How sensitive is focus on a CRT RPTV

Discussion in 'Displays' started by Dave H, Feb 27, 2006.

  1. Dave H

    Dave H Producer

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    Yesterday I carefully cleaned the CRTs and mirror. However, when I had to roll back my set, the set took a slight "bump" as the wheel of the TV moved into the groove of the carpet.

    I checked convergence and it looked pretty.

    I know my ISF calibrator said he tighted the bolts well on the CRTs when he adjusted for focus a couple of months ago.

    My question is, how sensitive is focus? Can it get knocked out of place easy?
     
  2. ChrisWiggles

    ChrisWiggles Producer

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    Focus can drift, I suspect it probably drifted before rather than the slight bump but it's possible. If you don't see any focus or convergence errors, I wouldn't worry about it.
     
  3. Steve Berger

    Steve Berger Supporting Actor

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    You could probably take it for a ride in a truck without affecting focus. Twisting the cabinet could alter convergence but focus is determined by the distance from the face of the CRT to the screen (bouncing off the mirror) and is actually a fairly coarse adjustment.

    Obviously, if screws were loose it could shift a little but mechanical focus is pretty durable.
     
  4. ChrisWiggles

    ChrisWiggles Producer

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    I don't know if I would characterize it as a coarse adjustment at all. If you are really picky about focus issues, you can get to the point where just tightening the bolts of the lens alters things, and there is some thermal drift and other things that happens. This is why when I do an optical focus with a CRT FP, I do it several times, because usually after the first attempt by the next day things have changed slightly. This is really only noticeable if you are focusing the phosphor grain, which is extremely precise. The lenses themselves, even when the nuts are tightened, are a lot more mobile than you'd like to think. This is why you should not touch the lens body once everything is setup because you can shift things around and adversly affect convergence and things like that. It is also good to be aware of this if you ever clean your lenses, it is a good idea to check convergence after that.

    Rather, my point was more that there are many things that will cause focus to drift, and there are probably more pressing concerns. Moving the TV doesn't help, but it's probably one of many things that have caused focus to drift slightly so unless a person is really picky and has an extremely fine-tuned setup, there are many reasons why your focus has already long since drifted slightly from where it once was and this is just one more straw. Unless you actually see focus issues or are concerned about them and are doubting how well focused things are, I wouldn't worry about it. Of course, it should be easy to check just how focused things are by getting inside the set and checking the grain.
     
  5. Dave H

    Dave H Producer

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    Afterward, I turned off each gun (in the service menu) to look at one color at a time and each color looks as well focused as before. I'm familar with just how sharp the service menu text looks with just one color on at a time (I've checked this before). When I dusted off the CRTs, I used one hand holding microfiber cloth to lightly clean the lenses --- without touching the gun. In other words, my hand was raised over the lense doing it. Also, convergence didn't change one bit.


    When my ISF tech did my focus a couple of months ago, everytime he tightend the screw on the red gun, it would very slightly shift. So, he had turn it only so much....there by letting the tightening move it to optimal focus if you know what I mean.

    I would have no problem re-focusing every 6 mos. myself, however, I know it affects grayscale each time you do it...so I'll just let my ISF tech do it once year for my yearly ISF tune up.
     
  6. ChrisWiggles

    ChrisWiggles Producer

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    Sounds like you have things under control. However, optical focus should not (doesn't) affect grayscale. Beam focus of course would.

    If there were any change in focus, convergence would be off too. Depending on how stable your convergence is, this could help hint that optical focus may have drifted. However, you'd have to be familiar with your convergence, because some people might not pay attention at all, and then check really closely after something had occurred, notice their convergence was off or drifty (though it may be that way normally) and then conclude that something had shifted even if nothing had at all. In any case, it sounds like you are familiar with your display, which is good as it helps you know the starting point to tell whether things have changed or not.
     
  7. Dave H

    Dave H Producer

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    Yeah, I go into the service menu about once a month to tweak red and blue convergence. My "Flash Focus" is disabled since my overscan is under 4% (it's now between 2-3%). I had did a convergence tweak a couple of days before the cleaning and as mentioned, the convergence looked just as good after the cleaning, so it looks like focus didn't change.

    In regards to grayscale, I didn't state it clearly. As you mentioned beam focus would affect grayscale...as that affects the throw of color, right? I was trying to say that if one re-focuses, he may want to check grayscale to make sure that it hasn't changed.
     
  8. ChrisWiggles

    ChrisWiggles Producer

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    beam focus affects grayscale because the spot size can determine at what beam current the phosphor will saturate. This is especially the case with blue. An extremely well-focused blue beam will saturate the spot, while a slightly defocused beam's energy will be spread over a larger area, so the current can continue to ramp up before the larger area begins to saturate. This is why blue is often slightly defocused, to increase blue output capability to maintain better grayscale tracking at high %.

    Optical focusing wouldn't affect grayscale basically at all. But it sure could make your picture blurry! [​IMG]
     

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