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Re: Positonal Hum



Before ripping into your +70 volt supply too deep, be aware that there are
lots of MkIII-C's running around out there that have no positional hum and
are still running on their original stabilizers.

Did this problem just pop up one day?  If so, I'd suspect one of the big
filter capacitors or a leaky bridge rectifier first.  Also, I'd check that
there is no way hum could be getting into the zoom and position circuits via
the remote control lines.  Finally, the possibility that something
"environmental" changed, like somebody put a new power conduit behind the
machine, needs to be considered.

If this has been an ongoing problem, a lot of things could be checked first.
 Certain older Mk III C's were known for positional hum, particularly those
that had x-y zoom upgrades added in the field.  A lot of that has to do with
sheilding and cable routing in the frame of the machine.  Sometimes swapping
the mains control panel and the little vent panel beneath it helps, since it
gets the power cables out from under the tube.  Again, the possibility of
something like a big power transformer or an air conditioning motor operating
nearby and setting up magnetic fields has to be looked into as well.

If you do feel that the +70 volt supply is the culprit, there are any number
of places you could buy something that would work, or you could make your own
easily enough (any reasonable analog regulator circuit will do as long as it
is adequately bypassed to ground).  An amp or so of capacity should be
plenty.  Since this voltage is effectively "regulated" on the boards
themselves, it isn't necessary to provide an exceptionally precise degree of
regulation externally.  It might be worth obtaing a supply and "clip-leading"
it in place of the normal voltage source to see if it will help. The one
thing to be aware of is that you should take precautions to make sure that
the 70-volts does not come on until the 12-volt power has gotten there,
otherwise you might croak the line timebase board.

Good Luck,

Christopher Bacon
DuArt Video