[QuadList] Capstan Size Modification

[Chill315 at aol.com]

George
 
Interesting that CBS would try to turn the capstan.  The diameter was  set 
for monochrome standard at 60 Hz.  The color standard is 59.94  Hz.  This is 
less than 1 one hundredth of a percent difference.  The  size of the 
capstan determines the length of tape that is pulled through per  revolution.  
 
The SMPTE RP for the layout of tracks on the tape is what is critical for  
interchange.  There has to be the correct distance between tracks.   The 
angle of the track from vertical.  The distance from the track to the  position 
of the control track head.  The placement of the Vertical sync on  the tape 
with the frame pulse.  Even the distance to the erase and audio  heads.  
There is a shape that is required called the canoe.
 
Fortunately there is enough slop in the servos to take into account the  
miniscule difference in the master sync generators used during record and  
playback.  Think of video tape as a mechanical frame sync.  Because  the 
capstan and heads compare the off tape sync to the house sync they will  adjust 
the speed to compensate.  
 
Regarding the capstan belt.  I have never had to replace one.   The never 
broke on the machines that I had.  I did look at one when  the capstan was 
pulled and I took the cover off just to see it.  I did  replace the capstan 
motors on an AVR 2 that was damaged in a fire so that was an  easy fix.  We 
thought about rebuilding the shafts but found out that the  diameter was so 
critical we could only buy new.  
 
Chris Hill
WA8IGN
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