How can I find out whether my PSU is too weak, degraded etc., say for my GPU?

[flexy]

I have a very old Toughpower 750W which served me well for MANY years. It is a 4 rail CPU. There was never a problem with my overclocked i7 and a GTX 660 TI.

Now with a GTX 970, in some heavy overclock situations I am seeing that the card crashes and the PSU could possibly be one reason. Short of replacing my PSU, I want to know whether it could be too weak, maybe from degrading.

* When I do my GPU stress test and start Prime95 in the background, nothing special happens. One would think that the additional load would immediately crash out the PC if the rails would be too weak?

* When I observe the voltages w/ various HW monitors, hwinfo, OCCT etc. I only see VERY little drop on the rail, and then NEVER below +12V.

Without load on the GPU it says +12.192 and when I stress the GPU it drops to +12.096, so it basically only drops 0.100 which I THINK is totally negligible?

I actually got a new XFX 750W PSU with one 60A 12V rail but I am now using this on wife's comp and dont want to exchange PSUs if I really don't have to.

Edit: Logic says the PSU should be fine since the GTX970 doesn't use that much more power than the GTX 660.

The 970 is on the two PCIE rails from the PSU (which are in fact split from one single +12 rail, this is how the PSU is). However, I also tried out with using the rail for the drives for one of the 6pin PCIE connector, same black screen issues w/ heavy GPU overclock. Likely because it's all on the same rail regardless as with most so called "multi rail" GPUs. Oh, and before I forget...the PSU does NOT "shut down and click" which would be a sign that something is overloaded. The PC simply reboots with black screen.

 

[Cerb]

If it is the PSU, and the PSU is old, there is yet another possibility: it may not be offering good enough transient response (which may or may not be related to aging). Haswell and Maxwell both change power states all the time, and do so quicker than prior CPUs or GPUs. The power draw numbers are average, but this can lead to many Amps worth of loading and unloading, to the PSU, in very small time spans (FI, the erroneously interpreted power numbers in that Toms GTX 970 review). If the PSU either could never handle them, or is not able to any more (IE, high ripple, overshoot, ringing, etc., during transients), that would explain stress testing working fine, but not actual use.

Either way, you really have to try another PSU to tell.

 

[Nehalem_420]

How many amps on the 4 rails?
If it's that old, I'd just replace it.

 

[Nehalem_420]

If it's this one:

The rails are too weak to power a gtx 970.
If you get black screen + reboot,replace your PSU