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DC Voltage on Vibrato Line output of preamp

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  • DC Voltage on Vibrato Line output of preamp

    When nothing is attached to the "C" Terminal of an AO-28 or AO-29, or on an AO-10 the "VIB LINE" terminal, is it expected to have a somewhat high DC voltage present when measuring that terminal (say, 80 V DC)? It slowly tapers off to zero as I hold my voltmeter probe on it.

    Instinct tells me that the capacitor behind this terminal is leaky, yet the capacitor by itself passes the leakage test on my Heathkit IT-28.

    Apparently, both AO-10 preamps I'm working on do the same thing. Is this an expected anomaly, or are both leaky?

    Footnotes: I am using a Heathkit IT-28 to measure the leakage, all wires are disconnected from the capacitor ends, so it is "stand-alone".
    -- Also, when I touch the VIB LINE terminal while the preamp is on, I feel absolutely nothing.
    -- I thought this was odd, mainly because I don't recall ever having to replace these 1uf capacitors. I would not be the least bit surprised if they are bad, though.

    I think either I am misunderstanding a very fundamental lesson of how capacitors work, or I need to re-think how I am measuring leakage in general.

  • #2
    No trouble.

    This is a R.C time constant discharge circuit.
    Where R is the input resistance of your VTM (~10MOhm supposed)
    C13 the capacitor (1µF)
    That is : R.C = 10M.1µ = 10secondes.
    If initially the capacitor is discharged you have to wait a long time (>10s) before finding it at full charge and Zéro Volt Out (C Terminal).

    If you hurry, ground the C terminal a short time to charge C13. And renew your measurement.
    So if C13 is good you will measure zero....immediatly.

    In circuit this problem does not exist because terminal C is loaded by the resistor termination of the delay line.



    • #3

      Yes, my meter is a 10Mohm impedance.

      I had a feeling (a very strong feeling) this was all normal, but I wanted to confirm.

      There must have been a slip-up in my testing yesterday, because I thought one preamp did this and one did not, but turns out they both do.

      The other times where I test a capacitor for leakage in this manner is with small value coupling capacitors (in the .01 uf range, more or less.) For these, the same equation results in 0.1 seconds, which explains why no DC is observed, as it goes away too quickly.

      I knew it was something stupid and simple. Basic math.


      • #4
        Turns out one of these preamps 1uf caps was causing an intermittent static / crackle on the VIB LINE output. Replaced that, and crackle went away, after repeat testing, can't hear it.

        Thank you JP for the sanity check.