Adcom gfa5500 noise right channel

KimSalabim

Member
2020-02-28 9:02 pm
After adjusting bias and dc offset i now have a high static noise on right channel.
The left channel is just fine dead silent, no noise. Meassuring 0,7 mV dc offset.
The right channel meassured 115mV and had no noise. Maybe a little bit if I put my ear close to speaker.

After I adjusted dc offset on right channel stable down to 4,8mV there is now high static noise in speaker.

I've got 2 gfa5500's and the other one is just fine. Dead silent no background noise at all.


Anybody know what or where the problem is located? ��
 
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KimSalabim

Member
2020-02-28 9:02 pm
I’ll make guess - is the adjustment pot dirty or defective? If you turn the adjusting screw, does the noise change? If so, try spraying the pot with contact cleaner and twirling the adjustment some.


Yes, the noise gettings stronger when adjusting offset as near as possible to recommended specs. Adjusting other way the noise getting lower and almost completly gone when offset way out (115mV).

Thanks for answer, I'll give it å try😎
 
I can only guess that by the time you have the offset nulled out, one half of the input pair is running at such a low current that its voltage noise is through the roof. You should check whether there isn't an open resistor that would cause a major imbalance in DC resistance (inverting compared to noninverting input). From the presence of an offset pot I infer that there isn't a capacitor in the feedback network's ground leg? (If there is, this could be leaky / shorted.) Otherwise you may be looking at a faulty transistor.
 
I can only guess that by the time you have the offset nulled out, one half of the input pair is running at such a low current that its voltage noise is through the roof. You should check whether there isn't an open resistor that would cause a major imbalance in DC resistance (inverting compared to noninverting input). From the presence of an offset pot I infer that there isn't a capacitor in the feedback network's ground leg? (If there is, this could be leaky / shorted.) Otherwise you may be looking at a faulty transistor.



Thank you. Think I possible found the problem. Transistor R51 meassured 4,7mV. The rest around 35-50mV ish. So I'll start here with changing r51.