I am currently looking at a compaq presario model #4420CA for a
friend. When I turn the power switch on, nothing happens, the green
lights in the front of the tower do not come on at all. I have
replaced the power cord,and checked the switch to no avail, I have
replaced the power supply with my power supply from my HP 6746C
computer and it is still not working. Could someone pls let me know
if this means that my mother board is bad and is it feasible to
replace it. If anyone had any other ideas on what the problem may be,
it would be much appreciated.tks scott
There are two switches on the PC. The switch on the back of the PSU,
turns on the +5VSB. Many retail motherboards have a green LED on the
motherboard itself, and on those, you can look at the LED and see if it
is glowing when the PSU is switched on. That tells you the
+5VSB is present (and you are not supposed to add or remove
components from the computer, when the green LED on the motherboard
is lit).
The rest of the power rails are dependent on the power switch on
the front of the computer.
Pushing the buttom on the front of the computer, produces a
momentary pulse. The momentary pulse travels to the PANEL
header on the motherboard. A chip on the motherboard "latches"
the pulse, and that chip produces a steady logic level on
the PS_ON# signal. PS_ON# goes to a wire on the 20 pin ATX
power harness. When PS_ON# gets inside the PSU, that tells
the PSU to switch on the rest of the power rails. That
will cause the LEDs on the front of the tower to light up.
In terms of faults:
1) If +5VSB is overloaded, or if the power supply cannot make
+5VSB any more because of an internal failure, there is no
way for the motherboard chip to "latch" the power signal.
That chip needs +5VSB, before it can latch anything.
2) If the motherboard chip is defective or has been damaged,
that may prevent PS_ON# from happening.
3) A faulty tower power button can prevent it from working.
Disconnect the power switch and use the reset switch instead
as a temporary power switch. Both switches produce pulses
and do the same kind of job.
Since you swapped the PSU, that leaves the motherboard or the
switch as potential suspects.
HTH,
Paul
