Re: PC doesn't start

Before a computer can start after power supply turns on, first, all
electronics must receive a reset signal. This reset signal is created
on motherboard. If the reset circuit does not work on power up, then
power can be applied and computer will .... well, strum your lips as
you hum. Computer is just as confused if it does not get a power-on
reset signal (sometimes called 'Power On Reset' or POR).

When you pressed the front power switch, then green wire voltage
dropped to zero AND orange, red, and yellow wire voltages remained
sufficient (correct). Therefore voltage on that switch must have
dropped to near zero. 'Reset circuit' on motherboard' then resets and
'boots' a computer.

If reset circuit does not properly reset all motherboard
electronics, then motherboard may ignore future front power switch
presses. If that power switch is ignored (electronics not properly
reset on power up), then computer can only be powered off by
disconnecting power (ie. switch off rear power supply switch).

Meanwhile, when power switch is pressed, then green wire voltage
drops to near zero - orders power supply to power voltages on orange,
red, and yellow wires. That can only happen if front power switch
voltage drops to near zero - can only happen if switch works
correctly.

To review, that switch voltage is measured with red meter probe on
one switch wire and black meter probe on other wire (neither meter
probe connected to chassis). If switch voltage does not drop to near
zero when switch is pressed; then green wire voltage cannot drop to
zero and those LEDs will not light. IOW I suspect something is wrong
when measuring that switch wire voltage.

Well, apparently motherboard 'reset circuit' is not working
properly. If purple wire voltage remains sufficient as previously
posted (and that purple wire voltage must be measured only when
connected to motherboard), then motherboard 'reset circuit' is
defective. Even ACPI circuits will not function properly if reset
signal is not generated.

I assume that computer does power (does light LEDs) first and
everytime the front power switch is pressed. Therefore switch voltage
does drop to zero whenever switch is pressed. Therefore motherboard
reset circuit is probably defective. Visual inspection may reveal a
stray metal fragment, locate a slightly bulging capacitor, or may
identify a standoff somehow shorted to a printed circuit trace on
underside of motherboard. This visual inspection might explain the
'reset circuit' failure.

'Reset circuit' can be repaired. But skill level and cost to do so
means that replacing a motherboard is cheaper and quicker.

Sorry that I cannot provide a less expensive solution. But if those
orange, red, and yellow voltages rise and remain correct (in
specification), then the power supply *system* (power switch, power
supply, power supply controller, and connectors) is functioning
properly.

On Feb 13, 9:31 am, "Andrea"
<andrea.delzoppo(removethis)@fastwebnet.it> wrote:
Hi Tom,
alas I haven't solved the problem. There might have been a misunderstandig.
Yes the mesurements gave good results, but the PC doesn't start as due;
i.e., when I push the case front switch, normally nothing happens, except
that the
led on the DVD/r turns on (and if a cd is in, it starts running), the
case green led turns on too and so does the the orange one ( but stays on).
With the switch pressed the voltage are within the specs but, but if the PC
doesn't boot, I cannot "turn it off" using the front switch. So I turn off
the power, using the back ( line ) switch, turn it on and try again using
the front button. After a few attemps the PC starts and works fine.
I measured the voltage at the Power switch plug, when connected to the MoBo,
it says 3,17, but I have the same value with the switch depressed and when
the PC finally boots and is running fine. However, after I have the Pc boot,
I can "restart" and even turn it off and on ("cold restart") with no
problems, but if I leave it off for a long time , let's say 1hr or so, the
problem is back again.
Thanks again for your attention,
Andrea

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