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<DIV>Chris:</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: 'Arial'; COLOR: #000000">Don
Norwood<BR>Digitrak Communications, Inc.<BR>www.digitrakcom.com</DIV>
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<DIV style="BACKGROUND: #f5f5f5">
<DIV style="font-color: black"><B>From:</B> <A title=Chill315@aol.com
href="mailto:Chill315@aol.com">Chill315@aol.com</A> </DIV>
<DIV><B>Sent:</B> Wednesday, September 24, 2014 11:26 AM</DIV>
<DIV><B>To:</B> <A title=quadlist@quadvideotapegroup.com
href="mailto:quadlist@quadvideotapegroup.com">quadlist@quadvideotapegroup.com</A>
</DIV>
<DIV><B>Subject:</B> Capstan Information</DIV></DIV></DIV>
<DIV> </DIV></DIV>
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<DIV>
<DIV>Don</DIV>
<DIV> </DIV>
<DIV>That probably is the correct answer for the reason I asked. Hope you
find the drawing. Then we can find out the dimensions.</DIV>
<DIV> </DIV>
<DIV>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. </DIV>
<DIV> </DIV>
<DIV>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. </DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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. </DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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.</DIV>
<DIV> </DIV>
<DIV>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?</DIV>
<DIV> </DIV>
<DIV>Chris Hill</DIV>
<DIV>WA8IGN</DIV>
<DIV> </DIV>
<DIV>
<DIV>In a message dated 9/23/2014 1:24:37 P.M. Eastern Daylight Time,
quadlist@quadvideotapegroup.com writes:</DIV>
<BLOCKQUOTE
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT
style="BACKGROUND-COLOR: transparent" color=#000000 size=2
face=Arial><BR><BR><BR>-----Original Message-----<BR>From: "Don Norwood"
<quadlist@digitrakcom.com><BR>To: "Quad List"
<quadlist@quadvideotapegroup.com><BR>Subject: Re: [QuadList] Capstan
questions<BR>Date: Tue, 23 Sep 2014 13:24:40 -0400<BR><BR><BR>In reply to the
original question that Chris asked, I think I have a drawing <BR>that shows
the tolerance for the Ampex VR series capstan. I'll look for it. <BR>RCA
used a smaller diameter capstan, so tolerances might be different in <BR>their
case.<BR><BR>I think everybody understands that the goal in record mode is to
move the <BR>tape at a constant and precise rate. However, that doesn't
happen in real <BR>life with the VR series as evidenced by the experiment that
Chris did. I've <BR>seen the same thing. I'm not sure about the
capstan tolerance issue, but I <BR>do know that tape slippage can affect the
speed. This can be caused by <BR>incorrect tape tensions, bad pinch
rollers, slick capstans, or some <BR>combination of all of the above.
Further, the tape itself can slip <BR>differently depending on the
backing.<BR><BR>Also, in the VR decks, remember that the capstan is not
directly driven. So <BR>the complete system includes the motor pulley,
the mylar belt and the <BR>capstan pulley. Remember that you have to
properly set the tension on the <BR>belt.<BR><BR>More precise control of
record speed can be achieved by using a high <BR>resolution tach that measures
the tape motion, not the capstan speed. That <BR>technology wasn't
practical in early quad machines, and as others have <BR>pointed out, within
practical limits, the error is corrected in playback <BR>anyway.<BR><BR>Don
Norwood<BR>Digitrak Communications, Inc.<BR>www.digitrakcom.com
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