[QuadList] Ampex VR-1200 on ebay-in Apalachin, NY

[Don Norwood]

Hi Chris:

You are partially correct.  What you are thinking about is Module 13 which is standard.  It contains the dropout detector, the demod input switching, and the circuitry for inserting the peak white reference pulse from the AFC system.  However, the two AFC modules were separate and optional.

The "real" DOC requires the addition of Module 16 which is what appears to be installed in this machine.  This is a delay line which delays the RF by one line and then subsequently feeds it back to module 13 where it is substituted for the DO.  However, if the optional delay line module is not installed, then a fixed level as you described is inserted by the switch.

So there were two "levels" of DOC from Ampex, the freebie that you recall and then a "real" one as installed in this machine.  I use the quotes with "real" because the design obviously did not provide properly phased video for the dropout.  That's what led to the market for the 3M and Merlin (and others?) DOC's which delayed video instead of RF and then matched it to the proper phase.

Don
  ----- Original Message ----- 
  From: Chill315 at aol.com 
  To: quadlist at quadvideotapegroup.com 
  Sent: Thursday, December 10, 2009 4:11 PM
  Subject: Re: [QuadList] Ampex VR-1200 on ebay-in Apalachin, NY


  Don

  The AFC and Bare Bones DOC were included in all the machines at some point in manufacture.  The DOC was part of the AFC design.  It was a freebie as I put it.  The RF signal that was inserted back into the picture during a drop out was the frequency of blanking.  This frequency was used as it was the one that the AFC could clamp to a know point in the video.  Ampex did not use sync for a good reason.  Sync levels varied by a few between sources and thus with 100 IRE of video but different levels of sync, the video level would change at the output of the de mod if you clamped at sync tip prior to modulation.

  At least this is the way that I understood.  Can someone verify this or am I out to lunch on this theory?

  Chris Hill
  WA8IGN


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