After running with current config for 6 months my machine now won't even
POST. No beeps at all, but CPU & other fans running. HD runs and stops. Power switch when pushed and held now doesn't switch the unit off - I have to use the small switch on the PS to stop it running.

Testing with only graphics card and RAM in place. Both have been swapped
with no change. CMOS cleared also but no go.

Faulty PS or mobo? Any help appreciated.

If the fans aren't spinning then it is likely the PS.

If the fans are spinning it can still be the PS, but then the mobo could enter the equation.

Swap with a known good power supply.

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Have you had any heat problems in the past??

Have you tried testing the system outside the case? I would do that if you haven't. Try the minimal boot on a nonconductive surface (like a piece of cardboard or wooden table top). If still no POST with a different power supply and/or tabletop test, then the mobo or CPU are the most likely problems.

Send me a personal message with an e-mail address to which I can send alterable copies in Word Document files of my Diagnostic Flowcharts.

There are 10 charts covering:

START UP
SYSTEM
DISPLAY
NO POST
BOOT UP
HARD DISK DRIVE
FLOPPY DISK DRIVE
SERIAL PORT
PARALLEL PORT
POWER SUPPLY