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32.768 KHz crystal failure

Discussion in 'Electronic Repair' started by N_Cook, Jun 29, 2009.

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  1. N_Cook

    N_Cook Guest

    One of those tiny barrel watch-size ones. Measured 13 ohms across the pins
    before and after desoldering. Could not resist grinding the end off to have
    a butchers'. Heat from grinding destroyed whatever ohmic path there was.
    I've never looked inside one before. Tuning fork type form with 4 complex
    tapering tracks, silver looking, on each face, under a x30 microscope.
    Presumably silver migration/silver mica cap disease, only a few atoms
    bridging a 100 micron gap between tracks to cause failure.
    Previously I've come across ceramic resonator and filter failure due to
    ohmic , presumed Ag migration .
    The closure end with the wires, is the barrel swaged over a tiny paxolin
    disc exactly like can type electrolytic capacitor.
     
  2. Franc Zabkar

    Franc Zabkar Guest

    I've never observed that type of failure. I've seen crystals that
    resonate at an overtone rather than their rated frequency. Others were
    shattered by physical impact (you can hear the shards rattling
    inside), and others just wouldn't start oscillating without some
    coaxing, either by heating or freezing. I've also shattered a crystal
    when I cut up a circuit board with a band saw.

    The strangest failure was in an IBM PC whose graphics chip was
    outputting H & V sync frequencies that were much higher than normal,
    but were not an integer multiple of the correct frequency. I traced
    the fault to a crystal oscillator. It was suggested to me that maybe a
    quartz dag that was present during manufacturing had later fallen off.
    This would reduce the mass of the crystal, resulting in an increase in
    its resonant frequency.

    - Franc Zabkar
     
  3. N_Cook

    N_Cook Guest

    Certainly not the simple slab with simple silvering on each side. As the
    open ends of the tuning fork look ground, on a 30x ,then perhaps literally a
    tuning fork , actively tuned to frequency, and the odd trace patterning is
    for modulation of capacitance. I don't suppose a 13 ohm build up of Ag over
    100 micron gap would be observable, as stable at 13R via soldering/movement
    doubtful it was some mechanical breakage/movement.
     
  4. N_Cook

    N_Cook Guest


    My 30x microscope+camera can only capture about 1mm diameter
    http://home.graffiti.net/diverse:graffiti.net/quartz.jpg
    overal quartz sliver is 6.5x1.5x0.3mm
    White is the silvering with some illumination flare, some connected to one
    pin and some connected to the other pin, similar but not the same on the
    other side. G is the area of quartz ground down end and face, a bit ,
    compared to the lower finger of the "tuning fork".
    Area near the notch , dimension between + and + is 0.75mm , half the width.
    So minimum gap between silverings about 60 micron.
    Width of notch about 0.25mm
    There is also silvering along some of the 0.3mm edges, so very complicated
    silvering geometries
     
  5. It is a tuning fork reference. Bulova type, transistor switched to
    Non-sequitur. The Bulova Accutron, dating back more than 45 years, used a
    rather large tuning fork. I still have my father's.
     
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