Ok some background:
I have had this PC for over 2 years, with no problems to date. Nothing in the setup has changed recently. I have not installed / removed anything or done anything unduly stupid. The PC is a SCALEO 600, motherboard Gigabyte GA-8SGXL with Celeron 2.6GHz CPU.
The problem:
When I press (and release) the on / off button the power, HDD and DVD lights come on, as does the fan. Nothing else happens. The HDD and DVD lights do not go out (as they should), and the HDD does not start whirring. No output is sent to the monitor at all. Holding in the on / off button does not shut down the PC. Pressing the DVD eject button does not open the DVD drawer. The PC stays in this state indefinitely, until I turn off the power on the PSU.
This problem just started yesterday, and is 100% consistent. Same result every time I try. I figure some piece of hardware has broken, but I'm running out of ideas on how to isolate what it is.
Things I have tried:
1. Checked the on / off switch is not jammed in any way. All looks fine.
2. Removed all leads from the back of the PC. Held in on / off button for 15 secs. Reconnected power and monitor leads. No change in behaviour.
3. As above, plus removed CMOS battery for several minutes before reconnecting everything. No change in behaviour.
4. Disconnected all IDE devices (HDDs, DVDs, Floppy). I disconnected the data cables to these devices, not the power cables. When the PC was turned on the HDD and DVD lights no longer come on. It is now possible to eject the DVD. Still no output to monitor. No apparent change in behaviour.
The only thing I have not done is short the CMOS jumper to reset the CMOS settings to default.
Other than that I am running out of ideas.
Has anyone seen this before, or know what it could be? Or any other ideas for things to try?
I'm thinking CPU or motherboard problem.
Can't be any of my IDE devices, as they are all disconnected.
Can't be any of my peripherals (mouse / keyboard / USB bits) as they are also disconnected.
Don't think it is IDE controller, as surely I would still get something on the monitor, e.g. POST checks?
Don't think it is RAM either, for the same reason.