Audio Amplifier

After you have completed and checked the power supply section, the next stage to be assembled is the receiver audio amplifier. The demodulated receiver audio output is available at the pin 12 of the IC1 (CA7611). Resistance R17 with Capacitor C23, offer a 50 Ohms load to RF mixing products from the IC1. However a shorting link in place of resistance R17 can be used.

 

 

 

The capacitor C67, C68 and the resistance R52 acts as a passive low pass filter to filter out high audio frequencies and are used to reduce hissing noise from the receiver. The audio signal from the filter (point D) is coupled to the input of the audio amplifier IC2 (LM386 in version V1 of LM 380 in version V2) via the capacitor C 69 and volume control R53. The volume control R 53 is a 10 K Ohms Log. carbon potentiometer which is mounted on the front panel. The R 53 is connected to J10 with two miniature audio coaxial cables.

The output from the IC2 is connected to a 4 Ohms loudspeaker via capacitor C75 and the headphone jack. The connection for the headphone jack is wired in such a way so that when the headphone jack is inserted, the connection to the loudspeaker is cut off. Verify the connections with a Multimeter before applying power to the Audio amplifier. Be careful and do not short the output of the IC2 to ground by mistake in wiring of the phono jack.

The connection to the volume control is made in such a way so that the audio volume increases when the knob is rotated in clockwise direction. If it is other way round then just interchange the connections on the outermost terminals of the volume control potentiometer.

 

After checking and removing any solder bridges, short circuit or dry solders, insert the audio IC2 in the Base (be careful in inserting the IC in the base, check if by mistake you have not bend +any lead and Verify whether the pin 1 of IC2 is in the pin marked 1 on the PCB i.e. make sure that the IC is not inserted the other way round) and switch on the power supply. The Audio stage remains powered in both TX and RX mode, if every thing is correct, then touching the point D with a live soldering iron will produce humming sound from the loudspeaker. The volume of this humming sound can be varied 14th the volume control.

IF you do not hear anything then again check all the connections and components, their value, their polarity, all the solder points etc. and test again. If nothing is happening consult some friend who has experience in making electronic circuits work.

Only after the Audio stage is working properly proceed further.

 

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