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Valve Amplifier fault - nasty buzz.

Discussion in 'Electronic Repair' started by [email protected], Nov 15, 2008.

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    My Fender guitar amp has developed a fault and I'm pretty good with
    electronics, safety (discharging caps etc) and soldering, but I don't
    have the circuit knowledge to work out which components are faulty.
    Could someone here offer any help please ?

    The symptoms are that it's making a horrible buzz on the drive & more
    drive channels, as well as letting some sound through on the clean
    channel when the volume is at zero.

    Here's the full schematic.... amp_schematics/pdfs/Hot_Rod_Deluxe_Schematic.pdf

    And here's just the preamp section: HRDxpreamp.gif

    Here's a Wav file of the noise it's making... Amp_Buzz.wav
    Be careful when listening to it, it starts off very quietly then
    gradually gets loud - so don't turn up your speakers because you can't
    hear it very well when it starts !

    The increase in oscillation frequency is caused by me turning the
    knob from 12 down to 0. Turning the Bass knob from
    12 to 0 also has a similar effect. Adjusting any of the other
    controls does not cause a change in frequency - only in volume / tone
    of the buzz.

    This is happening *only* on the Drive channel, and gets nastier with
    the More Drive button pushed.

    The recording was done with nothing plugged into the amp.

    With the amp on the clean channel, you can hear a little output from
    the speaker with the Volume set to zero - which it never used
    to do. Apart from this anomaly, the amp behaves fine on the clean
    channel and I used it for practice this week.

    I've tried swapping positions of the preamp valves and also a
    different set of power amp valves - no difference. I've also tried
    resoldering R78 & R79, which apparently have a tendency to develop dry
    joints, and all the big electrolytic caps.

    I'm more than happy to give test point readings to anyone who can
    help... the schematic gives TP voltages so hopefully that might be of

    Anyone ? Many thanks in advance - I really can't afford to take this
    anywhere to be repaired.
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