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Positional 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)?
> Thanks
> Alan Burnham
> WickerWorks Video

Greetings Al,

I am more aware of positional hum than Id like to be. Although I am
not familiar with your particular machine, I will suggest the following
that should help your problem.

You seem to already conclude that the problem seems to stem from the 70
volt supply.  First, I highly recommend that you positively determine
whether or not the positional hum is really caused by the deflection
supplies or the control voltages.  This can be done by completely 
lifting the incoming x,y,z control lines on the zoom board and 
temporarily placing 3 trimmer pots on the board just to set usable 
x,y,&z voltages.

If the hum is caused by a ground loops involving your programmers 
D/As, I wouldnt mess around with trying to fix it, as that can  be 
more trouble than its worth and may just prove to be a futile effort.  
Instead, I would highly recommend completely isolating the control 
lines with opto-isolators. Its the only sure fix, given that 

1.  I agree with changing out the Rank 70volt supply with a real 
regulated supply (such as a Lambda perhaps).

2.  If your machine is using the original Rank Zoom board, locate the 
ground trace at the front of the board and tie it to the rear connector 
ground with some solid buss wire.  It typically helps, and with hum, 
every amount helps. Also, add a couple of 100uf. caps in addition to 
the .1ufs on the supply rails of the zoom board, and to the frame 
timebase as well.

3.  I also highly recommend breaking away the +12v and -12v on the zoom 
board and the frame timebase and powering them both from an added 
separately regulated linear supply.

4. Make sure that the ground from the zoom board is tied (via 10 gauge, 
or at least 12) to the frame timebase.

Be aware that the problem may also be related to magnetic rather than 
electrical influences.  I have even seen such stupid things as a 
soldering iron left on under a nearby floor tile.  Most typical though, 
would probably be power system ground loops within the facility, not 
within the telecine (as Hans writes).  My experience has been that 
people tend to under estimate the amount of contribution, or influence
that excessive ground currents can have on a Rank telecine.  Hans 
Lehmann describes it well in his posted reply.

My regards to Terry and the guys.


  Michael C. Kaye