N
N_Cook
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/downloadsm/22385/Audiolab_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.
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/downloadsm/22385/Audiolab_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.
