w_tom wrote:

It takes less time and would have taught more by not removing
supply AND using the so important 3.5 digit multimeter.

I don't have one of those - how much would it cost, where could I get
one, and which do you suggest? And how exactly would I use it -
attaching one lead of the multimeter to the little metal contacts in the
wires coming out of the PSU and the other lead to ... where?

Procedu Motherboard turns power supply on and off. But
controller circuit requires power. When power supply is
plugged into wall receptacle, then purple wire (from power
supply to motherboard) provides 5.0 volts (within limits of
chart).

When power button is pressed, then motherboard controller
tells power supply to power on. Green wire is above 2.4 volts
when power supply should be off. Green wire should measure
less than 0.7 volts when power button is pressed - to command
power supply on.

Three components - power supply, motherboard controller, and
power switch. Using meter, discover which is or is not
working. Only then were you ready to replace something.
Voltage Wire Color Min V Max V
+5 V Red 4.75 V 5.25 V
-5 V White -4.75 V -5.25 V
+12 V Yellow 11.4 V 12.6 V
-12 V Blue -11.4 V -12.6 V
+3.3 V Orange 3.135 V 3.465 V
+5VSB Purple 4.75 5.25
!Power On Green 0.8 2.0
Power OK Gray 2.4 when power is good

BTW, if you did not pull plug from wall receptacle before
removing supply, then motherboard failure is a new
possibility.

No, I definitely always totally unplug everything from my box before
opening it up and screwing around with the insides. Also, the physical
location of my box (on a high shelf above my desk) and its length from
the power outlet makes it a near physical impossibility to work on the
machine while it is still plugged in.

By the way, thanks for the quick and thorough reply. I'm going to get a
new power supply this weekend anyway - if it fixes the problem, great,
if not, I can stick it in one of my other machines that I think may be
suffering from less current.