This informative video, which Thin Air Audio recommends watching before buying a used machine, starts with a real-world example of the uncertainty involved with buying a used reel to reel tape recorder. Also, beginning around 12:30 there’s good information on operation and maintenance of recorders in general and multitrack (4 channels or more) recorders in particular. https://www.youtube.com/watch?v=TDrGkRAIafA