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
http://www.eserviceinfo.com/download...lab_8000A.html

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
http://home.graffiti.net/diverse:graffiti.net/

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
http://home.graffiti.net/diverse:graffiti.net/

On Tue, 7 Jul 2009 12:47:26 +0100, "N_Cook" wrote:

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
http://www.eserviceinfo.com/download...lab_8000A.html

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

--
Geo

Geo wrote in message
...
On Tue, 7 Jul 2009 12:47:26 +0100, "N_Cook" wrote:

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
http://www.eserviceinfo.com/download...lab_8000A.html

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

--
Geo

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
http://home.graffiti.net/diverse:graffiti.net/

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
http://home.graffiti.net/diverse:graffiti.net/

Geo wrote in message
...
On Tue, 7 Jul 2009 12:47:26 +0100, "N_Cook" wrote:

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
http://www.eserviceinfo.com/download...lab_8000A.html

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

--
Geo


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
http://home.graffiti.net/diverse:graffiti.net/

On Thu, 9 Jul 2009 08:10:54 +0100, "N_Cook" wrote:


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.

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

--
Geo