Fans work /no post - black screen

[sickster]

Firstly my front usb's stopped working for a day, no other problems at all b4 this.

Then i was away for a few days (computer off) on return computer would not boot. no beeps. no post, just fans and black screen.

I tested the two ram modules and both have some led lights, i tried taking one out/ then swapping them etc no difference.

I thought then it was the mobo, i replaced it w a (untested) secondhand mobo.
This time fans start for a moment then stop, still no post no/black screen etc.

I tested the power supply in another mobo/old sys and it powered up and there was post sound.

I have now found that the fans work when the 4pin atx12v is not plugged into the board. as this powers the chip i am wondering if the chip is fried or if it may be something else?

 

[sickster]

more info:

I have also replaced the power supply with 600w jeantech

also i have since tried to disconnect all power to all but the mobo and 4pin atx 12v - still same problem

i have removed all the pci cards and graphics card - still no joy

So what do i have left to do? disconnect any other drives completely and check for poss shorting of mobo?

The cpu and ram have been reseated when i replaced the original mobo

any help much appreciated

why do fans work when atx 4pin is not connected but not when it is connected?

obviously the system will not post w this disconnected but why do the fans run?

thanks

 

[Tedster]

have you tested your ram?

 

[edddder]

pls check also your video card is properly inserted (if you are using one)