Dell OptiPlex GX270 desktop - won't start up.

[Analogdevice]

Hello friends,

I've got a problem with this one and looks like a step down converter on CPU. When I plug PC into mains and press the ON button, power LED lights amber, HDD LED lights and diagnostic LEDs are off. Hard drive won't start up when IDE is connected. Chipset inside and CPU are cool. After few attempts computer goes ON normally, everything works fine. After that the computer is OK, I can turn off and back on machine and its OK. But when I unplug device from mains and after some time plug it back, the problem repeats.

I updated BIOS to version A07 (last version), battery is good (3V), power supply gives voltages in tolerance without ripple.

Any ideas?

Thanks!

[PA0PBZ]

Check the caps around the CPU, I brought one back to life replacing a few of those.

[edpalmer42]

When a PC has trouble turning on, it's often a problem with the 5V Standby power from the power supply.  Check the voltage with a scope if possible for correct value and minimal noise/ripple.  If there's a problem there, check the power supply for bad capacitors.  Use appropriate caution when working on the power supply.  Line voltage is present in there.

[Analogdevice]

Caps around CPU are OK (they have approx. 1780uF from 1800uF).

Standby voltage is present, real voltage is 5,1V without ripples. There isn't problem 

Don't worry, I'm qualified electrician up to 22kV.

[wblock]

Caps around CPU are OK (they have approx. 1780uF from 1800uF).

Caps can keep their capacitance while the ESR goes high, making them incapable of absorbing ripple.

Standby voltage is present, real voltage is 5,1V without ripples. There isn't problem 

Without ripple at all?  Most PC power supplies have easily-measurable ripple.  10 mV would be pretty good.  How are you measuring it?

Look in the power supply.  Failed caps do not always bulge, but usually there is more than one, and some will.  And the Wikipedia page on the Dell Optiplex specifically mentions the GX270 for failed caps: https://en.wikipedia.org/wiki/Dell_OptiPlex

[crispy_tofu]

And Wikipedia page on the Dell Optiplex specifically mentions the GX270 for failed caps: https://en.wikipedia.org/wiki/Dell_OptiPlex

+1
GX270s are notorious for capacitor issues, I had a few that blew up. Unfortunately I don't own them anymore...

[AKM]

How do the capacitors in the PSU look like?

[Analogdevice]

Caps around CPU are OK (they have approx. 1780uF from 1800uF).

Caps can keep their capacitance while the ESR goes high, making them incapable of absorbing ripple.

ESR was approx. 800mOhm, measured by RLC bridge at 100Hz.

Without ripple means that ripple was less than 5mV, measured on Fluke 123 with AC coupling in input.

I look to the power supply for the caps, thanks.