On Sun, 09 Dec 2007 23:03:34 GMT, "Arfa Daily"
wrote:


"Steve" wrote in message
.. .
I'm having issues setting the bias voltage for the right channel. The
service manual says to set it to 2.5mVDC. I can set the left channel
with no problem, but the right channel drifts badly with warming
up/cooling down etc. The left channel drifts around 2.1-2.8mV whether
it is just warmed up from cold or just cooled down from hot.
If I warm up the amp completely, set the right channel to 2.5, turn it
of and let it get cold, turn it back on & let it warm up, it will be
1.1mV. If I then set it to 2.5, let it sit, it will be up around 9mV.
These aren't exact figures, it drifts pretty widely. It isn't a dirty
pot, the readings aren't jittery or erratic, they just drift with
temperature variations.

I'm not sure what class this amp is, it's definantely not D, it looks
like a standard analog transistor audio amp. What would a normal
cause be for this kind of drift?

Thanks for any help, I'll gladly get more specific definite readings
if they will help.

Thanks,
Steve

Poor thermal contact of the output transistors with the heatsink. Poor
thermal contact of the bias stabilizer transistor or diode with the
heatsink. An abnormally thermally sensitive bias transistor or diode in the
external part of the bias stabilization network. An abnormally thermally
sensitive output or driver transistor. An abnormally thermally sensitive
resistor. In fact, any amount of things. Shouldn't be that hard to find with
the tip of a soldering iron, and a can of freezer. A set of schematics would
be helpful, but not essential, with another channel for comparison. I would
think that the amp is probably a standard class B or AB type.

Arfa

Thanks, that's what I figured. You're probably right, I just need to
bite the bullet and start heating/cooling components. Luckily, I do
have a schematic, I was quite suprised that the service manual was
available and has parts list, board layout, & schematic.

Thanks,
Steve