G4 Quicksilver adn G4 dual 867 mirrored doors - same symptoms

Both a G4 Quicksilver that I've had for some time and a new-to-me dual G4 mirrored machine are displaying the same symptoms.

Pressing poweron lights the power button while pressed, it goes off immediately upon release.

The LED on the motherboard comes on and remains on.

No power seems to make it to the USB jacks.

After a minute or so the fan switches to high speed and the machine remains like that until powered off, the power button will power it off if held long enough.

Both machines use AGP graphics.

Things I have done:

Resett the PMU on both machines.

Replaced battery on both machines and reset PMU again.

Replaced logic board on dual 867 machine. (motherboard and dual CPU daughterboard.)

Symptoms remain the same on both machines.

Any advide would be welcome.

Correction - although I get no keyboard response my cell phone does register a computer connection through the USB and automatically switches into connected mode.

Do you get the chime or any beep codes?
Great article:
Apple - Support - Discussions - G4 MDD won't start after a power outage ...
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