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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 Fender guitar amp has developed a fault and I'm pretty good working
    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
    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. Guest

    Well, thanks for that vote of confidence ! ...but I *will* fix
    it. ..of that I am certain.
    I've successfully repaired many things 'remotely' with the help of
    kind folk like Sam Goldwasser.
    This is only an amplifier after all.
    Err...Ok..... Thanks.
  3. Arfa Daily

    Arfa Daily Guest

    Well, thanks for that vote of confidence ! ...but I *will* fix
    it. ..of that I am certain.
    I've successfully repaired many things 'remotely' with the help of
    kind folk like Sam Goldwasser.
    This is only an amplifier after all.
    Err...Ok..... Thanks.

    Sadly, I'm with Meat on this one. I fix these things for a living, and I had
    one of the same model doing exactly the same thing a few months back. I
    spent literally DAYS on it, and was no nearer when I had finished.
    Eventually, as a result of some complicated negotiation between the owner of
    the shop it came via, the Fender rep, and Fender themselves, their service
    department agreed to take a look at it (normally, they will only take stuff
    back that is in warranty, and this was out).

    Some weeks later, it came back, and was apparently fixed, although I didn't
    see it again myself to confirm this. We tried to find out what exactly the
    problem was, but all we could get out of them was a vague-ism along the
    lines of "it was a couple of resistors".

    It seemed to me that the dirty channel is designed with altogether too much
    gain in it, in the first place, made even worse when the 'More Drive' button
    is engaged. Personally, I think that the HRD is one of the worst designs
    that Fender have ever come up with, but others may think differently.

    I wish you luck with it, and if you happen to find out what it is, or
    luckily drop on the problem, please come back and tell us.

  4. Arfa Daily

    Arfa Daily Guest

    That is a pretty fair description of the problems that would normally be
    associated with an unstable amp, but assuming that this example is the same
    as the one of the same model that I had exhibiting *identical* symptoms,
    then a power supply filtering / rail decoupling issue such as a bad cap, is
    not what is causing it. It certainly wasn't on the one that came across my
    bench. The rails remained very nearly as clean when the problem was
    occuring, as when it wasn't. Bear in mind also, that the problem only
    affects the dirty channel. You can drive the clean channel as hard as you
    like, with any ancilliary control settings, without the amp exhibiting a
    problem, which 99% knocks out the power supply, as a cause. As I said in my
    earlier post, it eventually went back to Fender's own service department,
    but the only comment that they made was that it was "a couple of resistors",
    which probably means that they couldn't find the problem in any reasonable
    period of time either, so just jammed a replacement board in it, and came up
    with the 'couple of resistors' story ...

  5. neon


    Oct 21, 2006
    IF it is motorboating to me it means a decoupling capacitor has given up probably on a tube cathode. c1 i suspect. to isolate short from the beginning the input and see it goes away. then repeat going deeper into the amp. to isolate the stage . it coul'd be power supply cap one of them give up the gost with power off inspect to make sure you have no bulgy capsand with a meter check the ac on the dc component.
  6. Guest

    Well, I managed it - despite all the naysayers. It was one of the
    22uF caps - C33, C35 or C36. I replaced all three and the problem is
    gone !

    Many thanks to all who tried to help, and ner-ner-ner-ner-ner to all
    who doubted my abilities as a mere mortal. ;-)
  7. Jamie

    Jamie Guest

  8. Arfa Daily

    Arfa Daily Guest

    Likewise. It's written down now, so if I get another, I'll try those caps.
    It's got that 'feel' about it, that it might become a stock fault.

  9. Arfa Daily

    Arfa Daily Guest

    Additional note.

    I have had a look at the schematic now to check what those caps were, and I
    was surprised to find that they were the HT filter caps. Although I didn't
    actually change the ones in the example that I had with the same problem, I
    did give them exhaustive checks for ESR and value, and they checked out just
    fine. The belief that they were indeed OK, was borne out by the fact that
    the rails at 'x', 'y', and 'z' were all fundamentally clean, and became only
    very little worse, when the amp was unstable. It's also hard to see how a
    problem with filtering on these rails, which feed all the tube HT supplies,
    would affect only the dirty channel. Given the fact that these caps are
    relatively small in value in the first place, I guess that it's possible
    that only a small reduction in performance of one that filters the rails to
    the preamp tubes, might be enough to cause a stage to become unstable, when
    it's working harder under 'drive' (dirty) conditions as opposed to when it's
    switched to 'clean'. If this is indeed the case, then it's a pretty lousy
    bit of design work on Fender's part, that a stage could go unstable with so
    little provocation.

    Just as a matter of interest, did you manage to actually fault-find the
    problem to these caps, and if so how, or was it a case of gut instinct and
    shotgun replacement ? Always interesting to know how others arrive a
    solutions to odd problems.

  10. Jamie

    Jamie Guest

    Didn't ask for the happy ending ?"
  11. N_Cook

    N_Cook Guest

    Of course with this sort of problem it could be just fiddling inside
    disturbed something , even just wiring looms and the onferlying problem is
    still lurking there.
    Unless something definitely found wrong with one of those caps
  12. Arfa Daily

    Arfa Daily Guest

    Likewise, based on a similar 30+ years of repairing said equipment.

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