TK70 maintenance--swapping capstans

Chris Zach cz at
Sat Feb 13 21:10:59 CST 2021

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...


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