I have on loan temporarily an Onkyo A-9050 until my Electrocompaniet is fixed.
It has these tiny cheap rotating things inside, one for each channel, which change the values across large three-legged resistors and if I turn them all the way to one direction, they make the transformer buzz.
I already touched them and don't know the original intended position. Now I'm not sure if the really shrill sound is due to the adjustment or just characteristic to this amplifier.
Anyway, how would I find out the best position for these? This amplifier is a "three-stage inverted Darlington" circuit, so is it not so clear compared to other designs, what the best value is and where? Is it even necessarily the same as "bias trim" in this case?
Originally I touched them to find out if they'd change the measured quiescent current at the terminals. They didn't affect it.
It has these tiny cheap rotating things inside, one for each channel, which change the values across large three-legged resistors and if I turn them all the way to one direction, they make the transformer buzz.
I already touched them and don't know the original intended position. Now I'm not sure if the really shrill sound is due to the adjustment or just characteristic to this amplifier.
Anyway, how would I find out the best position for these? This amplifier is a "three-stage inverted Darlington" circuit, so is it not so clear compared to other designs, what the best value is and where? Is it even necessarily the same as "bias trim" in this case?
Originally I touched them to find out if they'd change the measured quiescent current at the terminals. They didn't affect it.
Oh dear. Cheap rotating these, perhaps they need tightening down... (hint) DON'T 🙂
The transformers buzzes because are causing the output stage to pass a massive current... and you are likely a moment away from catastrophic failure in that state.
What terminals? Current flows 'in' things, Do you mean DC offset at the speaker terminals? That is a totally different thing.
Turn the presets to the end that doesn't make the transformer buzz (which is minimum current) and then either follow the procedure in the service manual or just play it safe and set for about 20 millivolts (that's millivolts) as measured across the two outer legs of those three legged resistors. Disconnect the speakers while setting this. Do the same for both channels.
20 millivolts is 0.02 volts so a very small amount.
