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Have a question about Variac diode in a HW24T/C528 Hand held.

Discussion in 'Electronic Repair' started by Jamie, Apr 22, 2007.

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  1. Jamie

    Jamie Guest

    This radio is dual band Heath Kit hand held, Same as the
    Standard C528/A
    It appears that the VHF Tx has low audio compared to the
    UHF., Using my SM, I noticed that I need to turn up the
    audio drive that goes to the VCO much more than the UHF
    Both VCO are of the same design, just using different
    size components due to freq differences.
    There is a Variac DIODE there being injected via the
    Mic audio for the FM deviation.
    Both Audio sources for UHF and VHF come from the same
    Mic pre amp source using a trimmer from the output for
    each band..
    The UHF needs only about 50% set point to get the max
    5K spread how ever, I need to turn the VHF up to 100% to
    make it satisfactory.. This unit has been repair three
    times by some one else that has replaced all of the smt
    electrolytic caps due to being leaky and thus claiming to
    cause the low audio problems. Now I am looking at it.

    I guess my question here is this:
    Have any of you ever seen a Variac Diode lose/reduce it's
    capacitance dynamic range?
  2. Arfa Daily

    Arfa Daily Guest

    That's a vari-cap diode ...

    Unless it has developed a physical fault such as having gone leaky, which
    you could of course easily measure, it's hard to see how a varicap could
    lose swing, and I can't recall ever having seen one do so. At the end of the
    day, there is nothing special or magical about varicaps. All diodes exhibit
    the effect to a greater or lesser degree, as it is purely down to the width
    of the barrier layer at the junction, which is in turn dependant on the
    level of reverse bias that's being applied. The only thing that makes a
    varicap a varicap, is that the doping and construction has been specifically
    fine tuned to maximise and linearise the effect.

    I think that my first move in a case like this, would be to take some
    careful measurements of both the DC bias conditions and AC drive levels at
    the diode, specifically using a high z measuring device - ideally a 'scope,
    if you have access to one. A x10 low capacitance probe will probably be
    needed to stop the input capacitance of the scope and probe lead, from
    loading and stopping the oscillator.

    I would be very suspicious of board damage - partially eaten and hence high
    resistance thru' plated holes, and corroded / partially eaten surface mount
    Rs and Cs, if the replaced sm electros really had leaked.

  3. Jamie-

    You mentioned SMT and the Standard C528/A. Does this mean the Heath was
    not built from a kit?

    Based on my experience with Heathkits, I'd question whether it had ever
    worked correctly if built from a kit. I'd be examining it for incorrect
    components, backwards electrolytics, solder bridges, et cetera.

    One thought - if the VCO is multiplied to get to the output frequency,
    less modulation would be needed for the higher frequency due to the
    additional 3X multiplication. The design should have taken that into
    consideration. One way would be to use a different vari-cap diode for
    VHF, that had a 3X greater delta C/delta V ratio. Is it possible that
    the two vari-caps are interchanged?

    I noticed that when adding a PL oscillator to an older radio, a series
    resistor is used between its output and the radio's FM modulator. In
    your HW24T, how are the trimmer resistors isolated from the varicaps?
    If by a high-value resistor, could the VHF resistor have gone high in
    value? Or be the wrong part?

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