Capstan Information

[Chill315 at aol.com]

Don
 
That probably is the correct answer for the reason I asked.  Hope  you find 
the drawing.  Then we can find out the dimensions.
 
I did find out more information.  Both RCA and Ampex used motors that  had 
a pulley and belt arrangement to the capstan shaft.  Thus the  rotational 
speed of the motors was reduced to drive the capstan shaft.  In  RCA's case 
the motor is specified to turn at 1800 RPM.  I do not know the  Ampex speed 
but it might be the same looking at the drive waveform in the  Intersync book. 
  
 
Another interesting fact is that the Ampex shaft is machined to a spec  of 
plus or minus .0002 inches.  There is a note in the Intersync  book about 
replacing the 1000 series sine wave capstan motor with a square wave  motor.  
It talks about protecting the shaft to prevent any damage and how  damage 
can cause wow and flutter.  
 
Now as an extreme example.  Suppose you have the  nominal circumference of 
the VR capstan at 1.5 inches.  The 10  revolutions a second will pull the 
tape through at 15 inches per second.   The servo during record always expects 
this size and is told to turn it at 10  times a second.  As a result 
everything is OK with locking to sync, control  track pulses, etc.  Go to play on 
a VR machine with the same size  capstan and it all works out.
 
Now if the circumference were a little smaller, say 1.4 inches, then the  
tape is only going to be pulled through at 14 inches per second.  The  
recording will be off standard and a big problem.  Does the electronics  know? No, 
because during record it is expecting the normal 1.5 inch  capstan.  If the 
capstan were 1.6 inches, then the tape is being  recorded at 16 inches per 
second.  
 
These are extreme numbers to show the issue.   Fortunately we are  playing 
back tapes and not recording so the servos can  handle minute differences 
between machines.  We do not have to  worry about making tapes to SMPTE 
standards.  Only playing back.  But  if the capstan were worn down a lot, this 
could become an issue if it got out of  the capture range of the servo.
 
Now another thing that went through my mind.  All speed designs were  based 
upon a black and white sync generator.  So the vertical is exactly 60  Hz.  
So when taking into account the 3.58 MHz color sync generator, the  speed 
is reduced because we are now at 59.94.  The speed would be 14.985  inches 
per second.  Also everything above is stated using the 15 not  the 7.5 speed.  
A little quirk and almost too much detail.
 
Now the interesting question is what are the capstan differences between  
the VR series and the AVR-1 plus the RCA machines.  Was there a change to  
the TCR-100, TR-70C and the TR-61?  How about the TR-600?
 
Chris Hill
WA8IGN
 
 
In a message dated 9/23/2014 1:24:37 P.M. Eastern Daylight Time,  
quadlist at quadvideotapegroup.com writes:




-----Original Message-----
From: "Don Norwood"  <quadlist at digitrakcom.com>
To: "Quad List"  <quadlist at quadvideotapegroup.com>
Subject: Re: [QuadList] Capstan  questions
Date: Tue, 23 Sep 2014 13:24:40 -0400


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  



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