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@mcnick50 

15 year old windows XP/Vista Machine. Most likely failure is power supply.

Replace CMOS battery and power supply with EVGA 850B

EVGA-110-BQ-0850-V1 B01FYDUCA0

evga  direct purchase 110-BQ-0850-V1

i've changed psu and battery nothing changed...same error

@mcnick50 

Replacing CMOS battery will NOT repair a physically bad power supply.

the power button is amber and the diagnostic led 2 blinking. i've used another same model power supply

@mcnick50 

Unless you used specific Power supply from Dell or a unit with 150W or more on the 3.3v/5v rails its not working.

Blinking amber power button indicates incompatible or bad PSU.

Solid amber power button is dead cpu / motherboard.

as i said i also changed power supply and the same error

How many addon pci-e cards do you have installed ?

Try remove them and check if there are differences

i have removed everything... and the problem there

Hi @mcnick50,

With PC off and plugged in, and with external peripherals disconnected, push the test button on the PSU.  Does it give you the green light?  If it does, then your internal peripherals are good.

If not:  How to Run a Power Supply Unit Self Test On a Dell Desktop or All-in-One Computer 

And:  Dell BIST Test Video 

yes i have a green light

Normally the error codes can be helpful.  But in this case, we're not landing on something more tangible.

Precision T3500/T5500/T7500 Workstations Diagnostic Lights and Beep Codes 

Try reseating the RAM one stick at a time, or try each stick individually in the first DIMM slot and see if the PC will light up.  Just a reminder to have PC off and unplugged anytime working on the inside of it.

Not sure what you have for RAM.  But if you find the T3500 works with certain sticks, granted you have more than 1 - Try 2 sticks in the DIMM slots with white tabs first and/or the same with black tabs.

One can also try a known working stick of RAM and try it one a time in all the DIMM slots in case of bad slot.

Also, if more than 1 stick of RAM, make sure all the specs are the same - mfr., capacity, volts, and speed.  Sometimes some Dell's will have a little flexibility with mixing mfrs. and/or capacity, but if they're different, that's something to rule out.