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XPS 730 Will Not Posst


Please stick with me on this lengthy explanation.

 My XPS 730 ran well for almost 4 years. One day when I went to push the power button, it booted up but gave 5-beeps as an error code. I looked through the manual and recognized that this indicated an RTC failure. I checked the button cell battery and it was at full voltage, so I concluded it must be a motherboard error. The motherboard was displaying "26" on the LCD indicator. 

I purchased an identical motherboard and replaced the "failed" motherboard. The computer started with no beeps, but the fans stay at 100% and I get no video signal from the video card. I tried resetting the BIOS and resetting the Master Control Board. This did not change the issue. The motherboard currently displays "FF" when the computer is on.

The CPU Hot light on the motherboard does not illuminate when the computer is left on, but the fans remain at 100%. I tried removing the thermal sensor, but this does not affect the issue. The only way I can affect the issue is by unplugging either the fan to the heatsink over the processor (it is an air-cooled processor), or by unplugging the cable on the motherboard labeled as CPU fan. When either of these cables are unplugged, I am able to turn the computer on and the fans run at normal levels, though I do not leave the computer on in this state since the processor may not be sufficiently cooled. In all of these tests, I have never seen the CPU hot light come on. Even with these cables unplugged, I still receive no video from the video card.

I have tried using another video card but have not had any success. One odd thing I noticed is that I can turn the computer on without any  memory installed, or without the hard drive plugged in, or without a video card installed, and I receive no errors as I expect I should. The motherboard keeps displaying "FF", which I understand indicates normal operation.

My theory is that something is wrong with the Master Control Board that is preventing the computer from POSTing. There is an orange light labeled "DS1" on the Master Control Board which remains illuminated whenever the computer has power, whether turned on or off.

Should I replace the Master Control Board, or will this not help the issue? My PC is far out of warranty, but I believe it is repairable and would prefer to repair it than to purchase a new one. Can Dell realistically help me repair it at a reasonable cost? 

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