Dell Dimension 2400 -- Won't Power On

I'm working with a friend to try and resolve a problem with a Dell
Dimension 2400 PC: the system will not power-on.

In an earlier phone call to Dell, my friend was told that the problem
was either a bad power supply or a bad motherboard. The system was out
of warranty, so my friend ordered replacements for both components.

Those components arrived a couple days ago, and my friend asked me to
help install the parts, which we completed earlier today. Much to our
surprise, the system still refuses to power up.

We do see a green pilot light on the motherboard (the LED is located
adjacent to the motherboard's main power connector). But the four
diagnostic lights on the back of the system remain unlit.

We tried the power supplies on a second (non-Dell) PC; that PC seemed
to work just fine with both of the Dell power supplies.

We've swapped the CPU in the Dell with an equivalent CPU chip from
another system; the Dell still refuses to power on.

We've tested the Dell's power switch using a digital multimeter; it
appears to be functional. When the button is pushed in, we get zero
ohms resistance; with the button released, we get infinite resistance.

At this point, we can't figure out what the problem might be. Reaching
the "grasping at straws" stage, we started poking around the non-Dell
system and found one interesting difference between the two boxes: on
the non-Dell PC, we measured 3.3 volts between the two motherboard
power-switch jumper pins. On the Dell, we measured 1.3 volts between
the same two jumper pins.

We've started wondering if this anomaly might be the source of our
problem. Since the 2400 uses an ATX motherboard and power supply, we
assume that the power-on sequence is triggered when the motherboard
(or the power supply) sees a jump of 0 volts to some non-zero value,
caused when the power button is pressed. We're wondering if this
circuitry expects to see some minimum voltage level in order to
recognize that the power switch has been pressed; 3.3 volts seems like
a reasonable value, since that's one of the voltage levels coming out
of the power supply. If our measurements are correct, then we're
guessing that 1.3 volts might not be large enough to trigger the
power-on sequence.

If our guess is correct, our next question would be: any ideas on what
could be causing the under-voltage condition?

Thanks in advance for any help you can offer!

Eric

"Eric Chevalier" <> wrote in message
news:...
> I'm working with a friend to try and resolve a problem with a Dell
> Dimension 2400 PC: the system will not power-on.
>
> In an earlier phone call to Dell, my friend was told that the problem
> was either a bad power supply or a bad motherboard. The system was out
> of warranty, so my friend ordered replacements for both components.
>
> Those components arrived a couple days ago, and my friend asked me to
> help install the parts, which we completed earlier today. Much to our
> surprise, the system still refuses to power up.
>
> We do see a green pilot light on the motherboard (the LED is located
> adjacent to the motherboard's main power connector). But the four
> diagnostic lights on the back of the system remain unlit.
>
> We tried the power supplies on a second (non-Dell) PC; that PC seemed
> to work just fine with both of the Dell power supplies.
>
> We've swapped the CPU in the Dell with an equivalent CPU chip from
> another system; the Dell still refuses to power on.
>
> We've tested the Dell's power switch using a digital multimeter; it
> appears to be functional. When the button is pushed in, we get zero
> ohms resistance; with the button released, we get infinite resistance.
>
> At this point, we can't figure out what the problem might be. Reaching
> the "grasping at straws" stage, we started poking around the non-Dell
> system and found one interesting difference between the two boxes: on
> the non-Dell PC, we measured 3.3 volts between the two motherboard
> power-switch jumper pins. On the Dell, we measured 1.3 volts between
> the same two jumper pins.
>
> We've started wondering if this anomaly might be the source of our
> problem. Since the 2400 uses an ATX motherboard and power supply, we
> assume that the power-on sequence is triggered when the motherboard
> (or the power supply) sees a jump of 0 volts to some non-zero value,
> caused when the power button is pressed. We're wondering if this
> circuitry expects to see some minimum voltage level in order to
> recognize that the power switch has been pressed; 3.3 volts seems like
> a reasonable value, since that's one of the voltage levels coming out
> of the power supply. If our measurements are correct, then we're
> guessing that 1.3 volts might not be large enough to trigger the
> power-on sequence.
>
> If our guess is correct, our next question would be: any ideas on what
> could be causing the under-voltage condition?
>
> Thanks in advance for any help you can offer!
>
> Eric



Test the old power supply as well if he hasn't sent it back. See if your
measurements are consistent.

You need to have only the new board, the CPU and heatsink, and the new power
supply in and connected to test for POST beeps. All cards and drives need to
remain unplugged.

From what you've replaced, you could still be no POST due to any number of
bad components - beginning with the CPU all the way down to a bad drive or
card.


Stew

"Eric Chevalier" <> wrote in message
news:...
> I'm working with a friend to try and resolve a problem with a Dell
> Dimension 2400 PC: the system will not power-on.
>
> In an earlier phone call to Dell, my friend was told that the problem
> was either a bad power supply or a bad motherboard. The system was out
> of warranty, so my friend ordered replacements for both components.
>
> Those components arrived a couple days ago, and my friend asked me to
> help install the parts, which we completed earlier today. Much to our
> surprise, the system still refuses to power up.
>
> We do see a green pilot light on the motherboard (the LED is located
> adjacent to the motherboard's main power connector). But the four
> diagnostic lights on the back of the system remain unlit.
>
> We tried the power supplies on a second (non-Dell) PC; that PC seemed
> to work just fine with both of the Dell power supplies.
>
> We've swapped the CPU in the Dell with an equivalent CPU chip from
> another system; the Dell still refuses to power on.
>
> We've tested the Dell's power switch using a digital multimeter; it
> appears to be functional. When the button is pushed in, we get zero
> ohms resistance; with the button released, we get infinite resistance.
>
> At this point, we can't figure out what the problem might be. Reaching
> the "grasping at straws" stage, we started poking around the non-Dell
> system and found one interesting difference between the two boxes: on
> the non-Dell PC, we measured 3.3 volts between the two motherboard
> power-switch jumper pins. On the Dell, we measured 1.3 volts between
> the same two jumper pins.
>
> We've started wondering if this anomaly might be the source of our
> problem. Since the 2400 uses an ATX motherboard and power supply, we
> assume that the power-on sequence is triggered when the motherboard
> (or the power supply) sees a jump of 0 volts to some non-zero value,
> caused when the power button is pressed. We're wondering if this
> circuitry expects to see some minimum voltage level in order to
> recognize that the power switch has been pressed; 3.3 volts seems like
> a reasonable value, since that's one of the voltage levels coming out
> of the power supply. If our measurements are correct, then we're
> guessing that 1.3 volts might not be large enough to trigger the
> power-on sequence.
>
> If our guess is correct, our next question would be: any ideas on what
> could be causing the under-voltage condition?
>
> Thanks in advance for any help you can offer!
>
> Eric

Have you stripped everything else out of the machine (video card,
memory, drives, sound card, any other add-ons) and tried to power it up?
If not try that first then add in pieces until it doesn't power up. That
will show you the bad part.

Steve W.



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Eric Chevalier <> writes:
>Those components arrived a couple days ago, and my friend asked me to
>help install the parts, which we completed earlier today. Much to our
>surprise, the system still refuses to power up.

Make a list of the other items in the system that use power.

1. CDROM
2. Disk drive
3. CPU fan

unplug each item and try to power up.

I had a Dimension 4700 arrive DOA a few weeks ago - same symptom - would not
power up.

Dell service came out twice - swapped cpu, motherboard, power supply. It still
would not power up.

When I pulled the CPU fan - bingo - POST.

Good Luck!

Mark Hittinger