Ok, thanks for all the excellent suggestions.
My wife and I performed a couple of tests on this old PC and it has me thinking: failed motherboard.
Test Power On switch. We removed the switch connection from the motherboard. We measured the resistance of the wires leading to the case power ON switch. It is nominally an open circuit when we measure it. When we pressed the power on switch on the case, we could see the resistance briefly drop, and then go back to an open circuit. I think < not sure > that transient is nominal.
Then we shorted (very carefully) with a screw driver the adjacent connector pins on the motherboard where the power On switch wire/connector would attach (I was wearing rubber gloves AND had a good screw driver for this). This are two very specific unmarked pins in the pin out area marked “CTRL_PANEL 1” in the bottom right corner of this sketch:
Its the pins that the case power cable goes to, and its clearly marked ‘PWR’ on the mother board (note it is NOT the green arrow … I’m just reusing an old sketch).
Shorting those pins with the screw driver did nothing. So it suggests to me the problem is NOT with the switch but with the motherboard or power supply.
Test power supply.
We removed all drives (except the CD drive and motherboard) from the power supply. ie Next to no load. Same symptom when we switch the PC ON. Nothing. No indication of a power switch ON (other than green light on motherboard, and brief fan movement when master power switch on power supply switched OFF).
We then removed all the power supply’s connections from the motherboard and from all other hardware devices. ie power supply was connected to nothing. Now see sketch below (click on sketch for larger view).
On the 20-connector cable coming from the power supply (which is the cable that nominally connects we shorted the connection #14 (green) to one of the black ground cables. We switched on the power supply. The power supply’s fan came on. We then measured the power from ground (black) to each of the colour connector pins on that ATX Main power connector, and obtained expected voltages readings, (orange = ~ +3.3 volts, red = ~ +5 volts, purple = ~ +5 volts, yellow = ~ +12 volts, blue = ~ -12 volts, white = ~ -5 volts). All that reads what it is supposed to read according to what our research suggests it should read.
I think that means the rather old power supply is good.
Any contrary views as to this assessment given the tests?
If I am correct that the power supply is good, then I think the Asus A7N8X Deluxe (~7 to 8 year old) motherboard is dead.
And ergo that’s the end of this PC.
Replacing the motherboard is not on the menu for me. The old 2GB of DDR 400 memory won’t work in a new motherboard (its too old). So I would need a new motherboard and new memory. Plus it means too much effort to take the motherboard out of the case, and put a new motherboard in the case (as noted, I would pay someone 50 euros to do all this without hesitation). Ergo, by the time I add up the cose of new motherboard, new memory, and installation fee, and its simply not worth it given this PC uses a low performance nVidia 8400 GS PCI (not PCI-e) graphic card, and it has relatively small sized hard drives, old DVD writer/readers … AND ~ 7 to 8 year old power supply.
Don’t forget we have 5 other functional PCs in our place.
I guess I will just take out the working parts, and put them aside.