Barco Data 808s : Horizontal Edge To Edge Focus Problem

Discussion in 'Projectors, Screens & Video Processors' started by Leon Rees, Apr 10, 2001.

  1. Leon Rees

    Leon Rees
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    I'm running a low hours, Barco Data 808s against a 1.3 gain 90 inch diagonal 16:9 screen.

    I'm having difficulty getting the left and right edges of the screen in perfect focus on the green tube. The problem is limited to the green tube only - I can get razor sharp focus on the blue and red tubes. I hear that edge focus on 16:9 screens is difficult to achieve, but this obviously out of focus and could be sharpened.

    Using the mechanical lens focussing, I can get one of the sides in focus, but not the other. Electromagnetic focus isn't helping me, but its probably worth mentioning that I can't even get the centre of the screen in focus unless I use a centre EM focus of 80. Again this is only for green.

    I've tried making astigmatism adjustments, but it would seem the problem doesn't lie there. My projector has no Scheimpflug adjustment (that I'm aware of - only the BG808 has this) to call into play. It is firing at a slight downward angle, but top to bottom focus is perfect, so I don't anticipate that being a problem. The projector is also perfectly square to the screen... I re-triangulated it yesterday.

    Is the next logical step to use the focus CT at the back of the tube? I've left this as the last resort. Does anybody have a diagram of the various yokes on the BD808s, where they are and what they do?

    Is there anything else you recommend trying?

    I'd appreciate any help you can offer.

    Thanks,

    Leon
     
  2. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Leon,

    I'm unfamiliar with the 808 at present. My immediate thought was that you weren't square to the screen but you seem to have checked that.

    When you say it's out of focus, how bad and is it impossible to focus it even at the expense of de-focussing other parts of the screen (I know you don't want to do this but it helps understand the problem more)

    Gordon
     
  3. Leon Rees

    Leon Rees
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    Thanks for your fast reply...

    I can get the left side of the screen in focus at the expense of the right side being terrible and vice versa... so yes I can focus at the expense of other parts of the screen losing focus.

    I can't help but think that having to set EM centre focus for green to 80 (out of 100) to get any kind of centre focus has something to do with this.

    Leon
     
  4. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Leon,

    I guess that the astigmatism adjustment you are talking about is an electronic one.

    I think you'll have to set astigmatism and electronic focus to mid points all round and then then do a mechanical focus. Next you'll have to either eployee someone or learn how to adjust the focus and astigmatism magnets on the neck of the green gun.

    I see you've posted over at AVS about this so I expect some 808 users will explain the best way of going about this on your unit.

    Gordon
     
  5. Leon Rees

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    Thanks Gordon,

    That was my next step... electronic astig and focus to mid point and use yokes to make the baseline adjustments.

    Yeah, I posted to the AVSForum... if it's OK with people, I'll cross-post a summary of the outcome... we really should be building our own knowledge base of this kind of stuff.

    Uncle Eric - anything to add from your 808 experiences here?
     
  6. Oasis

    Oasis
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    Leon,

    From your description of the problem, i.e. that you can get the left in focus or the right, but not both, using the mechanical focus, then I think the problem has to be a mechanical/optical one so I wouldn't touch the electrical bits at all for now.

    If you couldn't get good focus at all one one side then it could be electrical focus - i.e. the electron beam isn't focused correctly on the CRT faceplate.

    But as you can get both sides focused then it has to be an mechanical alignment between the CRT and the lens assembly. It's quite possible that the projector has been put down lens end first at some point (e.g during it's extraction from it's previous home)and bent the chassis or lens mounts.

    I would suggest calling Barco in Reading and chatting to the service chaps to see if there are any mechanical adjustments possible before trying to move yokes and the like!

    Good luck in your quest!

    Oasis
     
  7. Jenz

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  8. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Oasis,

    Smart man. Don't know why but I just never assumed that someone could do that sort of damage. Seems obvious though.

    Leon let us know what happens.

    Gordon
     
  9. uncle eric

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    Hi Leon,
    Sorry for the late reply, just got back from Hong Kong, phew what a horrible 14 hour flight!!!
    From your discription, I would almost certainly take the view that its a damaged lense mount. However, it doesent seem logical that the green would get bashed because, as the Red and Blue are either side, they would be more prone to a knock than the Green.

    Heres something else to do. 1) Average out the mechanical focus on the green as much as possible. 2) Set digital focus to midpoint.
    3) Open the main (big plastic) cover. Of the two metal covers, open the smaller metal cover at the lense end. This should swing down (I'm assuming you have yours ceiling mounted). On the Green tube yoke you will see
    black hand turnable screws (they look like those keys that were used to open sardine tins long ago). Depending on your eyesight, you may have to use binoculars or a video camera to check your progress.
    PLEASE BE AWARE THAT THERE ARE LETHAL CHARGES IN THIS AREA. BE VERY CAREFULL. AS A PRECAUTION WEAR A GOOD PAIR OF RUBBER GLOVES.

    Eric
     
  10. Leon Rees

    Leon Rees
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    Thanks Eric...

    Are those sardine key things for focus or for astig?

    I'm starting to think that this is a Scheimpflug problem ... this seems to be a diagonal corner focus problem. Left bottom corner and top right corner go out of focus in sync.

    Unfortunately my projector doesn't have Scheimpflug adjustments ...

    Leon
     
  11. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Leon,

    Even when you do get that type of adjustment it's only for the red and blue guns. Green is expected to be mounted centrally and shouldn't need adjustment.

    I think Oasis seems closest here. Check for damage.If none is eveindent then you may have to liove with the best averaged focus you can get.

    I'd post over at the Barco site as Jenz suggests though.

    Gordon
     
  12. uncle eric

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    Leon,
    They are for focus, and yes green DOES have these adjustments also.
    Last resort, swap over the Greens lense mount with either the red or blue and look again.
    let me know how you get on.
    Eric
     
  13. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Eric,

    I was talking about about the Schiemflug not being available for Green. Not astigmatism and focus. Sorry for any confusion.

    Gordon
     
  14. Leon Rees

    Leon Rees
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    OK... we're getting there!

    Eric... your mechanical (not optical) focus suggestion has nailed most of the problem

    I have a small astig problem throwing a corner off ... I'll tackle that next.

    Thanks for your help guys...

    Leon
     
  15. Roland @ B4

    Roland @ B4
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    Try this
    First you need to establish if this is optical or electrical issue.
    Because you can get focus left and right but not at the same time I suspect it is optical but let’s check anyway.

    Electrical.
    Go into random access menu.
    Select internal pattern and go for a low frequency say pal video.
    This will give you loads of small dots to assess the focus.
    Turn the brightness right down and the contrast too until you can bear to look down the lens at the tube surface. Be prepared to make marks on the lens with your nose here.

    Is the image sharp over the whole field?
    No, time adjust the electronic focus.
    Adjust the middle (centre) focus (in the random access menu) first.
    If it’s still soft in the left or right adjust for that area now.
    If you now have it sharp over the whole surface you can move onto the optical bit.
    If it’s sharp but has a "hallow" to the top/bottom or left/right of the main line then diagonal astigmatism is out. See comments below

    Optical
    Loosen the nut closest to the tube face (centre focus) and rack it back and forth to see if you can get the image sharp in every point of the screen. Assuming you can, get the centre sharp and then lock it off. Now use the front focus ring to bring the sides into focus. Still can't get it sharp! Sounds like Oasis may be right and you have a bent projector or the lens is squify.

    Pull the power cord (37,000v!) and lift the lid, lift the metal shield and measure from one side of the chassis to the centre of the green tube same other side. If it’s different - problem.

    The astigmatism is not one to fiddle with unless you know what you are doing because it is global adjustment for all sources. Play with this only if you know what you are doing that's why it's put in a service menu. There is no reset and can take hours to get back to normal. Great way to bugger up a good projector!

    Play with the magnets on the back of the tubes with rubber gloves (again 37000v) at your peril; these only need to be changed
    1 if you have just replaced a tube
    2 if you have moved the projector into a new part of the world northern/ southern hemisphere 3 it has a VERY strong magnetic field near to the projector. (Steel beam or mounting)

    With respect you run the risk of screwing up a very fine projector before it is mounted to the ceiling.

    A lot of tests I know but I very much doubt it is beyond hope.

    Let us know how it goes.
     
  16. uncle eric

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    Leon,
    Glad to be of help.
    Eric
     

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