Dimension 3100 Won't POST, getting amber flashing amber light at front panel and on the mother board getting steady light near the cmos battery note under the light says cr3h1 re-seated cpu and cards,...
I concur with your diagnoses.
Weak power supply due to a weak, or failed, voltage power rail.
There are three main voltage power rails for your Dell Dimension 3100 power supply.
1.The 3.3 Volt rail
2.The 5 Volt rail
3.The 12 Volt rail
Newer computer power supply's are doing away with the 3.3 Volt rail.
Your power supply has,
16 Amps on the 3.3 Volt rail,
21 Amps on the 5 Volt rail, and
21 Amps on the 12 Volt rail.
These are the main power voltage rails.
Voltage times Amperage = Wattage.
You have a power supply, that is rated at a maximum output of 400 Watts.
1) ALL the lights use less than 1 Watt.
2) EACH fan uses 2 to 3 Watts
3) A typical processor uses 55 to 125 Watts.
(A typical Intel Pentium 4 HT processor, Model 640, (3.2GHz), uses a maximum of 84 Watts,
With a weak, or failed voltage power rail you don't have enough power to turn on the processor.
No processor running there is nothing to run the operating system. (Windows XP is an example of an O/S)
The harddrive just sits there, and spins, or will spin a few times, and stop.
What causes this?
1) Cheap quality power supply, (SMPS), installed by computer manufacturer. Cheap components are used in the power supply. (Electrolytic Capacitors, gauge of wiring, rectifier bridge, and so on)
2) Inside of computer, and power supply hasn't been cleaned on a regular basis.
Causes havoc inside the computer.
The processor fan builds up with dust, dirt, hair, etc, and the cooling capacity drops tremendously. Eventually the processor turns off. (BIOS turns it off)
It's a Fail safe feature that is built-in, to keep the processor from burning up.
Before it get's to the stage of overheating, and turning off, it calls for more power from the power supply.
The power supply works overtime to keep up.
When the inside of a power supply, and it's cooling fan are clogged with the above mentioned gunk, the cooling capacity of the power supply also drops tremendously.
The heatsink's inside the power supply cannot get rid of the heat. The fins are clogged, and air cannot rush through them to disperse the heat.
When the blades of the fan, and it's center hub become clogged, the cooling capacity drops due to not enough air is passed.
Heat = Wasted Energy, and the power supply strains to keep up. Eventually components inside the power supply start to fail, or fail.
(Electrolytic Capacitors are usually the first to go. They are the weakest link)