After I replaced Power Supply 550W with one 650W my computer can't boot - no beeps, no BIOS, nothoing. From time to time it works but only 10 to 20 secconds. The new Power Supply is no demaged, it works on another computer.
I took processor (Athlon 1700MHz) and 768MB DDR (512+256) and put in another computer. All worked fine about 1 hour. Than the second computer stops also. It can boot, it is possible to go in BIOS, it is possible to stay in BIOS half an hour, but after exiting BIOS the computer works max 20 secs.
I replaced than all components with originals (Athlon 1400MHz and 2x256MB DDR) but the second computer "works" on the same way as worked with processor and memory from first computer.
Another CPU, another Power Supply, another Graphic Card - no success.
I tried with only Graphic Card, keyboard and bootable CD in CD Drive - nothing, it works max 15 secs.
Booth motherboards are dead ? If true, why?
Anu idea?

One item at a time. If the computer wouldn't POST after the PSU change then either the PSU is defective or you have some wires connected wrong or not at all. You may have also bumped womething. Reseat all cards, cables, RAM.

When you moved the CPU to another computer did you clean off the thermal paste and apply a small amount of new? If not, you may have damaged the CPU.

Swapping parts indiscriminately isn't the correct way to troubleshoot. You need to learn the basics.

Why did you replace the PSU in the first place?

Try clearing the CMOS and resetting BIOS values as needed. Use only one stick of RAM.

First check that there isn't a voltage switch.

It could be that you damaged something or the power supply damaged something.

You test power supplies with a voltmeter. Check DC and also test for AC on the DC line.

I read it wrong and answer it wrong too. So get off my case you peanut.

risp

What is the local AC voltage where you live?

A few thoughts.
1. Compare the the plugs (20 or 24 pin)to the motherboard on both power supplies. Are they same? Starting at on end.
Orange blue
,orange, orange
black black
green red
black black
black red

on and on...to the end.

If the color coded wires are different then thats a problem. Not all machines use a standard power supply.

2. First concentrate on getting the computer that was running ,up and going 100%.

Then drop back to the other one.

3> One machine and one part at a time.

Thanks to all, especially to OtheHill who gives me idea "clean off the thermal paste and apply a small amount of new".
I've cleaned but the new amount was not small. And I believe that my thermal paste was one of bad quality.
I tested 768 MB RAM for about 3 hours (MemTest86+, 4 pass, 0 errors).
I hope the problem solved :-)