[QuadList] Capstan questions

[Don Norwood]

In reply to the original question that Chris asked, I think I have a drawing 
that shows the tolerance for the Ampex VR series capstan.  I'll look for it. 
RCA used a smaller diameter capstan, so tolerances might be different in 
their case.

I think everybody understands that the goal in record mode is to move the 
tape at a constant and precise rate.  However, that doesn't happen in real 
life with the VR series as evidenced by the experiment that Chris did.  I've 
seen the same thing.  I'm not sure about the capstan tolerance issue, but I 
do know that tape slippage can affect the speed.  This can be caused by 
incorrect tape tensions, bad pinch rollers, slick capstans, or some 
combination of all of the above.  Further, the tape itself can slip 
differently depending on the backing.

Also, in the VR decks, remember that the capstan is not directly driven.  So 
the complete system includes the motor pulley, the mylar belt and the 
capstan pulley.  Remember that you have to properly set the tension on the 
belt.

More precise control of record speed can be achieved by using a high 
resolution tach that measures the tape motion, not the capstan speed.  That 
technology wasn't practical in early quad machines, and as others have 
pointed out, within practical limits, the error is corrected in playback 
anyway.

Don Norwood
Digitrak Communications, Inc.
www.digitrakcom.com