I know this is a reply to a very old post but I just had the same 21-1C1 error. You remove the cover. There is a 2 screw plate to hold the carriage in place, remove the plate. Unplug the little multi-plug from the carriage. Remove carriage. Towards front of machine you will see 2 gears with a "worm" gear inbetween them. With your finger, turn the worm gear so that the gears run without sticking (this is the error). Just a couple of turns and it be loose. Re=assemble the deck and you should be able to play the big cassettes. Those gears should be cleaned and lubed by a pro but this fix will keep things running for a while.

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