bushouse@[EMAIL PROTECTED]
wrote:
> Once a year I volunteer to light a concert at a local church whose
> main set of fixtures consists of some old
> Altman-style 6x22's (mounted from the balcony ceiling - lovely
> access!). These fixtures originally used the
> typical FEL 1kw lamps, but a couple years ago the church folks decided
> to try to save some energy and
> switched them all to HX600 lamps. Along with a noticeable drop in
> light output, I also seem to have
> a really hard time getting those lamps to focus properly in the
> fixtures. No matter how much "bench"
> focusing of the lamps I try to do (where of course by "bench" I mean
> focusing from the top of a Genie lift)
> I can never seem to get rid of a dead spot in the middle of the field
> - just can't flatten it out.
>
> I compared the mechanical specs of the two types of lamps, and even
> held up two of them side by side
> for a visual check, and it appears that the location of the filament
> relative to the base is the same, so the
> light origin should be at about the same place within the optics of
> the fixture.
>
> Anyone have any clues (or experience) with what the problem might be
> here? Sure would appreciate any
> help getting rid of those dead spots (there's so little light left
> coming out to begin with, I can't afford to lose
> any more due to poor focus!).
>
> Thanks,
> Howard
Dear Howard,
No matter how close the two lamps appear, they probably don't 'seat' the
same way. I do not know the LCL of the HX600, but a dark center in the
field usually means the lamp is too far forward in the Altman reflector.
While a bit dicey, try loosening the brass cap screw and pull the cap
slowly back out while it is lit - gloves on, of course - and see what
difference it makes. If each 6x22 is benched so the lamp is on dead
center axially in the reflector, and the HX600s have both the same
filament size and orientation in the envelope, the remaining variable
can only be location of the filament on that axis.
Also, check to see that the rough casting edges on the Altman cap are
not keeping it from perfect seating, even by a little. The 6x22s are
very squirrely that way and the barrel length cuts down lumen output
anyway. Often the grounding wire is run over the cap focus plate rather
than under it and that keeps the light gasket from seating true, which
keeps the cap from seating all the way, too.
At any rate, if no go, unplug the unit and try this time saver: open the
thumb screw at the top of the barrel, gently lower it down, look inside
the reflector and visually inspect the centering job you did with the
three screws on the back of the cap. If really centered, you will see
the image of the filament equally in all parts of the reflector. If not
you must re-bench. Then try the in-out slide until the field gets flat.
I doubt the output will ever match a 1K FEL, but hey...
Doing all this from a Genie is tough, but it sure beats straddling atop
the fully extended stick on an old wooden A-frame, while your buddies
below holler for you to hurry it up. [But we had fun!]
Regards,
Bert
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