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Open capacitor blows fuse.

Discussion in 'Electronic Basics' started by John Popelish, Mar 4, 2007.

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  1. I recently bought a vintage Tektronix scope (a T922) that
    was sold as a fixer upper. I found a blown fuse in the 100
    volt supply to the high voltage generator. It looked like
    someone had been soldering on all the semiconductors in that
    area, and failed to solve the problem. The fuse is a 0.062
    amp slow blow (normally with hundreds of ohms resistance).
    I looked at the board for a while before it dawned on me
    that the flyback type circuit would have pretty high peak
    and therefore RMS to average current ratio.

    There was a 10 uF 150 volt electrolytic bypass capacitor
    down stream of the fuse. I pulled that and found it
    measured about 500 pF. So I assumed that the fuse was over
    heating because it was carrying the high RMS current of the
    narrow flyback changing pulses, instead of the average
    (bypassed) current. I was very proud of my deduction.

    While a replacement was on order (I got a 10 uF 250 volt
    film capacitor, so this would never happen, again), I found
    a schematic on the web and discovered that the driver
    transistor does not operate in normal, saturated switching,
    flyback mode, but as a linear sine wave oscillator with
    variable bias feedback current from a measurement of one of
    the high voltages. The dried out capacitor, evidently
    loaded the oscillator Q down, with the resistance of the
    series fuse, enough to prevent oscillation, and the bias
    circuit just cranked the average current up till the fuse
    blew. But the dried out, open circuit capacitor was still
    responsible (indirectly) for the blown fuse.

    The scope is working fine now. :)
  2. MassiveProng

    MassiveProng Guest

    If an "open" electrolytic is presented with a high enough voltage,
    it can appear as a dead short at the moment if there is a breach and
    an arc between plates, yet it will appear as "open" under some

    This is especially true for HV Ceramics. They can be pierced by an
    overvoltage, and appear as fine under LV testing, but as a dead short
    in circuit or when used at the higher voltages they are rated for.

    This is why diagnosing an HV multiplier circuit problem can be very
    difficult. Thermal imagers help as the bad cap will always be warmer.
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