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Cold cathode

Discussion in 'Electronic Repair' started by [email protected], Jun 11, 2005.

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  1. Guest

    Hi;

    Was working on a Zenith RPTV, a 9-1302 based set. Starts bad and gets
    worse like some of these do. Bad waveform, bad flyback. I did check it
    with the HVB disconnected and it was stil bad. With the new flyback now
    the waveform is good diconnected. I don't know how it happened but it
    did need both. Changed the HVB and finally got somwhat of a green
    raster with intermittent vertical sweep, which was enough for us to see
    the phosphor burn. Nothing from the red and blue though.

    The HVB was used so I tested the G2 output, it was there. It hit about
    500V and with the meter loading that's about right. After the flyback
    didn't fix the set I checked everything off it and found nothing
    shorted, so I decided to try it with the yokes disconnected. Of course
    I disabled the filament supply first for obvious reasons. I had
    fogotten doing that and that's why it had no filament. The boss himself
    saw the weak green raster and what he calls buggers, referring to the
    phosphor burn.

    He stopped short of saying I was high (at the time :) ) but apparently
    has never seen this. I have. It is VERY rare but it does happen. My
    nightmare is over with that set, it might get a used green tube or not,
    but that is not the point.

    The main questions:

    1. Just how many of you have seen cathodes emit without heat ? I don't
    mean when they are designed to do so. I can remember a few RCSa doing
    it, but still working. I'm not sure of what other brands I've seen do
    this but they are out there.

    2. Does anyone know what the physics of this phenomenon are ? Is it a
    CERTAIN contaminant, or is it somehow the current of the leakage in the
    tube is actually heating the cathode.

    The plug to the standup board on the 9-1302 probably supplies the other
    voltages to the CRT boards. The 12V or something might come from the
    signal board, I'm not sure right now.

    Also, the raster, such as it was, was there immediately. I doubt a
    leakage path could get something hot enough to emit that fast. I mean
    immediately. Also, with the filament supply working, the green tube is
    leaky/shorted obviously and that's probably the cause of the burnt
    phosphor.

    Now you know I'm not stupid, but I am a bit befuddled by the multiple
    problems. I could see if the customer literally pushed the power button
    1000 times a shorted HVB might burn up a marginal flyback that may have
    had a month or two of life in it. The burn indicates they watched the
    set screwed up, all green. The intermittent sweep might clear up when
    hot. Other ideas are indeed welcome.

    Any valid responses are bound to be interesting and as such are greatly
    appreciated.

    JURB
     
  2. Guest

    RCAs not RCSes. Sorry.
     
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