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Drift/shading on a Mark III

Hello all,
    A question for all of the MK III gurus out there:
I have a Turbo with a 563A waveform generator, and standard shading
boards. The shading correction has a DC drift that follows the scan size
ONLY when a compound parabola (LP x FP or LP2 x FP2) is applied. I
traced this back through the festival board, and shading board, and
found that the 100563A waveform generator has a drift in DC that follows
the scan size only on these two shading waveforms (the DC drift takes
about 4 seconds to stabilize). All 6 other waveforms are rock solid when
the speed/scan size changes. The 563 board from my other turbo exibits
the same problem. On the schematic for the 563 I noticed that only these
waveformas are capacitor coupled to the output op-amps (indicating the
need for some removal of DC element I presume) The caps are polarized
tantalum, so I don't suspect a failure, especially in all 4. I would
like to implement a mod to stabilize this DC level if it is an inherent
design flaw. Or if someone could reccomend what might be wrong here that
would be great. I should re-iterate that this problem is independent of
the video channel signal flow, and burn correction. Also the DC drift is
present on the op amp side of the coupling caps, and not on the
multiplier side (op-amp input impedance perhaps)  Thank you all in
advance for the help.

Vince Forcier
Director of Engineering
Roland House, Inc.
2020 North 14th Street
Suite 600
Arlington, VA 22201
mailto:vforcier at rolandhouse.com
703.525.7000 - Voice
703.525.7001 - Fax

Thanks to Al Hart of Modern Videofilm for support in 1998.
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