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Audiolab 8000A amp oscillation

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

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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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook
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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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Surprised there is no decoupling caps across the 12 v zeners.

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Geo
Reply to
Geo

I

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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 wires. The wires to the speaker terminals is also mains cable

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

What was the amp used for?

Was it left on 24/7 unattended?

Reply to
Meat Plow

I

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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

Very weird they were not in the original design - perhaps they hired a new graduate with no practical experience.

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Geo
Reply to
Geo

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