M20J Pilot reported alternator problem inflight along with failure to obtain proper A&P sign-off for repair prior to further flight.

Narrative

On Date; I embarked on a flight from my home airport (ZZZ1); to (ZZZ2) intending to return to ZZZ1 the same evening. This flight required a fuel stop [at] (ZZZ3). This was a pleasure flight operated under Part 91. On the leg from ZZZ3-ZZZ2; I noted the ammeter began to show 'wiper' action; swinging from discharge to charge; but the volts shown on the engine instrument remained steady. While descending into the ZZZ4 nearing the ZZZ5 VOR; the low volts annunciator illuminated; and I noted that the voltage on the engine instrument had dropped from a nominal 12.9-13.1v down to 11.6v. I immediately performed load shedding; tried to reset the alternator field circuit breaker with no success. Air Traffic Control was notified that the aircraft had experienced an alternator failure; but that we had sufficient power to land safely without assistance. After the load shedding was performed; the voltage never dropped below 11.6v. Because my aircraft is equipped with battery backup for my primary attitude indication and the remaining voltage was sufficient to continue the short flight to ZZZ2 and extend the landing gear; I elected not to perform a precautionary landing at a closer airport. After a safe and uneventful landing at ZZZ2; I parked the aircraft; and removed the cowlings to look for any obvious deficiencies in the alternator wiring. Upon seeing this; a person I believe was an A&P; was working on aircraft nearby; and came over to offer assistance. I located a ground wire that had come out of its ring terminal on the F2 post of the alternator. This person said that they had a terminals and tools; and offered them to me to repair the wire. I performed the repair; asked them to look it over; they said it looked good; I performed a nominal run-up and charging system check; and re-installed the cowlings. Because of the time lost due to the maintenance defect; I was unable to return to ZZZ1 the same day as planned; and I was very worried about finding care for my dog that was left at home; as well as finding lodging for the night. In my haste to complete that; I neglected to get any information from the person who assisted me; and forgot about it. After an uneventful home the following day; I was contacted by the ZZZ6 FSDO who requested the logbook entry from the repair. When I attempted to find this person was and get a logbook entry; I was unsuccessful; and believe they may have been with a transient aircraft. Learning this; I brought my aircraft to my regular A&P at my home base to have them inspect the repair; as well as relocate the wire to prevent it from become defective again. From this occurrence; I have learned a lesson that I should not accept any maintenance advice; nor undertake any repairs until I am able to verify the credentials of the person offering said repairs or advice; and ensure that I obtain a logbook entry from anyone who performs work before I depart the area.