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Audiolab 8000A amp oscillation

Discussion in 'Electronic Repair' started by N_Cook, Jul 7, 2009.

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  1. N_Cook

    N_Cook Guest

    Anyone familiar with the tendency of thse amps to ultrasonic oscillation. I
    gave up on another one, years ago, because of unpredictable catastrophic
    oscillation probably initiated by mains spikes.
    Later form of the schematic available as a 20K file here

    The one I have here is the earliest manifestation than that schematic or
    the one I failed to cure. This one had to replace about 12 components (half
    of them burnt out) on one channel, other channel is probably good on this
    one. Previous one was a load of burnt stuff in one channel also.

    Removing the +/- rails from the bad channel and powering at about 1/3 of
    normal , no load, then there is high level oscillation and high current draw
    for a few seconds and then it disappears leaving working amp with a about
    50mA quiescent draw.

    Disconnecting that side and powering the ex-burnt channel the base voltages
    of all the transistors now agree (mirror version) near enough with the good
    channel. No oscillation with this channel but putting a DVM probe on the
    base of the BC546B at the bottom of the schematic (with paired 1N4148 to
    its emitter) will start the oscillation. Component values here are about a
    factor of 2 different to that schematic,( 3.3V zener instead of 2x 4148 etc)
    above but perhaps functionally much the same.
    Touching the good channel one, after it has stabilised will induce the
    oscillation also.

    Hopefully just a function of running at reduced rail voltages . Other than
    temporary adding some 10 ohm droppers in the 4 DC power rails for output
    device protection any other ideas for ramping up to full mains power and
    more importantly long term mod to reduce this propensity to oscillate ?

    Reason for failure is not known , could have been piece of metal unglued
    and adrift inside, but this could have occured while it was stored in a
    loft. When I got it the metal happened to be in a safe spot of the good
    channel. But it could have been due to 8000A oscillation, no abuse when it
    was working.
  2. N_Cook

    N_Cook Guest

    Running up to full mains, the oscillation goes, I've not tried deliberately
    inducing oscillation despite temporary droppers in place. Both amps working
    separately or normal but there is imbalance and crossover in the preamp
    somehow that now needs looking at
  3. N_Cook

    N_Cook Guest

    Yes thanks for that, I'd not noticed. 3x4k7 suggests quite a bit of current
    too. As it stands there are no electrolytics at all in the power amp, the
    100nF's at the 22 ohm main +/- rail droppers looks inadequate also.
  4. N_Cook

    N_Cook Guest

    Any thoughts on this as a failure mechanism.?
    I've had to replace the +ve rail 10,000uF as it had obviously been leaking ,
    probably for some time. I assume in those circumstances, in use, hum ac
    increases and the +DC rail decreases. I know from powering at +/- 15V the
    amp oscillates. Perhaps -44V/+20V or so, it also oscillates, hence
    catastrophic failure, but would one expect one channel oscillating to set
    the other one off too. ?

    I did not think the loose metal plate was the cause, as not the slightest
    "spot weld" mark or smoke staining from a nearby immolation.

    I will add 47uF over each of the 4x 12V zeners.

    Another noticeable thing about this amp being an early model. The pa power
    rails are delivered by mains cable and I thought the green must be 0V. Then
    I started hunting for the output lines to the relay and it is the green
    The wires to the speaker terminals is also mains cable
  5. N_Cook

    N_Cook Guest

    Electros over the 12V zeners looks like a generic cure, decades late, for
    all audiolab ultrasionic oscillation. I returned to variac powering giving
    +/-15V rails and no startup oscillation or initiating from touching that
    transistor base.

    No known history, inherited from an someone's expired relative's loft
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