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Anyone seen this? Toshiba HDTV CRT dead?

Discussion in 'Electronic Repair' started by John-Del, Jun 13, 2005.

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  1. John-Del

    John-Del Guest

    I did a house call on a Toshiba 36HF12 that the complain was no
    picture. I had repaired this about 6 months ago for vertical
    shrinkage, and it's been running fine till now.

    It develops 32KV HV, filament, 250 kine supply, G2, and all secondary
    diodes sources off the fly are normal. The cathode voltages on all
    three cathodes are high rail. I didn't have a scope with me, but the
    60hz vertical "growl" from the yoke is unmistakable, so I know it has
    vertical deflection. The G2 voltage varies from 200 to 850VDC.
    Carefully raising the G2 to maximum induces no light whatsoever on the
    CRT.

    I brought in my old B&K 467 and hooked it up to the CRT with clipleads,
    and the tube reads DEAD, like maybe the G2 or G1 pin is disconnected.
    The TV is just over 2 years old, but the customer is going to try to
    see if Toshiba will cough up a CRT.

    Has anyone seen one of these do this?


    John
     
  2. Jeff Rigby

    Jeff Rigby Guest

    Cathode voltage rail high means the tube is turned off. It's an electonic
    problem in the video out circuitry. Kine bias circuit?
     
  3. Guest

    John:
    Check the G1 voltage. If it is very negative, eg: -90V to -120V,
    the tube is biased off. Try connecting a resistor box from G1 to
    ground, and gradually reducing the G1 voltage. If picture comes
    back, you have a bias problem.
    I also try a 1K ohm resistor on a jumper wire to ground. Touching the
    other end of the resistor to a R,G, or B cathode should cause that
    color to appear on the screen.
    Hope this helps...
    John
     
  4. John-Del

    John-Del Guest

    An update, but first things first:

    Jeff, let me clarify that. The voltages right at the cathodes are
    exactly the same as the collectors of the kine outputs. In other words
    there is no voltage drop across the 560 ohm resistors that connects the
    kine outputs to the cathode. The voltage drop across the limiting
    resistor is typically a few tenths of a volt when the tube is drawing
    current. The collectors themselves show normal voltage drop across
    the 15K resistors. And remember, even if it was a video problem, the
    tube could easily be driven to raster using the G2 control (and this
    one wouldn't). Sorry for the confusion.

    Jdg, The G1 on most Toshibas (including this one) is actually grounded
    to the aquadag, so the voltage is zero, which I didn't mention but
    should have......


    Okay, I have a Panasonic 34 HD in the shop that has a propblem in the
    DG board, so I ran the Toshiba chassis on the Panasonic, and it runs
    perfectly ( slight geometry overscan issues with the Panasonic yoke :)
    , so the tube is most definitely bad. I've been doing this for over 35
    years, and this is the first one I've seen go cold dead despite having
    good filament.

    Thanks for the replies.
    John
     
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