QRV-08 Building and test

[peranders]

Check once more the voltages across T26 and also R18. Are you really sure it is a BC860?

[galouche]

Yes, I've verified here's on a schematic ...
it's a 860 and the other a 850

[peranders]

Do you have adjusted the DC-servo and also the input bias? How much output offset do you have? The whole amp should be fairly symmetrical in voltages. The transistors aren't active if Vbe is less than 500 mV and not more than 1 volt or so.

[galouche]

to adjust the input bias, I have to solder the jumpers on the back side of the pcb ? adjust the DC servo ? how ?

  I think I have a prob with a condensator because testing the input bias, I've seen 3.5 V going in 2 secondes to 1 V ( violtage shutting down in my tester ) and I every time i Test it the voltage is 3.5 V

  On the other channel the imput bias is 0.7 mV

[peranders]

The voltage across R1 should be 0 mV. This can achieved by adjusting the P1 with either positive or negative current. If you'll adjust this the DC-servo won't have to be working overtime.

[galouche]

THe voltage across the left channel is 0.0004 V but on the right channel it's 0.170 V. When I turn the pot, no change at all ( I've not soldered jumpers behind ).

[galouche]

It's ok now ... two transistors were responsable .... I'll study tomrrow why acting the input bias potentiometers don't change the bias ... for the mioment I listen to music  :-D

[galouche]

I've listen a lot of music ... as if I was discovering them ...  :-)

   The out of the super reg was on the left channel 13.9001 V and on 13.9005 V so it's ok but the out on the right channel was 13.856 V and 13.934 V so I've put a 100 k resistor in // with R55 and now it's 13.938 V and 13.934 V so it's ok too.

[peranders]

Trimming the supply voltage is very unnecessary in a technical point of view. 13.90, 13.93 and 13.85 is extremely good for an untrimmed power supply! The main thing is that the voltage is stable. I think you should concetrate your efforts on adjusting the input bias current and check all working points, verify the symmetry in the design.

[galouche]

I can't adjust the right input bias for the right channel ( I can for for the left channel) .... when I turn the pot, it's immediatly compensed by the opa134 ... I must have a prob with a condensator voltage I see in the right input ( begining from 3.5 V and shuting down in my tester slowly ) ... but for the moment I can't find it ...

[peranders]

You should measure over R1 = you inject current and nothing goes through R1, only AC signal.

You should also check that the opamp not is saturated. The output of the opamp (pin 6) should be max +- 10 volts.

[galouche]

I can easily adjust the input bias testing after C1 and it's ok but testing before C1 ( on the wago for exemple ) I can turn the pot, it doesn't change anything, if I turn quickly, I can see on the oscillo that it grows but quickly go back tothe first value as if the opa was having this on reference and was adjusting before C1. On the left channel I don't have this problem, turning the pot adjust before and after C1 DC bias

[peranders]

Do you really measure over R1?

[galouche]

over R1 it's ok ( it's after C1 )

[peranders]

Do you really have 560 ohms at R2?