"Tom" bravely wrote to "All" (03 Nov 04 22:53:27)
--- on the heady topic of " Technics SE-9060 DC on input potentiometer"

To From: "Tom"
[,,,]
If the amplifier is truly DC from input to output then the problem could
[,,,]
To The amplifier is truly DC.
To There is a switch just after the potentiometer. It says "DC or Low
To cut". If the switch is in "Low cut" the signal goes from the
To potentiometer through a capacitor to the input of the amplifier. If the
To switch is in DC, the signal goes directly from the potentiometer to the
To amplifier. If the switch is in "Low cut" the amplifier goes in
To protection mode, because then the potentiometer can no longer influence
To on the DC level. I have now replaced all 5 electrolytic capacitors in
To that channel, except for those in the power supply just after the
To bridge. They are a bit special, and their physical form needs to match
To for an easy replacement. I have checked some additional transistors in
To the middle section of the amplifier they were also fine.
To Since I have DC out on the input, I expect the error to be in the very
To first stage of the amplifier. Don't know if an error further back, can
To cause that.
To /Tom

The only other thing that could cause an offset is if an output
transistor is very leaky though not totally shorted. The feedback loop
would then inject the offset current into the stage after the pot. The
point here is that it is worse with the pot near the middle but gets
better at each end. Either it is a stage offset or the output pulling.
The low cut switch really confirms the serious offset.

Try to trace the feedback loop and see if anything else connects to
it. The stage after the pot is probably differential with one side to
the pot's wiper. The other side likely has a balancing adjustment or a
fixed resistor network. Perhaps even a pot across both emitters or
sources (if JFETS). Something is upsetting the comparison voltage and
causing a large error input. Maybe even the pot wiper side device is
simply leaky. I may have the schematics of a similar Technics model but
it is on a shelf behind some speaker bins... heavy buggers!

BTW a truly DC amp is not very kind to speakers because of the danger of
damage from infrasonic signals. The speakers will pump in and out with
every wobble on an LP and may even pop out if the tonearm is
accidentally dropped. Thumps and wind noise on mics are murder too. Then
there is aliasing from digital signals which might get in. Erk!

The point of the DC amp is to have zero phase shift at bass frequencies.
In fact true DC coupling is unecessary because an amp with capacitors in
the chain can be made to have zero phase shift by adding a simple
network to correct the phase error at low audio frequencies. However,
some amps with a capacitor in the chain have a problem with damping at
low frequencies but this is rare, for example Dynaco's would motorboat.

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.