Audiolab 8000A

Hate the things, pa schematic out there but not prea it would seem. Preamp
problem t'would seem. At switch on a pair of 47uF electros fed via 120K from
27V rail give increasing voltage that at about 8V , after 4 seconds, ends up
putting a 3 to 5V pulse on the preamp outputs , straight into the power amp,
both channels. This timed hold-off would seem deliberate, presumably muting
off, but the DC surge?.
Anyway this of course upsets the amp which then has to re-settle, otherwise
seetles in first second , but now outside its own time-off period and delays
relay turn on ,or worse , makes the relay chatter , via its DC loudspeaker
protection cct activating.
What to look for that the preamps , both channels oddly, come out of mute
with a surge ?

 

eserviceinfo has the non-integrated C suffix, same sort of age, perhaps near
enough the same to get some hints,

 

Beggars and choosers comes to mind, hardly worth uploading let alone
downloading. Loads of long tail pairs , about the sum of information
content.

 

Does the unit use muting transisors or FET's? These can get leaky, and are
often found right at the output.

Mark Z.

 

I will look into that. The eservice schema, cannot even see whether FETs
anywhere in the C version
As 2 separate "timers" I will add another 120K to one and see if the effect
is the same 2 sec in and 4 sec in.
As it stands they time in near enough together but only one may be at fault
and cross coupled into the other channel, otherwise 2 separate but identical
faults seems iffy. It may be a hf oscillation problem .
When I was scoping the amp output there was short bursts of 150KHz or so at
this 4 second point. Did not think of scoping the preamp then to see if the
hf was there also or just the DC pulse.

 

Yes, the timing certainly suggests this. At the end of the muting cycle, a
large DC bias is applied to the base or gate. With the drain or collector
sitting across the output to ground, this leaky transistor will then pass DC
to the output. It has been a relatively common failure on Pioneer and Yamaha
receivers, and I've seen it on Hafler preamps.

Mark Z.

 

not had time to power it up again , but no FETs in there BC550 and BC560
only.

Is there any recognised circuit supplemental to allow for the replacement
transistors going leaky over time?
I suppose 20 years before metalisation creep/migration re-occurs in the
replacement, is fair use.

 

DC pulse in the prea , not oscillation. Both channels the same , by
changing one to 2 sec timing in.
Using the eservice 8000C preamp schematic (bottom right splodge) as just
litterally a pattern and then following traces and filling in component
values, the stage around the 4 sec timer and nearby circuitry agrees with
the prea section of this 8000A.
Other than the pulse starting from a Q13 with its base at a zener 4.7V at
switch on and after 4 seconds turning on , will have to see why a jump and
not smooth , but design would seem to be for a step change.

 

As I cannot find anything wrong as far as broken wrong, I will assume this
is as designed. I cannot see why there is a low 4K7 dropper from 27V to the
4.7V zener that basically causes this 5V DC pulse, when only supplying 80uA
at most into the base of a TO92 transistor, Q13. I will try upping the
dropper and adding a C to give a time constant of 2 seconds and change the
main hold off time constant from 4 sec to 2 sec also but keeping 13V bias
point, both channels. A few strange noises in the first couple of seconds ,
if that and only if the phones are connected, is surely preferable to 5V DC
pulses fed , delayed, into the PA.
So both will rise approximately together and so no 5V surge is possible.

 

But DC is arriving at the PA 5V each ch for 0.1s or so every time it is
switched on . I cannot make matters worse than that. If you don't have
phones connected you never hear the clicks as it is over by the time the
relay clicks over for the speaker (normally), so perhaps everyone thinks it
is just a matter of the power rails still balancing up rather than
"designed" in.
Agreed on the zener. I see nothing wrong with putting 47K rather than 4K7
leaving plenty of headroom for functioning reference voltage and base feed.
I'm wondering if the designer, whoever it was, had an aversion to putting
electrolytic capacitors over zeners. Whenever I come across these Audiolabs
now I put a 22uF over each of the 4 , 12V zeners in the pa, as without them
amp is liable to hf oscillation (then self-immolation) with large dV/dt
spikes fed in the PA input eg fridge click.

 

Now a volt or so on output within first second, each channel, and PA can now
settle within its design alloted time span.