Mark, if you get a well restored Mellotron, you may never have to worry about it. The Mellotron I just sold and the ones I have rebuilt require only regular maintenance. There is very little to go wrong. I give my clients about three pages of instructions on maintenance and adjustment. Anything else is only a fax or a phone call away. All you really need is a head demagnetizer, some rubber rejuvenator and a demagnetized screwdriver.
Mellotrons today are very simple to maintain and very reliable. The only reason not to own one is if you don't have electricity.
Regards,
Frank