Why would a single receptacle fail after updating other wiring?
The short version of the solution is: when you see a very old junction that is taped together, unwrap it completely (while the circuit is off), and inspect the junction directly. Then, fix as needed, and replace the tape with wire nuts. The new upstream wiring had nothing to do with the actual problem, and the wiring inside the old downstream junction box must have shaken loose during demolition. Troubleshooting it the first time resulted in the accidental rejoining of the wires within the box, restoring continuity, and creating the illusion that the outlet was the problem. The first repair worked out on a fluke.



How I missed It: The issue was not with the outlet, but inside a junction box that was ruled out since it was not accessed until the problem with the receptacle arose. While the circuit was off, I removed the old receptacle and opened up the junction boxes between it and the panel. After restoring power to the circuit, testing the bare wire immediately yielded ~126v. So I shut it back off, installed a new outlet, closed the boxes and restored power. Done deal.

Three days later, no power again. Actually, very low power. After the second time the outlet stopped delivering proper voltage, I decided to diagnose the original receptacle before removing the new one. It was fine.



Checking for continuity and shorts with a meter: Disconnect and remove the outlet. Set your meter to resistance/ohms. This is a little more reliable than continuity (if you have that), because you can have a resistive short that may not register as full continuity. 

Test the receptacle in groups:


Neutral Group = long slots &amp; silver screws
Hot Group = short slots &amp; brass screws
Ground Group = round holes &amp; (usually) green screw on the mounting frame


Touching the probes to any two points within a group should read zero or nearly zero ohms. High readings or no response indicate a dirty, loose, or broken internal connection. Any points from two different groups should yield no response from your meter. If you get any reading, there is a short.



Finding nothing wrong with the original receptacle brought me to the realization that something else happened with the wiring. I removed the new one to confirm that the bare wires still read low. Working backward, the junction nearest to the the receptacle has an old porcelain light/power socket combo. Testing that power socket also yielded full power, so the problem was absolutely with the one cable between that box and the actual receptacle. Opening that box up immediately gave me a reading at the trouble spot. It became obvious that the issue was a loose hot connection in an old junction box hidden by tape.



One final tip... Testing a receptacle/wiring on a confirmed working circuit that is on. If you get a reading on your hot (black) to ground (bare), but nothing on your hot to neutral (white), then your neutral leg is open. If you get no reading from hot to either one, you most likely have a break in your hot leg. If you get a reading on hot to neutral, but not hot to ground, you have an open ground. In my case, I got about 36v to neutral, and 40v to ground. Ruling out the outlet, and getting nearly the same result on both made it rather obvious that a hot wire was loose somewhere.