[QuadList] Cap Disc

Tue Jan 26 11:52:29 CST 2010

Whoa!!  Hold on, guys,

I never said there was any speed change in the capstan at all.  The disc is only the freq. loop in the cap circuit.  The phase comp is sill there and does such a great job you don't even need the disc.  Actually I think they would fight each other in the beginning which is why it takes a little less time to lock up without it.  It creates no time changes at all and all the tracks are scanned as they should be.

My guess is that this effect might be demonstrated by listening to the head when you first hit "Ready".  I notice that the head seems to come up a little fast at first, then locks up at approx. the proper speed. At first it is somewhat out of control, then it locks up to the proper phase.  This may well be what happens in the cap servo when both the disc. (freq) and the phase control (phase) are both working.  I always wanted to eliminate the head disc. but never got around to doing it. You can also see the cap error voltage hunting a bit, especially if the cap osc is a little off freq. You might notice that it is the phase comp which switches to the various refs. while tape is rolling, (Vert or H or ref).

I'm sorry I didn't pick up on this on your earlier e-mails.

How you can make 2 tapes at the same time and still come out with 2 good recordings a couple seconds apart is beyond me, without having any kind of AST or DT.  The only thing I can think of is that on the AVR-2 the digital TBC may have made up for a slight difference in cap speed, due to it's wide window  and there may have been a frame or 2 missing that nobody noticed.

By the way, the take up and supply servos in the AVR-2's was neat.  The output of the servo system was an optocoupler in which an LED lit up a light dependant resistor.  This ldr was connected to the control terminals on a block containing a triac and it's phase shifting network (all embedded in epoxy). I think this block was made by Cutler Hammer, (I may be wrong, but who cares). The triac terminals went in series with the 110vac take up and supply motors and controlled the torque as needed, either holdback or full bore in rewind or FF.

These blocks were kind of expensive,  CBS ordered many or them and they were all used, although I can't remember replacing any of them in AVR-2's. They made great motor controllers for electric drills, florescent dimmers, soldering iron controllers, etc. All you had to do is put a 200K pot across the control terminals, where the ldr was connected for full control, and the AC in series with the two other terminals.. I can't imagine where they all went!!  Also, CBS ordered many 200K pots for some reason.

Insofar as the cap pully mod is concerned, look at the intersync schematics, there is one pot called "Cap servo gain" or something to that effect, I don't remember the number.  IF you bring this pot up a little too high the cap will go nuts and at the same time you had to set it very carefully, especially  if you wanted to make good edits. This was because of the cap fighting the servo, apparently the change in the speed from mono to color was enough to upset it. The new cap speed made a big difference.  While the servo probably had a good range, it may have been kind of close to one end of the envelope.  Also, now that I think of it that pot wasn't one of the "normal" tweeks you would adjust, only if you were doing a "long servo setup".

73,

George Keller

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