TK70 maintenance--swapping capstans

So.... TK70 is working again, and the solution was not what I thought. Yesterday I spent the afternoon adjusting the capstan bolts, first one, then the other to see if the height adjustment worked. I found that the rear one (also known as the encoder) was the most sensitive to bolt tension. Then I watched it run and realized something: When the TK70 starts up without a tape in the unit it moves the leader a tiny bit. I thought it was looking for the end of leader hole and was missing it due to alignment, but that's not what happens as there is only a single LED transceiver watching the tape. Thus it had to be something else, and the only thing it could be is the velocity sensor. The velocity sensor is a part of the rear capstan, I took it apart to see what the heck was going on and figured out the problem: The top bearing has an imperfection and is not allowing the capstan to spin on the bearing. Instead it spins under the bolt, which causes it to drag and read speed improperly. That little jog of the leader during loading moves the capstan a bit and allows the unit to verify it works. Thus different tensions on the bolt were allowing it to move (somewhat) but not well enough for the tape drive to work. So..... I swapped in the capstan from the broken TK70. Swapping it is a *job* and while there I took the old one apart and saw that it's pretty complex inside but if the bolt is not tight enough the sensor disc in there rubs against the diffraction grating below the LED. Put in the other capstan, put the drive together, and it works. Adjusted the bolts so they ride the tape with a minimum of edge touching and it still works (the head does adjust for the capstan height. The bad news is the tachometer capstan is in pretty bad shape, the chrome flaked off the top and the bottom and it's not perfectly smooth. I may try to sand the flanges with 2,000 grit sandpaper to smooth it out but unlike the front one it doesn't have to deal with tapes that have varying heights so as long as it is aligned with the take up reel (which doesn't go up and down) it might be ok. Very interesting little project. I guess my last question is this: Is there a difference between the tachometer capstan of a TK50 and a TK70? Maybe I can swap one from a broken TK50 I have (I have one good one too) and get the second 70 running as a spare. Or maybe I should just troubleshoot the broken TK50 now that I am smarter... Chris