Our little Mac Mini started turning off whenever we would move it or bump it at all.
Upon closer inspection, the cause was that the power cable's connection into the power jack on the back of the Mac Mini had become very sloppy and loose, and would fall out of the Mac Mini was disturbed in any way.
When you plug the power connector into the back of the Mac Mini, it would angle down and not grab on tight to the inner connector contacts. You can see in the second picture how the connector would sort of bend down and fall out.
A search of the Internet revealed that there are a lot of people who have this same problem, even with brand new Mac Minis and Mac Mini Servers. Apparently some of the Time Capsule units have this power supply and the problem of loose power connector cables too, but I was not able to verify that.
The only solutions that I found on the Internet were about replacing the motherboard, replacing the power supply, taking it to the Genius Bar, etc.
But I wanted to understand what the failure mode of this looseness was. I also did not want to buy a new motherboard or power supply. I wanted to know why it become loose, and how could it be fixed!
The solution was about as simple and cheap as you can get. Read on!
Step 1: Why Does the Cable Get Loose? Bad Design!
If you look at the opening in the end of the power cable, you see that it is like a squashed circle, or a racetrack shape. The flat sides of the racetrack-shaped opening are where the electrical contacts are located. These contacts slide onto corresponding contacts in the power jack in the back if the Mac Mini.
Unfortunately, there is nothing structurally in the power cable to keep the two flat sides from being pushed out to the sides, so over time, of the power cable is bent side to side or not inserted absolutely straight, the sides of the racetrack shape will be pushed out away from the center, and this will cause the connection to be loose on the inner plug on the back of the Mac Mini.
This is obviously a bad design. Apple's engineers should have considered adding a center bar that would connect the two sides of the racetrack together to prevent spreading.
Fortunately, there is a really easy way to fix this problem...squeeze it!
Step 2: Protect the Plug With Some Tape Before You Squeeze It
The metal band around the outside of the plug needs to be squeezed or crushed in just a little bit to flatten the sides of the racetrack back into shape.
We don't want to scratch the metal or the plastic on the plug, so we will wrap a layer of tape around the outside temporarily to protect against gouges or scratches.
I used green "Frog" painter's tape, but just about any tape will do such as black electrical tape, etc.
You can also opt to put the tape on the inside of the jaws of the pliers if you wish, so you can test-fit the plug into your Mac as you squeeze the plug more and more to fit properly. In my case since I wrapped the tape around the plug, I had to remove the tape in order to do a test fit.
Step 3: Now Squeeze Gently to Bend the Racetrack Sides Back In
I used Vise Grips for this since you can adjust them so they will only squeeze in a certain amount. I would recommend you do the same, and avoid using other types of pliers that do not limit how far you can squeeze.
Just crush the sides in a little bit, then try the plug for proper fit into the back of your Mac Mini. You'll need to remove the tape before you plug the connector into the jack on the Mac Mini unless you put the protective tape on the plier jaws instead.
Repeat squeezing and testing until the plug is nice and tight and stable.
Remove the tape when you are all finished.
Step 4: That's It! Your Mac Mini Is As Good As New (or Probably Better)
Now your Mac Mini's power cable will stay in securely without coming loose and restarting.
You just saved a bunch of money that you would have paid for a new motherboard, a new power adapter, or a trip to the Genius Bar.