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Re: Positonal Hum
a.burnham at genie.com wrote:
> Our MKIII/C is exibiting a problem that has been preplexing
> with the following solution recommended by Rank L.A.:
> Most visible with the SMPTE "Registration chart" (the one with
> horizontal and vertical lines) is a shaking of the horizontal
> lines with a scrolling upward of the effect. Rank called this
> "positional hum".
> The problem seems to stem from the 70 volt supply in the scan and
> Signal Power Supply as it is a very hummy source of power.
> The solution, as recommended by Rank, is to replace the
> unregulated 70 volt supply, in the Scan and Signal Power
> Supply, with a regulated supply supplying a no hum source
> of power to the Line Timebase and EHT power supply cards.
> This all sounds correct but before I build such a supply has
> anyone found this true and is there a power supply availiable
> (what does the Turbo do about this)?
Scan hum can be also be caused by sources other than power supply ripple. A
common culprit is stray magnetic fields in the vicinity of the telecine
itself. Encore Video had a problem when an new wing was added to our
existing building. The power grids for the two halves of the building had
their grounds tied together through the floor structure in our machine room,
almost directly under a Mk-III. As soon as power was connected to the new
building, huge amounts of positional hum appeared in the picture from the
telecine. Currents approaching *40 amps* were measured running through some
of the steel floor support beams! A wiring change and addition of isolation
transformers to the building electrical supply solved the problem.
We have also had problems in the past with magnetic fields emenating
from the telecine power supplies. A few pieces of strategically placed
mu-metal solved that.
However, I don't remember seeing positional hum that was directly
caused by ripple on the scan/signal power supply, except of course at those
times when a component (usually one of the large electrolytic filter caps)