Sony TA-F6B DC Bias problem

[Mooly]

Hmm

C303 is just a rail decoupler (across the neg 8 volt rail to the chip). Most likely residual charge on the rails affecting the meter reading... you have to measure the voltage on pin 2 to check it alters OK and even saying that assumes it does change in a reasonably linear fashion.

 

[nyogo]

Yes, you're right of course. I wanted so badly to narrow the issue to that cap, that I forgot about that At least almost every cap is new.
Anyway, powered up and measured.
Right channel: pin 2 voltage changes from 0 to -6.7V(-8.8V rail), on pin 8 and 5 the voltage not really changing, stays around 20V.
Left channel: pin 2 voltage changes from 0 to -9.3V(-9.4V rail, bit higher, idk why), on pin 8 and 5 the voltage stays around 21.4V.
I'm not familiar with this CX171 and it would be weird if they both bad, but still the amp board is working and without measurements you cannot tell that something is not ok.

 

[Mooly]

Left channel: pin 2 voltage changes from 0 to -9.3V(-9.4V rail, bit higher, idk why),

Probably a normal tolerance on the shunt regulator in the chip (which may be little more than a Zener). It would be too weird for both channels to have the same fault if it were the chip.

Is the 15 ohm (R308) OK that goes to pin 7?

I really don't know what to suggest given it actually all works and sounds OK.

on pin 8 and 5 the voltage stays around 21.4V.

I found this (sorry its a small image) that shows a measured 9v on the chip and also shows what looks like an NPN/PNP output in the chip.

 

[nyogo]

Those 15 ohm resistors are ok. Today I managed to measure only around the ICs, every component looks ok.
I have no idea either. The last thing is to measure everything, but it will take some time.
Beside that it works and sounds nice I don't want to leave it like this: cannot set the dc offset to absolute 0V, slowly drifting on left channel(the one with higher rails and outputs) and the bias is fix 1.6-2 mV on both channel.

 

[Mooly]

I'm not sure how close to zero the offset should hold but remember we used to reckon historically that -/+100 mv was actually acceptable. So a few millivolts drift is no issue.

If you have a low distortion and low level test tone (test CD or MP3 files etc) then it might be worth playing that at very low volume and seeing if it sounds absolutely pure. Turn the preset and see if you hear any difference in the quality of the tone. There should be no audible harshness at all which signifies crossover distortion. Use something like 1 or 2kHz. Listen at a very low level via speakers. Try one channel at a time.

The 'conductivity' between pins 8 and 5 are what sets the conduction of the output stage. The two Zener's hold the base (and so emitter) of the first drivers at a constant voltage set by the Zener.

 

[nyogo]

Thanks. Later today I’ll test these, but I’m afraid it developed a new issue. Yesterday I was fiddling around with the bias/offset presets, measuring voltages and at one point on the offset preset the output protection relay clicked. I set back the offset preset a little and it switched back again. So clearly something happened, because never did it before.
That was the point where I gave up the work for that day 😄

 

[Mooly]

Sounds like fun

As long as you had no load attached it should be fairly safe from damaging output transistors. Even an offset of many volts will not cause damage in itself.