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CRT dims gradually after power on

Discussion in 'Electronic Repair' started by Vedran Vuk, Dec 9, 2013.

  1. Vedran Vuk

    Vedran Vuk Guest

    Hello,

    I have a question regarding a CRT monitor which I couldn't quite find the answer to on http://www.repairfaq.org/.

    I don't believe it's the usual brightness loss of a CRT over time where phosphors lose their efficiency but wish to confirm my doubt.

    The problem manifests in the following way: After a power-on the display seems to have solid brightness levels. However, displaying a full white picture dims the display over a period of some 30 seconds - the picture noticeably loses brightness.

    Now if I display a full black picture for a few minutes and then display a full white again, the brightness is back, but starts to fade again like described in the prior paragraph.

    Can you please confirm that this is a problem with some driver circuit, or is this in fact an aging CR tube?

    Thanks,
    Vedran.
     
  2. mike

    mike Guest

    How old is the monitor?
    Is this a new problem?
    Or been that way since you acquired it?

    When it goes dim, can you reduce the brightness?
    And, by that, I mean does the first step down from max reduce the
    brightness more?
    What happens if you set it to the level to which it dims and wait?
    Does it dim proportionally more?

    May be some kind of screen protection function.

    I'd also peer at the filament to see if it also dims.
     
  3. Guest

    It is not the CRT.
     
  4. Guest

    Does the image size get bigger as it dims?

     
  5. Vedran Vuk

    Vedran Vuk Guest

    No, no picture deformations whatsoever.

    Mike,

    I tried reducing the brightness level then did the same test, the problem doesn't occur then. Also, this monitor has a "Highlight" function where you can define either a portion of a screen or the whole screen and additionally bump the brightness in the defined area. But regardless, if the "highlight" is turned on, or just the standard brightness control is too high, it will dim to a certain level below its standard 100% after a short period. With reduced brightness setting this doesn't occur.

    So, I'm guessing some circuit is at fault here rather than the tube. Now, I'm not a professional, but I do know the principle of a CRT and all the dangers that come with servicing it. So my question is, where do I start looking for the fault? I'm guessing something that feeds the flyback transformermight be giving out?
     
  6. Guest

    Nope, if the geometry is staying put, it is something in the video.
     
  7. Sjouke Burry

    Sjouke Burry Guest

    Check whether something overheats. See if cooling cures the
    brightness change.
     
  8. mike

    mike Guest

    Well, my remote diagnostic ability has been stymied by your snipping
    and ignoring most of my questions.
    Suggest you disclose the make/model and contact the vendor.
     
  9. Guest

    Just an FYI, measuring the G2 voltage on the CRT will require a high voltage probe not because of high voltage but high impedance. The 11 megohm load of a digital meter will give inaccurate values so you need the gigohm resistance to get valid readings. Drifting values on the CRT grids can cause allsorts of issues. Anode voltage change will cause the size changes but gridchanges will not - within reason.

     
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