Studio XPS 7100, steady orange light on switch on

try replace cmos battery w a new one then clear cmos by motherboard RTCRST jumper

Thanks. I'd changed the battery but not cleared the CMOS. I'll give that a go.

Nope, did't work. I assume the CMOS reset jumper is the same as the  RTCRST one you refer to?

I assume the CMOS reset jumper is the same as the RTCRST one you refer to?

Correct. Studio XPS 7100 Service Manual page 46 =

probably bad motherboard if clear cmos, replace ram and psu makes no difference.  old board not worth replacement.

Yeah, I think so. Thanks for the help.

I have ordered a cheapo 2nd hand one from EBay, just so I can try and familiarise myself with this stuff so that one day before I die I can build my own spec machine. £20 for the board.

I got a motherboard of EBay and fitted it, same problem. I wonder if the on/off switch unit could be defective? I tried that "cutting the green wire and connect ot to ground" thing and still got the orange light and nothing on the screen, but the fans seemed a LOT faster.