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drift (forwarded)



--- Forwarded mail from m.c.kaye at ix.netcom.com (Michael Kaye )

Greetings Al,

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 
awful.

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.

Regards,
Mike

--- End of forwarded message from m.c.kaye at ix.netcom.com (Michael Kaye )