[Electronic M]

Jamming, loose bits of plastic or metal that obviously should not be, wheels and gears that don't want to turn.

I generally look for things on the mechanism that should be moving (tape, reels capstan, etc), but are not then try to trace what moves them till I find a bad belt or idler. I'll often manually try to move/turn those to see if it is really rubber or if anything is stuck, jammed or broken.
Sometimes things get stuck from grease turning to glue with age. Be ready to re-lube if/where needed.