I) Let's start with the Dell. The computer was subject to a power surge 
caused by lightning. You have replaced the PS with a new power supply. 
When you press the case power switch nothing happens.
1) Check that the PS is set to the proper voltage and plugged into a 
known good AC source and that the PS switch is turned on. Unplug till 
item 6.
2) Check that the case power switch is properly connected to the 
motherboard.
3) Check that the 20 pin power connection to the motherboard is properly 
connected.
4) Check that the 4 pin power connection to the motherboard is the right 
one and that it is properly connected. If the motherboard does not use 
the 4 pin connection, this item does not apply.
5) Have no other items connected to the computer, the motherboard 
including cards,RAM and CPU, and the power supply.
6) Momentarily press the case power switch. The PS fan should come on 
and, if the motherboard is equipped, the green LED on the motherboard 
should light.
7) If nothing happens, try reversing the orientation of the case power 
switch connection to the motherboard.
8) If nothing happens, either the case switch is bad or the motherboard 
is bad. OR The new power supply is bad out of the box. For a test of the 
power switch and since they are both momentary contact switches, you can 
temporarily substitute the reset switch for the power switch using the 
connection at the motherboard. If needed, you can buy a power switch 
replacement.
9) If the PS fan comes on and, if equipped, the green motherboard LED 
comes on, start rebuilding your computer one item at a time. There are 
several good web sites with step by step directions on building a 
computer. If I start with known questionable components as you probably 
have after a power surge, I always test my build after each step even 
though it is more time consuming. That way it is a lot easier to 
identify any bad components.

II) Now let's go to the HP. The PS "tests" good on a tester, but when 
connected to the motherboard and the case power switch is momentarily 
pressed, the PS fan does not come on although the PS is supplying at 
least 12v power because the CPU fan comes on. I've never run into this 
before, but I would try to isolate the problem. I would try the above 
steps with the existing PS. If the PS fan still does not come on after 
step 7, replace the PS with a known good one and repeat the steps.
a) If the current PS fan works, keep going on the rebuild one step at a 
time. Reset the BIOS/CMOS by removing the battery on the motherboard for 
about a minute and then replace it back on the motherboard. If your BIOS 
is not corrupted, this action will reset it to the default settings. If 
your BIOS is corrupted, you will need to either replace the BIOS chip, 
get a replacement motherboard, or consider replacing the computer.
b) If the current PS fan does not work and the known good PS replacement 
fan does not work, the problem is most likely in the motherboard. You 
will need to replace the motherboard or replace the computer.
c) As I remember your previous posts, the motherboard may not be the 
only problem and repair costs will start mounting making the computer 
replacement option more advantageous. And you might be able to use some 
components of the HP in the rebuild of the Dell (compatibility may be an 
issue).

I hope this helps.

Tom Mayer

Russ Cox wrote:
> I have two computers (one HP, one Dell)  that will not power on.  The 
> Dell suffered from some lightning effect, but nothing obvious.  The HP 
> went thru a series of "freezes" (HP Toast? update) before a freeze 
> during flashing the BIOS killed it. Both power supplies test good, all 
> voltages present and the PS fan runs when plugged into the tester.  
> When I plug the PS back into the motherboard and press the Power on" 
> switch, nothing happens on the Dell.  On the HP, the CPU fan comes on 
> but not the PS fan. +5v is present on both units since the mobo LEDs 
> are on.
> The power switch on the HP does close when pressed as checked by 
> ohmmeter.  Didn't check the Dell.  All cables are in place on both 
> units and not damaged. I thought the power was turned on by the 
> momentary ground applied from the power switch to the "Power Good" 
> line (pin 8 on the 20 pin power connector) to the PS.    It appears 
> the power supplies are good, but something isn't working. I did try a 
> new PS on the Dell, same result.
> I must be missing some step in this analysis. But I'm trying to figure 
> out what failed.
>
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