[…] managed to squeeze maybe six more months of use out of that old rig by putting in a new video card, until the motherboard died in August. In September I got a brand new machine, and it started having issues on December […]

So, seeing a reading that indicates that the 12V line is too low either means that it’s too low or that you don’t have it connected to the meter right. Your power supply brick does indicate that it should have 4 accessible +12V lines.

assuming that this is your problem:

Depending on your usage and the power supply’s pinout that might just mean that even if one of the +12V lines is down, it could still be usable.
I’d recommend looking up some pinout charts like http://www.smpspowersupply.com/connectors-pinouts.html for your connectors. (heck that one might even have the needed pinout for you)
As it looks like the pinout styles only end up having two +12 V lines per connector, you might be able to find one that works fine.
If you are able to eliminate the ones that have a damaged then you might be able to reconnect and boot away.

I’d still shove the voltmeter on the pins that the PSU tester indicates are bad to see what sort of voltage actually is there, as it does eliminate ambiguity due to bad documentation on the PSU tester’s part.

Guys, I troubleshooted (thoubleshat?) some more.

I went out and bought this PSU tester device:

Using this contraption to test a desktop PSU. It's flashing LL and beeping. I have no working PSU for base comparison pic.twitter.com/dLGhqf9QNC

— Luke Maciak (@LukeMaciak) August 8, 2014

It did give me some interesting readings that I am not entirely sure how to read. The manual that came with the device is more than useless:

Anyone can tell me what this output of a PSU voltage tester means? The "manual" is useless 1 page POS. pic.twitter.com/qMRPuEKno1

— Luke Maciak (@LukeMaciak) August 8, 2014

The LL symbol under the +12V2 is flashing. The 1 page manual claims it either means no voltage or voltage lower than acceptable minimum. Greg on Twitter suggested my PSU might be missing that rail so I checked it:

This is the spec block on my PSU. @Vanderdecken it looks like it's supposed to have 2nd +12V rail pic.twitter.com/wUWmWBEhb8

— Luke Maciak (@LukeMaciak) August 9, 2014

The +12V2 rail is listed on the PSU spec block so it does seem like it is supposed to have it. So am I correct interpreting this as my PSU having developed a fault on that rail and needing to be replaced?

you should check to see that you havn’t grounded out your mother board as that could cause the effects you have listed…

I did have similar symptoms (nothing happening, just a light on and fans, I’m not sure about beeps, since that machine has no speaker). Unplugging and removing the BIOS battery for few minutes actually helped.

@ Luke Maciak:

I’d imagine that you should be able to find some information online as to which pins should be at which voltage levels in some sort of guide format. If you do go doing this though be aware that (IIRC) power supplies will expect some pins to get shorted out for them to power on and that the tolerances on what an acceptable voltage is will vary quite a bit if you’re testing an unloaded power supply.
If you can see something like the 12V line is only registering 3V though and the capacitors within the supply are bulging, then you can at least know that that supply (or at least that one cap) is shot.

A power supply tester might manage all of this easily, but I’ve got at least 3 voltmeters sitting around and this is the first time I’ve heard of dedicated power supply testers, though their existence makes perfect sense.

@ fundamental:

I think my dad has some voltmeters lying around somewhere, but I probably wouldn’t know what to do with them.

@ Douglas:

Nice! Thanks for the suggestion. :)

@ Jason *StDoodle* Wood:

TIL that PSU Testers are a thing. :P

@ Chris Wellons:

Uhgh, thanks for reminding me of the EUFI bull-crap. Do they only enable that for machines that ship with Win8? We’ve been getting laptops with Win7 at work and they all have fairly standard (albeit proprietary) DELL BIOS setup not unlike what I’ve seen shipping with XP and Vista. I’m guessing that I get a system that’s can be “upgraded” to Win7 on the store page, it will have it disabled?

The ideal way to solve this type of problem is with identical hardware. One approach to get similar results is to bye all possible replacement parts from ebay and sell what you don’t need back when you are up and running again.

Yeah, I absolutely dread troubleshooting computer hardware too. Troubleshooting software is interesting because if I can’t solve the problem immediately, I usually learn something new, interesting, and valuable. Troubleshooting hardware is time-consuming, expensive, non-deterministic, and generally doesn’t teach me anything useful. To avoid it, I just go the brand-name route and pay a little extra to not have to deal with the headaches. The risk is that I might buy something hostile to Linux (or to re-installing Windows), which currently an issue now that everyone’s switched to their own half-assed implementations of UEFI. It can be difficult to impossible to boot install media with this. To avoid getting stuck on this I spend a bunch of time researching before making the purchase.

For example, in February my wife’s computer’s cooling system died. It would shut off from overheating after about 3-5 minutes. It was a cheap iBuyPower computer, which I strongly discourage anyone from buying. Their builds are total crap, they have no quality standards, and their computers are noisy. Stay away. Since it was a couple years old and was kind of junky we just decided to replace the whole thing. She’s even more of a gamer than me (in terms of hours played), so we got a Acer AG3-605-UR38. It’s listed at $1,400 now, but back in February it was only $975 (why?). Outside of disabling UEFI in order to “upgrade” to Windows 7 — and so I could boot my personal live Linux system on it in case I ever wanted to use it myself — everything worked out without a hitch. I plan to do this every single time I buy a new computer in order to avoid messing with hardware.

@ fundamental:

They even make specialized testers for PSUs; have one myself, dead simple to use. If you know any hardware-centric folks you might be able to borrow one to quickly narrow down that particular possibility.