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Burn cell overload



     I've noticed that in some Mark IIIs the burn cell has a ND filter taped
to it, some have Scotch tape, others have none.  We have also seen some that
overload and create pretty nasty shapes as the beam current is increased
past 150uA.  The burn correction is Festival. We adjusted the burn cell
symmetry for the best possible waveform, but we noticed some hooking of the
edges of the burn signal, and increase of vertical errors as the beam
current is increased. On one machine recently, adding a .7 ND filter reduced
the problem greatly. Does anyone have any similar experience with this, or
can you enlighten us? Dave Walker told me that the new Glen Gundlich derived
burn corrector "Accuglow" he is revising will address this. It also offers
compression of blacks and whites. It is being optimized for use with
Scatterscan and Digital Deflection.
     BTW...I've tried using all 3256 PMTs in a Mark III and had success. I
do know that in an Ursa if a 2154 is used in the green and a 3256 in blue,
it can be very noisy.  Someone, for some unknown reason, decided to do this
(without telling anyone) at one place I worked, and it contributed to some
unwelcome head scratching and the dreaded D word (downtime)! ;-)

Craig Nichols
Telecine Engineer