Seemed like a good idea at the time, when I needed to use a spare power supply to run an LED fixture. I needed about 8 volts so I figured what's the harm in joining a +3V and a +5V rail on the PSU. Immediately I then got 0V out of any rail - D'oh!

Given this scenario, what's the most likely component to look into to bring this thing back to life? It's a high efficiency Seasonic 650W job and I'd like to resurrect it if possible.

 

I should mention that I've been using this power supply for years on another project and the green and ground have been jumpered to allow it to run apart from being in a computer. Also, even the fan doesn't spin up as it always had when powered on.

 

If you're really, really lucky the problem will be the rectifier(s) on the 3.3V and/or 5V outputs or the capacitors on the 3.3V output.
Otherwise

 

How can you combine +3V and +5V from a PC power supply to get +8V?
They don't add like math 3 + 5 = 8.

Look like you blew the 3V supply and then that shut down the whole power supply.
Time to junk that power supply and start over with another solution.

 

Seemed like a good idea at the time, when I needed to use a spare power supply to run an LED fixture. I needed about 8 volts so I figured what's the harm in joining a +3V and a +5V rail on the PSU. Immediately I then got 0V out of any rail - D'oh!

If you found reasonable to do that, repairing that PSU is not what you could attempt and expect success, sorry to say.

 

You might be lucky to find a fuse blown on the mains side , otherwise you won't repair it without a diagram and even then these are a nightmare to fix.

 

Some great responses, thanks to all, even the snarky ones. It should be apparent to all that I know just enough to be dangerous to electronics. Though I did cobble together a self professed masterpiece of engineering, a DSLR based array stitching film scanner. Sadly it became inoperable when I re-purposed it's power supply and attempted this ill conceived, LED bank powering sub genius feat of engineering.

I am of mixed feelings to hear that a computer power supply isn't something so readily repairable. Yes, I watched some utube videos from folks that put my knowledge to shame and got a bit lost in the fine points but I fully intend to (discharge all caps and) try my hand at replacing a few cheap components to see if I can reverse the damage. With your help. So thanks again to all.

This won't go far in convincing my detractors that I'm in any way capable of doing so, but I'll be damned if I can find the 'fuse'. It's a newer job so must not be using a fuse but rather the new self resetting thingy, I forget the name. All I can find is a thermal resistor at this point that might be implicated.

 

https://www.techrepublic.com/article/learn-to-re-fuse-a-power-supply/
But note that they are not always obvious. They are often covered in black heatshrink tubing with wires sticking of the ends which are soldered into the board.

 

Some great responses, thanks to all, even the snarky ones. It should be apparent to all that I know just enough to be dangerous to electronics. Though I did cobble together a self professed masterpiece of engineering, a DSLR based array stitching film scanner. Sadly it became inoperable when I re-purposed it's power supply and attempted this ill conceived, LED bank powering sub genius feat of engineering.

I am of mixed feelings to hear that a computer power supply isn't something so readily repairable. Yes, I watched some utube videos from folks that put my knowledge to shame and got a bit lost in the fine points but I fully intend to (discharge all caps and) try my hand at replacing a few cheap components to see if I can reverse the damage. With your help. So thanks again to all.

This won't go far in convincing my detractors that I'm in any way capable of doing so, but I'll be damned if I can find the 'fuse'. It's a newer job so must not be using a fuse but rather the new self resetting thingy, I forget the name. All I can find is a thermal resistor at this point that might be implicated.

I know one or two who do repair PSUs regularly.
Maybe you could try, why not?
They started somewhere as well.

Snarky? (Had to look in the dictionay)
Not at all. Really serious in my comment.
Buena suerte.