Mellotronists

I have to second what Doug and Frank said.  If the machine is set up 
right and doesn't have any nasty problems (such as the issues with 
the machines we've seen at the Mellotron Ranch), you won't be 
spending a lot of time on maintenance.

Keep a flat blade screwdriver and nut driver handy to adjust a key 
once in a while, and that's about it for anything semi-regular.  You 
may want to maintain the tape path by cleaning it up a bit and 
demagging the heads, same as you'd do with a tape deck, but this is 
every once in a long while and not a constant demand.  Now and then 
check out and oil the motor.  Else these things require little 
maintenance, provided they're good/stable to begin with (and there 
are several people on the list who can help set a machine right if 
it's in rough shape).

If you're moving the machine around a lot, using a variety of tape 
stock, experiencing humidity/temperature changes, and so forth, yes, 
you'll probably do some additional adjusting of the pressure pads and 
pinch rollers to compensate for this.

But it's not like these things blow up all the time or fail 
miserably.  They're probably just like anything else for reliability.

See what Duncan has to say about this topic:

http://www.kleonard.com/music/concerts/rmi040508.htm

...kl...
M400 #805 - knock on wood
M400 #1037 - reliable...?  tell that to my first motor...