Capstan Information

Wed Sep 24 20:42:29 CDT 2014

Chris:

I can’t locate the drawing but will let you know if I do find it.  However, I did measure three VR series capstans and together with the +/-0.0002 spec, we can conclude several things.  The measurement that I get is a diameter of 0.7495 inches.  So to get 15 IPS, depending on exactly how many decimal points you care to use for the calcs, the capstan speed has to be 382.2267 RPM. That’s easy enough to do from a standard speed sync motor by using the appropriate sized pulleys on the motor and the capstan.

I want to clarify one thing.  I said that the VR machines used a belt drive.  That’s true for all VR models after the VR-1000 (1100/1200/2000) except for the 3000.  The 1000 used a direct drive sync motor with a smaller shaft (capstan) diameter and the 3000 used a DC motor with a tach.

But back to your question and some calculations, if we reduce the diameter by the maximum allowed spec to 0.7493, then the tape speed is reduced by about 0.004 IPS which would be slightly over 14 inches in one hour, or less than a second of tape time.  Expressed as a percentage variation of speed, it’s less than 3/100’s of one percent.  In trying to imagine how much a capstan could be worn by the tape, and after looking at the numbers, I just don’t think that’s the problem.  I’d still vote for tape slippage based on the criteria I listed previously in the email below.

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

From: Chill315 at aol.com 
Sent: Wednesday, September 24, 2014 11:26 AM
To: quadlist at quadvideotapegroup.com 
Subject: Capstan Information

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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