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How do I test the power supply in my 2012 Mac P...

My mid 2012 12 core Mac Pro is randomly crashing.  It happens from time to time when I do a routine function, like moving a folder to the desk top. 

I have tested the 64 GB of RAM by just installing 2 strips at a time and running the computer.  It crashed with more than one RAM set while testing, so I don't think that it is bad RAM. 

When it crashes it just turns totally OFF.  Both monitors go dead and the front light goes out - BAM - off!  the computer does NOT reboot on its own.

I do not know how to get an error message of an instantaneous shut down.  I ran the hardware test and used TechTools 9.5 - and they reported no problems.

I have a used power supply on order from eBay --- Can it be that?

Power connectors[edit]

Standard connector[edit]

Pin #		Mating	Function
—			Coding notch
	1	3rd	3.3 V
	2	3rd
	3	2nd
	4	1st	Ground
	5	2nd
	6	2nd
	7	2nd	5 V
	8	3rd
	9	3rd
	10	2nd	Ground
	11	3rd	Staggered spinup/activity (in supporting drives)
	12	1st	Ground
	13	2nd	12 V
	14	3rd
	15	3rd