1. Join Now

    AVForums.com uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

"dirty screen" "darker patches"

Discussion in 'Televisions' started by Kula, Dec 16, 2003.

  1. Kula

    Kula
    Standard Member

    Joined:
    Nov 8, 2003
    Messages:
    38
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    8
    Location:
    Glasgow
    Ratings:
    +1
    Can anyone give me a definative answer on what causes these so called "dirty Patches" which are visible when the screen pans.

    After many hours of searching I have come to the conclusion that the shadowmask is involved in some way and degaussing either automatically through the tv's own circuitry or manually does nothing to help with this problem.

    It seems that many makes of Large screen crt's have this fault to a greater or lesser degree.

    Is there anything that can remove or lessen the effect???
     
  2. Kula

    Kula
    Standard Member

    Joined:
    Nov 8, 2003
    Messages:
    38
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    8
    Location:
    Glasgow
    Ratings:
    +1
    Are there not any TV engineers who read these forums who can offer any insight into this problem??

    I have searched various threads involving "dirty screen" and loads of people have the problem but I cant find any thread mentioning a solution :(
     
  3. LV426

    LV426
    Administrator Staff Member

    Joined:
    Jul 14, 2000
    Messages:
    12,789
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Somewhere in South Yorkshire
    Ratings:
    +4,975
    The problem is caused by a minor error in the way the scanning coils, mounted around the neck of the tube, are steering the beam of electrons towards the screen. This may be the coils themselves, some external magetism, or some residual internal magnetism.

    Working perfectly (and few, if any do) the beam of electrons from, say, the red gun, is exactly steered towards the shadow mask so that is passes neatly through the corresponding holes in the mask and lands on the red (say) phosphor spots on the screen itself.

    Working less than perfectly, the beam of electrons does not hit the shadow mask in exactly the right place and part of the beam is absorbed by the mask. Only the remaining part hits the screen and the phosphor is thus illuminated somewhat less than it should be. Hence a dark patch.

    Working even less well, and the beam passes through the wrong hole in the shadow mask and lands on the wrong phosphor. So, for example, the red signal actually (partly) illumintates, say a blue phosphor. And this is what causes colour staining.

    So, colour staining is the same problem you have, made worse.

    It can be fixed by a competent and experienced engineer, who can affix small permanent magnets to the neck of the tube, to correct any mis-steering of the electron beams.
     
  4. Kula

    Kula
    Standard Member

    Joined:
    Nov 8, 2003
    Messages:
    38
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    8
    Location:
    Glasgow
    Ratings:
    +1
    Nigel, thankyou very much for your in-depth response. This has provided me with some valuable information.

    I have recently bought a JVC AV32T25. In general I am very happy with the set. Picture is nice and clear, there are some minor geometry issues but nothing I can't live with. The problem I do hve as you might have guessed is "dirty" patches on the screen.

    I am debating with myself as to whether I can live with this problem or not ( I've already had a refund on a Phillips 32PW9308 which had "dirty screen" among other problems).

    One thing I did notice however was when the tv was switched off - if I go quite close to the screen there seems to be some minor distortions in the shadowmask in the areas where the "dirty" patches are. They are barely noticable but are definately there. With a light source behind me if I go close to the screen ( less than 1 foot) and shift my position to the left or right (like when looking for a dent in a cars bodywork) the reflection of the light behind me shows definate distortion in the shadowmask.

    DO you think that these distortions in the shadowmask would cause the problem and not/as well as the scanning coils??

    Also is there a tolerance level for this sort of problem??

    Thx again for your info.
     
  5. LV426

    LV426
    Administrator Staff Member

    Joined:
    Jul 14, 2000
    Messages:
    12,789
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Somewhere in South Yorkshire
    Ratings:
    +4,975
    I doubt very much that you can actually see the shadow mask itself. More likely you can see the 'gaps' between the phosphors on the tube surface. I'm not sure, but I'd guess that any irregularities there are more likely to be part of the design of the tube, than a fault. The shadow mask itself is a small distance back from the tube face and sight of it is normally wholly obscured by the phosphors themselves.

    Tolerance? - yes, almost certainly. Every consumer product has these and, typically, manufacturers view tolerances with greater 'tolerance' than users do.

    Not wanting to seem unsympathetic, your issues here are a good example of why, whenever anyone says 'CRT is best' I'm apt to respond with a general statement about why every technology has its shortcomings, including CRT. It's just that they differ from technology to technology. And CRTS (especially big ones) are prone to the type of issue you describe.

    I hope you get it resolved to your satisfaction.
     
  6. cosaw

    cosaw
    Active Member

    Joined:
    Jun 14, 2003
    Messages:
    753
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    31
    Location:
    Stockport, Manchester
    Ratings:
    +19
    Kula,

    On my sony KD32DX40 I too can see what look like distortions in the shadow mask when viewed up close/telly off/and with light source behind. This does create some shadowing and a pinkish hue when viewed at more extreme angles (past the normal angle of viewing thankfully). I've had this on every sony set I've had to some extent. However it is rarily noticed when the telly is on so has never affected my viewing pleasure and I've been happy with all my Sony's.

    I couldn't agree more.

    cosaw
     
  7. Dan666

    Dan666
    Standard Member

    Joined:
    Jul 4, 2002
    Messages:
    144
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    18
    Ratings:
    +2
    Kula, I have this exact same priblem with my JVC AV32R25.

    The areas in the top and bottom left of the screen seem to be the worst affected - that said, its not too noticeable (only on scenes with large areas of solid colour) and I could probably live with it if its too difficult a job to sort out.

    But if you decide to get yours fixed, please post your experiences here.

    TIA
     
  8. blindlemon

    blindlemon
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    I'm still waiting for a visit from an "official" Panasonic engineer to tell me whether the colour purity/dirty mark problems on my 36PD30 are a defect or a "feature".

    When he/she finally arrives I'll ask if fixing permanent magnets will cure it - as, if it will, then so long as it doesn't cost me anything, I'd be happy to let Panasonic give it a try.

    Seems to me that if you pay £2k for a "top of the range", "flagship model" TV - they should bloody well do everything in their power to make sure it's up to scratch.

    I bet Mr Matsu****a's PD30 doesn't have dirty marks!
     
  9. LV426

    LV426
    Administrator Staff Member

    Joined:
    Jul 14, 2000
    Messages:
    12,789
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    166
    Location:
    Somewhere in South Yorkshire
    Ratings:
    +4,975
    I bet Mr Matsu*****a has a massive Plasma, LCD or Projector, and not a whopping great CRT.

    (Damn clever that profanity filter. Too clever, in fact!)
     
  10. blindlemon

    blindlemon
    Guest

    Products Owned:
    0
    Products Wanted:
    0
    Ratings:
    +0
    Unfortunately, and against my instincts (as I still feel that in absolute terms 'CRT is best'), I'm starting to see why....

    I suspect I'll end up with a plasma too - as I reckon the chances of Panasonic sending a genuinely skilled engineer round to my house to fit little magnets to my tube until I'm happy with the picture are just about next to zero. :(

    And the chances of getting a perfect 36PD30 on an exchange are pretty remote too....:suicide:
     
  11. Kula

    Kula
    Standard Member

    Joined:
    Nov 8, 2003
    Messages:
    38
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    8
    Location:
    Glasgow
    Ratings:
    +1
    Thx again guys, appreciate the input.

    I have sent a "customer service fault form" to the company I bought the tv from. I contacted them within 3 days of taking delivery of the tv and I am waiting for a response from them.

    I basically detailled the problem as clearly as possible with the help of Nigels post above (thx again m8).

    But I basically have 2 dilemas now :-

    1) IS the aforementioned problem really bad enough to warrant a repair/replacement? My girlfriend does'nt really notice the patches - she says she is too busy watching whats going on in the screen to bother looking for minor faults, but the fact I can see the fault means I look for it instead of just watching the blooming tv :( Also the engineer/manufacturer may say it is in tollerance.
    and....
    2) If it is deemed to be faulty and they need to take the tv away
    to repair it/replace it i'll probably have to go back to the 15yr old 21" I had before through the whole of christmas :(

    Anyways I'll keep you all updated as the situation progresses.

    PS DAN666 FYI the worst patches on my tv are on the upper and lower right hand side, there is a bit on the left hand side and a sort of v shaped section left of centre coming down from the top of the screen.
     

Share This Page

Loading...