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Valve amp fault - nasty buzz

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

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  1. Guest


    My HRD 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....

    And here's just the preamp section:

    Here's a Wav file of the noise it's making...
    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
    DRIVE 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.
  2. N_Cook

    N_Cook Guest

    buzz at mains frequency F , 2*F or 3*F ? or random
  3. N_Cook

    N_Cook Guest

    I would try touching a grounded 1nF, HV cap to various points until a change
    of oscillation frequency and then change whatever gives the gain in that
    part of the circuit.

    I only read down to wav file ref before, sound card in my other pc
  4. Arfa Daily

    Arfa Daily Guest

    Trust me - I tried all of that to no avail. The problem is in the preamp
    section, around the opamps somewhere, but no amount of additional decoupling
    or alteration in the gains of the relevant stages, made any appreciable
    difference to the problem. I checked every component in the area, both
    visually, and electrically. I did at one point find a resistor which was not
    the value that it stated on the schematics, but changing it to the correct
    (?) value, again made no difference to the problem, which I kind of expected
    would be the case anyway, as the amp had apparently worked correctly before
    developing this problem.

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