Hello. I have an FBT MXX 4 active speaker and although it works, but it has a very strong hissing noise in both speakers, bass and treble. With no input signal, if I turn the volume to minimum I have absolutely no noise, but if I increase the volume potentiometer the noise is heard. I checked all the electrolytes and they are ok, cleaned the potentiometers, the voltages are ok. If you can help me with an advice.
Link to the schematic https://fastupload.io/7f7d2c326ab80f3d
Link to the schematic https://fastupload.io/7f7d2c326ab80f3d
These are the voltages measured on the integrated.
The supply voltages are +16.8V and -16.7V
U2 = pin 1,2,3,5 and 6 =0V, and pin 7 = -0.27V
U4 = pin 1,2,3 = -0,4V, pin 5,6 7 = 0V
U11 =pin 1,2,3,5,6 si 7 = 0 V
U1 =pin 1 =15,3V , pin 2 ,3 = 0,4V , pin 5,6 =0V, pin 7 = -0,5V
I replace U1 and the voltage is now
U1 =pin 1 = -0,4 V , pin 2 ,3 = - 1,05V , pin 5,6 =0V, pin 7 = -0,4V
The problem is with the preamplification, up to the volume potentiometer.
If I put the oscilloscope, it enters the limitation as if the input signal is too high.
https://i.postimg.cc/cHPXZ3Vy/Preamp.jpg
The supply voltages are +16.8V and -16.7V
U2 = pin 1,2,3,5 and 6 =0V, and pin 7 = -0.27V
U4 = pin 1,2,3 = -0,4V, pin 5,6 7 = 0V
U11 =pin 1,2,3,5,6 si 7 = 0 V
U1 =pin 1 =15,3V , pin 2 ,3 = 0,4V , pin 5,6 =0V, pin 7 = -0,5V
I replace U1 and the voltage is now
U1 =pin 1 = -0,4 V , pin 2 ,3 = - 1,05V , pin 5,6 =0V, pin 7 = -0,4V
The problem is with the preamplification, up to the volume potentiometer.
If I put the oscilloscope, it enters the limitation as if the input signal is too high.
https://i.postimg.cc/cHPXZ3Vy/Preamp.jpg