For now, this is the short version; I need some sleep and a chance to reflect on what I have found.
I have been troubleshooting my parents' PC, which is a Acer Aspire X3400 with AMD Athlon II processor.
After much time going through the obvious and getting more obscure, I finally have some interesting observations.
By the process of elimination, I have removed all components down to the bare minimum.
The computer is configured as motherboard, CPU with fan cooler and power supply. Nothing else.
All RAM, peripherals and connections have been removed.
What I have noticed is that the 5V standby power of the power supply is 5 V when NOT connected to the motherboard.
When connected to the motherboard, before the computer is turned on, the standby power appears to be dragged down and measures only 3.8V.
When power is activated by briefly shorting the power switch pins on the motherboard, the computer starts and runs for 3 to 3.5 seconds.
During this brief time, the CPU fan runs. Very interesting is that the standby power is dragged down to 0V.
The computer turns itself off after the 3 to 3.5 seconds, and the standby power momentarily reaches just over 4 V and then settles back to 3.8 V.
This is measured with a fairly cheap handheld DMM.
Does anyone have any experience with symptoms like this? Googling while fatigued did not give any good results.
Also, any opinions on whether something this old (but not heavily used) is even worth the time and effort to chase down?
Thanks!