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High Voltage

Discussion in 'Announcements & Open Forum' started by kopcicle, Dec 8, 2020.

  1. kopcicle

    kopcicle Sr. Member

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  2. kopcicle

    kopcicle Sr. Member

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    and on a completely related note
    "Do the name Ruby Begonia strike a familiar note"?
    not_so_neutral.jpg
    PC-076 Transmitter Mixer and Driver.JPG
    So a brief explanation is in order. Operating the radio one afternoon there was a comment from a local on 10m "what happened to ya"?
    Turning to the radio and external watt meter I saw a jump on PTT then a fall to nothing. One more attempt to transmit and I heard a soft near imperceptible "phhhhhzzzt". Trying again and looking through the top cover I saw a brief arc on key up. Having been a welder most of my life the arc was very familiar but did not belong in my radio. Further inspection revealed the damage.

    C(2)87 shown above with a wart on it and on the schematic as C287 is inline with the neutralization wire from the PA board.
    Power_amplifier.PNG
    It's something we all see from time to time, the upstream leg of C287 developed an insulating ring of shmooze around the upstream leg and the subsequent open caused the HV to jump across the air gap to the capacitor.

    Fortunate that this is at the exposed edge of the board and only a small amount soldering gymnastics were required to replace the part.
     

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    Slowmover, tecnicoloco, TM86 and 2 others like this.
  3. DoctorAudio

    DoctorAudio Active Member

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    Slowmover likes this.
  4. nomadradio

    nomadradio Analog Retentive

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    Hmmmm. 3 capacitors in series. A 47pf, 10pf variable and the 5 pf fixed.

    The way I remember it, AC and DC voltage are divided between series capacitors in a proportion inverse to capacitance. Of those 3 capacitors the 5pf will get more RF and DC voltage both across it than the other two. Does suggest that scrimping on C87's breakdown-voltage rating could be the whole root of the failure.

    73
     
    Slowmover, kopcicle and tecnicoloco like this.
  5. TIN_CAN

    TIN_CAN Well-Known Member

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    Shadetree Mechanic likes this.
  6. Slowmover

    Slowmover Elmer

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    “High Voltage”. “Sufficient Amperage”

    Are those the secret password challenge answers to the boys-only Raccoon Roaster Club

    (Man, I’m never gonna stop laughing at that one).
     
  7. nomadradio

    nomadradio Analog Retentive

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    My latest favorite HV hijinks vid.



    73
     
    Shadetree Mechanic and Slowmover like this.

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