I've been teaching for a very long time, as some of you know, since the early 80's actually, and over that time new spinners have brought second hand wheels into the class that they haven't been able to get to work. All that needs doing to the vast majority is:
A little oiling on most of the moving parts.
Most modern wheels don't need oiling, but old models do!
To remove the old nylon fishing line Scotch Tension band, which has set in position and won't allow adjustment, to a long length of linen buttonhole thread. (Yes, this will fray, but cut off the frayed bit and re-tie to the spring/elastic band and it's as good as new!) This tip was given to me in the early 80's by Sue Hailey Harris at one of her silk spinning workshops and has been the best thing I've done on all my old wheels!
The linen comes in brown.....
A change of a worn drive band, correctly set so that it can be used with all ratios. (If you have a stretchy nylon one, don't forget to remove it when the wheel is not being used otherwise it will stretch too much and you'll have to take a chunk out!)
Hay presto! The wheel works again!