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Feedback in the percussion circuit of my A100 please help.

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  • Feedback in the percussion circuit of my A100 please help.

    I am still getting feedback when I turn on the percussion. (A100) I am at a loss and have going at this thing for days. Would someone take pity on me and give me some ideas or even walk me through the troubleshooting?? (on or offlist) I would be so grateful! Here is the sound, perhaps it will help for you to hear what im dealing with.


    Here are the facts.</p>

    - No feedback if I disconnect the H or J terminal
    - No feedback if I disconnect all terminals
    - Still feedback if I disconnect the P terminal
    - No feedback if I remove V5
    - only feedback when the percussion tab is fliped on.</p>

    Here is what I have done and still have feedback

    - Replaced all tubes one at a time
    - Replaced the entire percussion switch
    - Double and triple checked the wires going from the preamp to the percussion switch and to resistor panel for a short
    - Dissconected the drawbars, and preset wires so that the only thing connected to the manual are the tonewheel wires and the wire going to the matching transformer



    - Could there be a problem in the preamp that would cause the feedback to occur only when the percussion tab is turned on, if so where do I start?
    - Could this problem be caused by a problem in the Matching Transformer?
    - Could this be occuring because I DON’T have the drawbar wires and preset keys wired to the preset key panel?


    Thanks A Ton


  • #2
    Re: Feedback in the percussion circuit of my A100 please help.

    You really need a scope to troubleshoot... </P>

    Looking at the circuit IT APPEARS that V5 is probably a regenerative filter in the form of a starved oscilator.</P>

    Within that circuit, C29 might be suspect. it ties to the grid pin 6 to pin 7of V5 and according to the diagram is a 100 pf cap.</P>

    It appears that C31 is the timing cap for the decay of the percussion. There are SEVERAL high resistance (megohm) resistors in the circuit. It is possible that one of these has opened causing a sustain of the percussion...</P>

    The frequency of this feedback MIGHT give a clue, because the starved oscilator looks like it is supposed to be at 500Hz which is just a little above B4.</P>

    C30 could also lead to the oscilator being way too high and staying on. It is a 25/25 electrolytic which may have dried out. It connects pin 7 of V5 to ground.</P>

    A short of one of the contacts in the percussion on off or the third/second switch could cause problem as well.</P>

    These circuits are high impedance and leakage due to moisture, etc. can raise havoc.</P>

    Please note that I am trying to interpret all this from the schematic diagramat:</P>