Question Question about PSU testing

Feb 25, 2021
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Hey my computer is dead, not even booting to BIOS and constantly trying to restart itself. I suspect a PSU issue but it may MOBO too I guess.
I was trying to test my PSU with a multimeter like discussed here, or shown here .

Now here's a thing. I have this modular PSU where two cables are permanently attached and go to MOBO (24 pins) and CPU (8 pins), but the rest of the power cables (for GPU, HDDs etc) are optional. You know the drill.
Whenever I test 24 MOBO pins with all optional cables detached, every voltage seems to be ok.
However, when I connect the optional cable(s), the red +5V pins drop to around 4.3V which is outside the tolerance level.



I don't know if this is expected or does it indicate a fauty PSU.
Some 'How to test your PSU' sites and articles (like this) tell you to unplug all optional cables before testing but I don't know what's the resoning behind this.

All other pins seem to be fine and not change when optional cables are plugged in.
 
Feb 25, 2021
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"However, when I connect the optional cable(s "

When you do this....are you only connecting the end to the PSU....the other end is open?
Other ends are connected to things. I tried it with three optional cables conected to GPU/HDDs and then with only two cabes connected to HDDs. Both times the red mobo pin drops to 4.3 V.
 

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