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TV Missing color diagnosis

Discussion in 'Electronic Repair' started by TEX, Jul 30, 2006.

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  1. TEX

    TEX Guest

    Been working on this 29" TV and cannot work out why the voltage on the
    one of the driver transistors is low. Drive transistors located on the
    CRT neck board.

    Blue =120VDC
    Green =195VDC
    Red =195VDC

    Green does show up on the screen but very dull. Red/magenta hue. Am i
    correct to say that the blue it under driven causing the complementary
    color to produce the hue. Or are the other two guns over driven????

    Since there is green am i to assume there is nothing wrong with the RGB
    IC and it is still just a driver circuit problem

    Where to diagnose from next??? Just stuck on this cos never come across
    it before.

    Thanks

    TEX
     
  2. Guest

    You need to provide the rest of the troubleshooting voltage and
    waveform measurements at the crt for anyone to help.
    too high.
     
  3. TEX

    TEX Guest

    Are you saying the Blue driven at too a high a voltage? I would have
    thought it was too low, or the others too high, causing the imbalance.
    I have measured these with the CRT connected to the board and
    disconnected with same findings.
     
  4. Art

    Art Guest

    Blue drive to the video output device may be too high, causing the item to
    conduct too heavily, that will eventually lower the voltage because of the
    current draw.Check the condition of the video drive and output devices, the
    voltages at each device, and the voltage at the video processor ic. Also
    check the video signals at each device. Post the diagnostic results so we
    can evaluate those parameters and attempt to supply additional relevant
    information
     
  5. Guest

    No I did not say the voltage was too high. I said the bias or drive to
    the blue was too high.
     
  6. What he's trying to say is that high cathode voltage reduces brightness, all other
    factors being equal. But you need all the other readings.

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  7. Arfa Daily

    Arfa Daily Guest

    Worst case scenario ,,,,cut the trace to the pin and start adding 2M
    This, however, is not fault finding. It is fault condition correction by
    bodging ...

    ( or botching if you prefer )

    Arfa
     
  8. TEX

    TEX Guest

    I am trying to locate the actual drive circuit, but makes it quiet
    difficult without a schematic and if you don't know wtf you are looking for.

    I have done some more testing and found that although i initial get low
    readings of Blue = 120VDC, Green =195VDC and Red =195VDC at the
    cathodes, if i leave the unit on for about 5mins the voltages are all
    the same. But still has a slight red tinge and green is still low.

    Now these are the reading from the collector of the drive transistors
    and basically and cathode pins to the CRT. On readings i get on the
    otherside of the resistors for each gun is about 217VDC constant and
    equal between them all.

    Now in this unit it looks like the main guns are red and green for
    intensity setting and the blue gun separate, as it has a slightly
    different circuit layout and components.

    TEX
     
  9. You can see if it may be a driver problem by switching color drive feed
    wires to the crt board. As in switch the red with the green and see how
    bright the color is driven. This may eliminate the problem from the color
    output from the chassis and you can concentrate your effort to the neck
    board. If the green is bright after switching with red driver, then you
    will know the green driver is weak (not the output on the neck board). Jtt
     
  10. TEX

    TEX Guest

    Confirmed it is not a color driver feed issue. Found a schematic for
    another model TV and from that worked out which wires were the
    control/drive lines. 5 wire connection RBG gnd and 12v. All the input
    drives give value of around 3.1v. Swapped drives and confirmed they working.

    So confirmed it is either a CRT fault or neck board fault. I was hoping
    that since the voltages i was getting at the driver transistors are the
    same weather the board is connected to the CRT or not, then it is board
    board.

    Where to next??

    TEX
     
  11. TEX

    TEX Guest

    After checking the CRT neck board again and found no obvious faults. I
    checked the driver signals and found that the Blue drive is double of
    that of the other two guns.
    As previously stated I am sure i noticed that the cathode voltage had
    equalised after a few minutes of running, but coloured hue was still
    present.

    Will trace back further and see what else I can find. So maybe it isn't
    a CRT output drive board then...


    TEX
     
  12. Perhaps it has developed a small short do to oxidation in the picture tube.
    If you can find an old shop around with a picture tube rejuvenator, or if
    you might have one, hook up the tube and do a short test and zap the grids
    with it. Might not take much to get it back to good shape, since you say it
    work fairly well and only has a slight dulling to the green.
     
  13. judges123

    judges123 Guest

    Worst case scenario ,,,,cut the trace to the pin and start adding 2M
    resistors,,,,a pot would be nice .I've done it on the screen voltage on
    monitors and corrected a BUNCH of them,,,,second thought 2Mohm might be
    a little excessive,,,,,,,;))) start w/whatever you have. Good Luck
    joe.
     
  14. GPG

    GPG Guest

    Also check that the filament supply resistor has not gone high.
     
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