[maplej6220]

HP Pavillion Desktop Model 700N

Pentium 4 CPU, 2.53Ghz

2 G Ram

Intel Graphics 82845

WD 800AB Hard Drive

 

My problem is when the power button is pressed  nothing happens.  The green light is on steady on the power supply.  The power supply fan is running.  At the present time, I have the computer completely disassemble and hooked together on a table.  I have the power supply 24 pin connector connected, also the 4 pin.  The HD, Floppy, DVD and CD are all disconnected.  The monitor and keyboard are connected.  The ram is installed.  The power button is connected.

 

The green LED on the power button does not light at all.  Fans do not move.  The voltages out of the power supply are 3.3 V, 5.01V and 11.69V. measured at the board.  I noticed the CPU heat simk was warm.  I have followed the instructions given to rest the CMOS.  I have swapped the memory board one at a time.

 

Nothing has helped.  I would appreciate any help available.

Thank You

 

 

[Advertisements]

Replace the power button.

[dbreeze]

Replace the power button.

[iammykyl]

Disconnect the power on and reset plugs to the front header panel then, with a small bladed screwdriver, short out the 2 pins for the power on.

[maplej6220]

Shorted the two pins  nothing changed.  Checked power button, working fine, also reseated the connector to board

[123Runner]

Beginning to sound like you have a board issue.

The power supply won't run unless the power button is pressed. And you state the psu fan is running.

You won't get your voltages to the board unless the psu is running, which seems to be since you have voltage.

Shorting the 2 pins as iammykyl had you do was a way to test the psu.

Make sure you have nothing grounding out since you have it on a table.

 

Do you get any beeps out of it. Removing memory generally gives a beep.

[maplej6220]

OK, I  am beginning to believe I have a bad board.  Here is what I found.  The power supply turns on as soon as I plug in the AC cord.  There is 3.3, 5.0 and 11.69 volts on the regulators all over the board.  I aso discovered that  the CPU fan would kick on for a second when I unplugged the AC power.   Wierd !!!  There are no beeps, nothing when I plug it in.  The surface I have it on is nonconductive but I will double check.  I am tempted to remove  the CPU and reseat it.  Do you know antone who could use a 3.5 inch floppy, CD Drive, DVD-R Drive, ATX power supply, 350 GB HD, 2 - 1GB Memory sticks.  !!!  I mean come on!!! this computer is only about ten years old!!!

 

By the way, this is far and away the best computer help site I have ever found!!!

[123Runner]

When you plug in the power cord the psu fan (nor the psu) should not turn on.

You will get some power on the board (I believe 3.3vdc) and that is all.

Most boards have a blue or green light on the board to indicate you have some power.

You should not get 5vdc or 12vdc till you push the power switch.

[maplej6220]

I am going to reseat the CPU.  I will get back to you tomorrow one way or the other.  Why the PS is turning on by itself really bugs me, leads me to believe the CPU is the problem.

[123Runner]

CPU shouldn't be the problem unless it got jarred and moved.

It wouldn't hurt to remove/ and reinstall. That would be 1 more thing out of the way.

[iammykyl]

Posted before seeing your latest, sorry if there are repeats. 

"Here is what I found.  The power supply turns on as soon as I plug in the AC cord.""

A.   That should not happen even if the PSU switch is in the on position. only when the power on circuit has been activated should the PSU fan start,  (shorting out the 2 pins or pressing the case button if the 2 pin plug is connected)    Swap in any known working PSU to rule it out as the cause of the problems. 

 

" I aso discovered that  the CPU fan would kick on for a second when I unplugged the AC power.   Wierd !!!"

A.   Yes, if the power on circuit has not been activated, doing anything with the PSU should have no effect except for power to the MB LED indicator.  

 

So I may have missed something here. In your first post, " I noticed the CPU heat simk was warm." (should not have been because you say nothing was working) and "CPU fan would kick in"  Possible the CPU has received power but fan is faulty or the CPU fan header.

Try the CPU fan on a different header.   You may have to use some sort of adaptor and directly test the CPU fan.

 ***********

 

Edits.

"Why the PS is turning on by itself really bugs me, leads me to believe the CPU is the problem."

A.   Don't think so, either a faulty PSU or a fault in the MB. swap in a known working PSU first.

If removing the CPU you will more easily be able to test the cooler fan.

Make sure to remove all traces of the OLD TIM from both the heatsink and CPU before applying some new TIM. 

[Plastic Nev]

For my two pence worth I am also suspecting the PSU, even though you see voltage readings it doesn't mean that the PSU can actually provide power. The one thing giving a clue is the fact that the CPU fan kicks in when power is disconnected. That sounds like a capacitor discharging across the fan as the mains power dies.

If it is the PSU, I am just hoping it hasn't damaged the motherboard, a new or borrowed known to be working PSU is all that can prove it one way or the other.

 

Nev.

[maplej6220]

Well,  "WE"  have found the problem.  It was the PS> I should have caught that when I only read 11.6 volts on the 12v line.  the 3.3 and 5v were right on.  Thank You all for the help and advice!!!  You are part of a great site!!!.

 

This one is solved.  Some really weird symptoms!!

[iammykyl]

 

 

I should have caught that when I only read 11.6 volts on the 12v line. 

I would not have made that as a first call. 

Although low at 11,69v, possible indication of an issue, but was still within specs.  More likely, "Nev, That sounds like a capacitor discharging across the fan" or such.

 

Good to see it fixed  

 

You are most welcome  

[123Runner]

11.69 vdc is not out of spec for the 12 volts. Although you were indicating its fan turning on when you 1st [lugged it in is not normal.

 

Glad you are working though.

[Plastic Nev]

Before I retired, I worked with CNC controlled plastics machinery and I have seen some strange things happen when a power supply for the control system fails. The flicker of power, in this case making the display flash on, was traced to the main ripple smoothing power capacitor discharging as the mains power was turned off..

 

Nev