I am reminded of the time, several years ago, that I purchased a used Brother label maker that was defective because it worked only on batteries but not with the usual ac adapter wall wart.
After using this for some time I decided to make it work with an ac adapter. I assumed the problem was in the closed circuit jack but that seemed okay. So I put a bench power supply on the unit and disccovered that it indeed worked fine via the jack, but required a narrow voltage range (too low or too high and it quit). I put in a little regulator and it's worked fine ever since.