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re< Positional Hum

                      Subject:                              Time:  4:53 PM
  OFFICE MEMO         re< Positional Hum                    Date:  6/3/96

I agree with what Mike Kaye added today.  In my experience Positional Hum is
more often caused by ground loops than anything else except the inevitable
problem from the cooling fan of the Hewlett Packard Digiscan Power Supply. 
The cooling fan problem was always solved by moving the Digi-PSU to the floor
behind the telecine as far from the CRT as the power cables would permit. 
Adding Digital Deflection with it's improved CRT shielding and power supplies
has cleaned up our positional hum problems greatly.  The last time we had a
positional hum problem was when we pulled out our old Classic DaVincis and put
in DaVinci Renaissances on our 2 upgraded Turbo-2-ized MK3's.  The hum was
coming from the Serial Digital video feed which comes from another part of the
building (in fact it is actually ANOTHER building next to the old main
building. It's hard to put additions on buildings in New York so we knocked a
hole in the wall and expanded in to the adjoining building).  The ground
current coming in to the film to tape area via the Digital router feed to the
DaVinci 888 was enough to add positional hum to the Rank. By adding a BIG
ground strap between the Rank and the DaVinci Chassis we got rid of the hum.
Dave Corbitt