can
never remember whether a full battery should be 1.5 or 3V.

One question: if you measure the various lines and find one or more that
fail your tests both in situ and when not connected to the PC, then the PSU
is faulty. But what is the best way of determining then whether it's safe to
plug in a good PSU to the motherboard? If the PC has failed because "the
computer has gone bang" (that's how the symptom was described to me by a
customer the other day), it could be that the old PSU has failed because of
a motherboard fault, in which case the motherboard could blow a new PSU.
Assuming there are no visual signs of burnt-out components on the
motherboard, what tests do you perform before risking plugging in a good
PSU.

.