[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
- To: telecine
- Subject: drift (forwarded)
- From: rob (Rob Lingelbach)
- Date: Mon, 10 Jun 96 07:51 PDT
--- Forwarded mail from m.c.kaye at ix.netcom.com (Michael Kaye )
Regarding your ground problems; Been there done that..
I had great success by strapping all of the tech equipment to a 2"
copper ground bus bar. You want to reestablish the copper bus bar as
your new ground for all of the equipment. If the situation is a bit
more out of hand than that, and the ground currents are excessive, then
I would suggest the following:
I am a firm believer in one good common ground. I think the concept of
maintaining "isolated grounds", especially in a post production
facility is not only unrealistic, but absolutely ridiculous because of
all the natural interconnects between facility equipment and the likely
potential differences throughout.
If you have an excessive amount of current through your neutral, I
would highly recommend splitting the tech load up by installing
isolation transformers. This will keep the current from collectively
adding on to the neutral. You want whatever potential differences that
may exist to conduct through the copper bus bar, not through all the
system interconnects, whether it be a telecine, or whatever. This
will allow you to provide a true common ground no longer causing havoc
to equipment not forgiving to AC hum.
Now, regarding color drift:
Most of the time those brilliantly placed coupling caps go undiscovered
and the facilities typically canít figure out why the color drift is so
Itís almost humorous that when those caps go unnoticed and unmodified,
the CRT ages, more shading correction gets added along with the error
from the floating RGB dc levels, the dc floats up and down with
run/stop size changes, and convinces the engineer that the CRT has
reached the end of its life when it really hasnít.
I have gone to many extremes in an effort to improve stability, such as
replace the RGB/burn PMT supplies with supplies of higher precision.
Replace the preamp supply with a supply of higher precision.
Certainly you can try replacing the burn cell. Upon closer
examination, you may notice that there is little latitude for the
output of the burn cell, it may reach saturation. You should probably
re-scale the input levels on the festival so the burn cell supply
voltage can be lowered (about 30%), each cell is slightly different
and may cause problems if this is not done.
I would suggest perhaps installing a new set of PMTís and adopting a
practice of not allowing the telecine to sit for lengthy periods of
time in negative mode with an open gate. This drives the blue PMT
through the roof, as well as the preamp. I was given this advise years
ago by one of the PMT engineering reps insisting that it permanently
damages the PMTís. Over time, this has proven to be true.
Remember, the only remedy once the damage is done, is to replace the
PMT, so make it understood to the ops.
Also, in case you donít do this already, do not shut the EHT off
between jobs, that also shuts down the PEC supplies which then has to
re-stabilize. When the telecine is not being used, adjust the beam
current to minimum.
--- End of forwarded message from m.c.kaye at ix.netcom.com (Michael Kaye )