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RE: Positional hum
Some thoughts about positional hum:
1. Digital deflection typically has far fewer problems with positional hum.
The original 761 zoom board has a differential receiver for frame saw, but
the 595 frame timebase sends it there single ended. This is one weakness of
the original system that sometimes causes problems.
2. The mu-metal shielding kit used on the Turbo II can help to reduce stray
fields from influencing the tube.
3. Ground loops, Ground loops, Ground loops......
4. Sometimes the color corrector interface can also contribute.
5. HP digiscan supplies...should be either on the floor behind the machine,
or better yet replaced with an Advance, Gould, or ACDC supply.