Lost of power in one channel of a STA 322 - diyAudio
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Old 18th August 2013, 10:55 PM   #1
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Join Date: Oct 2012
Default Lost of power in one channel of a STA 322

I am working in a power stage IMG, an STA 322 with a power stage based in MJ15024s and MJ15025s. A weird thing is going on in this unit. The right channel develops less power than the left one having both of them the voltage pot at the same mark. For example for half trip of the volume pot I am getting 850mV on the left one and 233mv on the right one. The signal is not distort or any fault LED on.

I checked the power stage and it seems that the left channel was repaired and they replace a MJ15025 with an MJ21193 and a MJ15024 with a MJ21194. I have checked the data sheets and they are pretty similar and do not explian why the right channel develops less power. I have not found the schematic for this unit so the work is becoming tedious.
Any suggestion will be appreciate and I will let you know if finally the problem is solved.
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Old 29th August 2013, 11:35 AM   #2
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Join Date: Oct 2012
Update. I restored both channels of the power amplifier putting back the transistors indicated by the manufacturer. Adjust the limitter circuit and finally obtain the same power in both outputs.

Now the next weird thing using the power stage in stereo mode:

Input to both channels with no load in the outputs - Signals are amplified correctly

Input to both channels with load in the outputs - If only one channel volume is activated (the other volume knob to 0) the signal is amplified but it is amplified also in the other channel with a 180 degree phaseshift.

Any ideas why this might be happening? I suspect the limitter circuit pots have being manipulated and might be producing this.
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