tony duell ard at p850ug1.demon.co.uk
Wed Dec 2 11:50:24 CST 2015

> Circumference and Diameter are linked by the constant Pi and therefore
> are an entity.
> Its a little more complex with a rubber wheel and its indented path.

Indeed. And that may even depend on the type of 'rubber' used and how
it deforms when pressed against the cartridge drive wheel. Remember that
the motpr on these drives is mounted on pivots, which allows it to move 
back as the catridge is inserted. So a soft rubber roller is going to deform
more than a hard one.

> However as we are using closed loop control
> when the measured term equals the target term there you are.

I disagree with that. The tacho disk is on the bottom of the motor 
spindle. So the control electronics will count pulses from that and 
control the motor voltage to get the _motor_ turning at a constant
speed. The drive roller in question is between the motor and the 
tape drive belt in the cartridge (effectively), so while the motor speed
might be constamt and correct, the tape speed need not be if the roller
is not right.

I wondered about using the 'tyres' used on the idler wheels in VCRs,
etc (which are still available). Unfortuantely they are sold by the machine
they fit, not dimensions, so I don't know if any are a suitable size. And of 
course the is no shop that still sells things like that (only mail-order :-() 
so I can't go in and look at them, make measurements, etc.

If the O/D is correct then that is all the really matters. If the I/D is different
then it would not be hard to turn a different hub.

I'll go back to the first question (as I also have 4 of these drive units, 2 
in a standalone TU58, 2 in an 11/730 CPU, that need repairing). Does
anyone have a UK source of tubing or whatever that will work


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