CurtPalme.com Home Theater Forum

[RVonse]

I have noticed that focus of my green tube changes very slightly from when I first power up to after a couple of hours of viewing. It is not an elctrical but a mechanical focus change, and its not a lot. I'm stumped about what would causes this. Could it be the c element distorting or shifting? Would it help to remove some of the fluid even though there is no sign of fluid leaking? The lense and c element were originally taken from an Ampro 4200.

[Tom.W]

I doubt the c element or glycol has anything to do with what your seeing...

Try swapping lenses and see what happens.

[r.bauer]

This is temperature related. I have experienced this with a 1209 setup I did for someone else. There was no temperature control in the room, and the temepersture in the room increased more than 8 degrees centigrade in the room, and the green lens was warm to the touch! The projector was ceiling mount, which makes it even worse. I had to re-focus green optically because of the increase in temperature.
CRT projectors 'behave' a lot better when the room is temperature controlled, unfortunetaley this is not the case in many Home Theaters.

[CZ Eddie]

Mechanical focus doesn't change in normal environments. It's set & forget.

If focus changes as temperatures rise, then you need to look at magnetics or electronics. If the problem is on all three tubes, then I'd start with magnetics. Try setting G2 to 100 and ramp focus up & down. If dots move around a lot, then you've got a magnetics issue.

If it's only on one tube, then I'd look to electronics.

[cmjohnson]

You guys are forgetting your basic high school physics classes.

EVERYTHING changes size as it gets warmer. Everything gets bigger as it gets warmer. Lenses included. As the lenses grow slightly larger, it slightly changes their focal point.

Why do you think that all the calibration specialists say to let the display or projector warm up for at least half an hour before calibration?

It's not to allow yourself a coffee and bathroom break. It's so the projector can stabilize at operating temperature, and that includes lens stabilization.


Anyone who ever spent any time at all in amateur astronomy knows just how far focus can drift on a telescope with a change in the weather. The difference is sometimes quite dramatic.


CJ

[zGman]

Yes - the entire pj chassis will shift a bit during warmup - but -
one thing to be aware of is that overtightening the lens base screws,
and especially tightening unevenly, can cause more shifting than
normal - also, there is no need to overtorque the focus knobs - just
snug is fine - not good to create unresolved stress in the system.

G

[RVonse]

[Curt Palme]

A customer complained bitterly on avs about this a few years ago with a G70 I sold to Sweden I think. all new tubes, etc. He finally came back saying it was optical shift, but his pictures were so poor that we could never tell anything from the\m. Over weeks of playing with it, he wore his green tube due to 50 hours of all white on the tubes (!!!) and then of course blamed me for that too.

We never figured out what did it, but based on the above responses, I'd say that mechanical shift is indeed possible due to temperature changes. I don't think most people would notice it though.

[Curt Palme]

A customer complained bitterly on avs about this a few years ago with a G70 I sold to Sweden I think. all new tubes, etc. He finally came back saying it was optical shift, but his pictures were so poor that we could never tell anything from the\m. Over weeks of playing with it, he wore his green tube due to 50 hours of all white on the tubes (!!!) and then of course blamed me for that too.

We never figured out what did it, but based on the above responses, I'd say that mechanical shift is indeed possible due to temperature changes. I don't think most people would notice it though.

[RVonse]

[zGman]

I had the thought back when I had my nice 9500LC mounted, that it would be nice
to have some temp controlled chassis heaters attached to the LC housing mounting
plate, as i was similarly bothered by slight focus shift during operation - never
followed thru on it though...Did find the issue with lens base screws, and switched
to stainless steel screws - which have a more compatible thermal coefficient of
expansion to the aluminum housings...

G

[ecrabb]

[Sparky015]

Is this why the Marquee has temperature thermistor in the focus coil? Sony and Barcos don't have this, so not sure if focus is temperature compensated like in a Marquee?

[Tom.W]

It could be a loose lens element...Did you try swapping lenses ?

[CZ Eddie]

[deronmoped]

Heck

It's even normal for your eyes focus to vary. Wake up first thing in the morning and you should be able to notice you can not focus as good. From day to day you will notice your eyes ability to focus will change. How your body is treating you, weather conditions... will change everything, even your PJ can be effected by the weather.

Set the PJ for best overall focus, trying to get it perfect will have you on the quest for adaptive optics.

Deron.

[Tim in Phoenix]

Guys

The first Electrohome ECPs (1000 and 2000) had lens elements mounted on bi-metal strips like those found in thermostats. When the lens elements warmed up, the strips would shift the lens position a few thousandths of an inch, effectively tracking the temperature shift.

.

[garyfritz]

[ecrabb]

[deronmoped]

I almost forgot about those days. Have not had a drop in ten years. The parties were always fun, but the next day was payback

Deron.

[Mark_A_W]

I've never noticed the focus change with temperature on my various NECs.

I've never really looked for it either.