TK70 maintenance--swapping capstans

Chris Zach cz at
Fri Feb 12 16:32:18 CST 2021

Well my little TK70 here has been squeaking and it looks like the 
capstans are frozen/bad. They don't move up and down and they don't spin 

Fortunately I have a dead TK70 with good capstans so I figured I would 
swap them. Unfortunately I don't have a maintenance manual (does anyone 
have one?) so I had to figure out alignment myself. Capstan alignment 
seems to be critical, if they are off the unit don't work...

Anyway here is my procedure so far to get the unit to load and unload 
tapes on the bench. Not perfect, but a start....

Pulling the capstans requires you to remove the two lock nuts on top 
first. I recommend you count the turns from all the way tight if 
possible as alignment is critical, and the front and rear ones can be at 
different relative heights.

Anyway if you didn't do this you need to adjust the rear height to trip 
the optical sensors and the front one to handle tape slew.

Note: All the below is done with the unit on a bench, with a PC power 

The first step is to adjust the rear capstan so the leader tape's wide 
hole for the end of tape-stop marker allows light from both LEDs/sensors 
to pass through. You do this by tightening the bolt down till snug, then 
back off 1/2 turn.

Turn on unit, see if it unlatches. It probably will try to turn the tape 
4 times then error out. Fine. Power down, back off the bolt 1/4 turn and 
try again.

At some point it will open the latch. Note the # of turns of the bolt 
then keep going 1/4 turn at a time till it doesn't work again (too 
high). The proper value for your unit in turns is halfway between too 
low and too high. Reset the bolt and verify it works several times. For 
my unit the right height was about 1.5 turns out.

Then you need to adjust the front capstan. The problem is if the front 
is higher or lower than the back you have tape slew errors. Start at the 
level of the rear one based on the # of turns minus a bit (1/2 turn). 
Then load a tape. It will load, but when you try to unload things will 
go bad, the tape will just move forward onto the take up reel 4 times, 
and the unit will error out. The reason is it has to read the tape as it 
turns to know a tape is loaded (as opposed to the leader where it looks 
for the end of leader light). If the capstans are not holding the tape 
level against the head it can't read.

Each time it moves forward a bit, try bringing the front up 1/8 turn at 
a time. Eventually it will speed up, that means it can read the tape on 
one of those moves. It should then unload. Now you have the front 
basically set. It will unload the tape, cycle it a few times to make 
sure it's working.

That's where I am now. Next step is to see if it will read the tape in 
the computer. I'll work on that tomorrow.

Note if you ever shine an led flashlight into a running TK50 or 70 all 
hell will break loose as the system will see the light on the tape 
sensor and think it has hit BOT. Even worse is if both LED sensors 
trigger, then it thinks it is at end of leader and it will throw tape 

Ask me how I know....

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