Found it!
The new power supply that I fitted did not provide -5 volts.
That appears to be dropped on later PSUs ???
The motherboard still worked but complained about the abscence by
displaying it as -61 volts in the bios. That must have been wrong
because there was nothing connected to the -5volts pin of the
motherboard power connector.
Changing the PSU for one that listed -5v and had a cable going to the
pin cured the problem.
>> Stay informed about: Alarm sounds on GA-6BXE?