If your audio playback has diminished in volume or fidelity, there might be a problem with your Harman Kardon car amplifier.
Troubleshoot your system. Turn the key half way so that the ignition is engaged but the engine is not running. Send a sound source to the amplifier, either by playing a CD or turning on the radio. Gradually increase the volume on the stereo's dashboard control panel. If there is no sound coming out, it is likely that the problem is in the power section of the amplifier. If sound is coming out, but it is poor quality or intermittently dipping in output, there is likely a problem in the output stage of the amplifier.
Turn off the ignition. Remove the amplifier and unscrew the chassis. Unscrew the lugs that hold the amp in place and carefully set it down in the boot. Do not disconnect the wires. Unscrew the top of the amplifier chassis. Models of the Mercedes E Class in 2011 are fitted with a multichannel 450 watt digital signal processing (DSP) amplifier. This amp is connected to the car's central electrical system.
Visually inspect the wiring and circuit board. Look out for loose wires, blown fuses and blown transistors. Blown fuses and transistors will have a brown or black discolouration.
Replace blown fuses and transistors. The power supply fuse clips out of the fuse holder. Fuses in the circuit board clip out of the circuit board turret. Transistors are side mounted on the board and soldered to the base. Melt the solder connection fixing the board to the chassis, flip the board over and melt the solder connecting fixing the transistor to the board. Replace with a new, identical value transistor. Reattach the circuit board.
Test the resistors. The standard Harman Kardon amplifier fitted in 2011 Subaru cars carries 440 watts. The stock amp in the BMW X1 series in 2011 is a 340 watt DSP amplifier. Be extremely careful when testing the resistors. Attach your meter probe to the first resistor on the board after the power supply. Locate this by finding the power supply and tracing the circuit until you reach a resistor. Turn on the ignition and note the reading. There is an allowable 5 per cent variance in relation to the power rating listed in the User's Manual. Any reading in this variance is fine. Any reading outside of this variance indicates a faulty resistor. A zero reading indicates a blown resistor.
Disconnect the circuit board and flip it over. Remove the faulty transistor and replace it with one of the same value.
Clean the inside of the amplifier with a plastic-safe contact cleaner and lubricant.
These amplifiers are extremely powerful. If you are unsure what to do, consult an electrician. Attempting repairs of your car audio system might invalidate the warranty.