TK70 maintenance--fixing capstans and making them work well again

Chris Zach cz at
Mon Feb 15 23:05:37 CST 2021

Since my last post I've gotten pretty good at getting these tape drives 
working and I think I see the reasons for a lot of the tape wear: Drag 
on the capstans.

The rear one (tach) is probably the biggest problem, but on 3 of the 4 
units I have here (2 TK50, 2 TK70) the bearings are not turning and as a 
result the capstan is dragging along the bolt at the top, tensioned by 
the spring on the axle. The result is a lot of drag at best, which will 
require the unit to apply extra tension to keep the tape moving across 
the capstan and encoder at a nice 75 inches per second. This translates 
into more wear at the head, more shedding, and more crap on the heads 
that people clean till the tape goes south.

Also the capstans should "float" on their axles, probably to deal with 
slightly misaligned tapes. If you push down on the capstan and it 
doesn't move then it is really bound up and needs a serious cleaning.

The solution is to clean out and lubricate the bearings. To do this for 
the front you can unbolt and remove the capstan. A hint on unbolting: 
Turn the bolt in a tightening direction and count the rotations before 
it stops at the bottom of travel. Should be a turn or two. Note this for 
each capstan, that way you can put the bolt back on at about the same 

Once the capstan is off, you simply wash top and bottom bearings out 
with 95% isopropyl to get out the old oil and gunk, let it dry, then use 
watch oil and a watch oiler to put about 6 small drops of oil on the 
inside bearing face. As the oil seeps into the bearing you will notice 
you can actually turn it with your oiler.

The rear one is a bit more complicated: The bottom bearing holds the 
encoder wheel on the shaft, so unless you want to take apart the encoder 
(moderately difficult, I'll write that another day if anyone's 
interested) focus on the top bearing. Remove the bolt, put a few drops 
of isopropyl first to loosen up the old oil, let it dry, then put six 
drops of new oil from an oiler tip on the bearing. It should now turn a 
*lot* easier.

Then assemble, clean off any excess oil, clean the tape head and guide 
and give it a try. I usually retract the leader before working on all of 
this to get it out of the way, re-hooking it is pretty simple. The 
result should be a unit that runs with just enough tension to spin the 
encoder (good) without drag on the front capstan (better) and minimal 
extra pressure on the tape head (best).

I could probably rebuild these for people if they would want it, what 
would be a fair price?

So far I'm happy: I am able to back up my 300gb ESDI disk on a single 
tape and the drives are pretty quick all things considered. RT11 backups 
are easy at 33mb, and I may just load all of my RL02's on tape images so 
I can have them around.


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