Another Attempt, Dell Optiplex SX260 Computer Won't Boot Up

Hi,

Regarding that small Dell Optiplex SX260 desktop type computer
that uses an external power supply that won't "boot up". Note: There
is a big difference between this SX260 and a GX260 model.

I checked to see if there was any operating voltage for the P4/2.6Ghz
CPU, and I measured 0V across the "bank" of 2200/6.3V filter caps after
I turned on the power. This CPU requires 1.25V - 1.4V to "run".

I measured +12V at the input of the V regulator(s) which I assume operate
as high speed switches (SMPS), but there was no voltage on the other
leads of those switching regulators in parallel.

Using a "low power" (prevents semi-conductors affecting in circuit
resistance readings) ohms meter, I measured about 5.5 ohms across those
filter caps. After I removed the CPU, I measured about 63 ohms. Of course
electrolytic caps normally have some "leakage" current that will affect the
reading.

It seems to me that the 5.5 ohm reading might indicate a CPU defect?

With the CPU removed, there is still no voltage across those filter caps.
I assume the voltage regulator has been damaged by the CPU?

Thank You in advance, John

wrote:
Hi,

Regarding that small Dell Optiplex SX260 desktop type computer
that uses an external power supply that won't "boot up". Note: There
is a big difference between this SX260 and a GX260 model.

I checked to see if there was any operating voltage for the P4/2.6Ghz
CPU, and I measured 0V across the "bank" of 2200/6.3V filter caps after
I turned on the power. This CPU requires 1.25V - 1.4V to "run".

I measured +12V at the input of the V regulator(s) which I assume operate
as high speed switches (SMPS), but there was no voltage on the other
leads of those switching regulators in parallel.

Using a "low power" (prevents semi-conductors affecting in circuit
resistance readings) ohms meter, I measured about 5.5 ohms across those
filter caps. After I removed the CPU, I measured about 63 ohms. Of course
electrolytic caps normally have some "leakage" current that will affect the
reading.

It seems to me that the 5.5 ohm reading might indicate a CPU defect?

With the CPU removed, there is still no voltage across those filter caps.
I assume the voltage regulator has been damaged by the CPU?

Thank You in advance, John


With the CPU removed, there is no VID code. The VID code floats.
On a VRD I worked on, the all-1's VID code (float value)
is "zero volts", which causes the chip to switch off.
(That means, with the CPU pulled, VCore switches off.)
On the regulator chip, take logic level readings on the VID inputs.
Compare to the datasheet VID table, and you'll likely find at least
two codes in the table that cause the regulator to be effectively
switched off.

The logic levels on VID codes now, can be quite low levels.
Like perhaps 1.5V logic. Looking at the regulator datasheet,
there may be pullups on the VID lines, and then all you'd need
is to short an input to GND to put a logic zero on it. At one time,
VID lines were open collector, but the processor may use totem pole
drive on them now. Since the processor is not plugged in, you
can do your own driving of the line.

Does VCore need a dummy load on it ? Is it open circuit stable
and properly regulated at no-load ? I don't know the answer to
that. I might use a small array of carbon composition resistors
as a dummy load. Say, twenty 10 ohm 1/4W resistors in parallel,
which would be a 0.5 ohm load across 1 volt output, for 2 amps
flow. And I^2R would be less than the 20/4 = 5 watt rating
of your array. Salt to taste.

*******

As for your ohm meter reading across the processor, I'll not
"go near that with a barge pole". If you need to do such things,
compare the reading to a known-good processor. There are
too many intangibles (sneak paths inside a multi-rail chip),
to go guessing whether "5.5 ohms = busted". My pure guess
would be, it's not dead with a reading like that. If you
read 0.0, I'd be suspicious... The VCore regulator
could still drive such a load - your ohmmeter likely can't
read low values reliably, to determine whether the
processor would actually cause the overcurrent to trip
on the regulator.

I've only seen one good failure on a VCore. Someone posted a
picture once, of a motherboard where the area around the
processor socket was charred (discolored). And that's a
plane to plane short, where the short wasn't close enough
to zero ohms, to trip the overcurrent. And VCore just
made a toaster out of that area of the motherboard.
If the internal plane short had been better quality,
and closer to zero ohms, there would have been no
charring, and the regulator would have switched off.
They do have overcurrent detection, but it's set
very very high. It takes a damn good short to trip it.

Paul