Have you ever done this?
I was recently sorting through several spare computer parts in my home office and decided to figure out if this mother board still worked. I know, it’s a risky thing to test out an old motherboard. It’s kind of like eating a box of chocolates – you know how the rest of the line goes. Anyway, I had a pretty good suspicion that this board was only lacking a power supply, but the only (ATX 24-pin) power supply I had available was in this other I had rebuilt within the past few years. So, rather than run out to MicroCenter and drop $15-20 on a power supply that I may or may not use, I opened up the working PC case and began to do a little surgery.
I disconnected the power from everything in working case, and I pulled the cables out to the test board just outside the case. I hooked up a spare CD drive to run a live Ubuntu disk, and I plugged in the basic KVM (keyboard, video, and mouse) hardware. Once I had all of that, I realized I need a way to turn it on, so I pulled the power switch cable from the working case and attached it to the test board. After a double check of all the cables, I plugged in the power and hit the switch. Thankfully, there was smoke and no sparks. In fact, the rig roared to life, booted Ubuntu, and even connected to the internet.
This test was a success. I don’t recommend it for timid or inexperienced, but in a pinch it can work. Now I just need to find another power supply…