Lesson 018 , Transceiver Construction - AGC Amp
This lesson covers the AGC amp for the MP20. Another easy lesson dedicated to Mikey...
Connect R18 (3k3) resistor to the 8 Volt pad. Mount R19, a 10k pot to the free end of R18 and to ground. Use a small piece of sticky tape to hold the pot in place on the board. Connect 100nF (C22) cap from the junction of R18/R19 to ground. Use the schematic as a reference.
Prep U4, the LM358 dual op amp. Bend all leads horizontal except pin #4 which is at a 45 degree angle. Clip the small parts of the pins. Solder the chip in plase by the ground lead #4. Mount the IC as far up on the board as possible. See Pix. The XTAL filter will be mounted right below the AGC amp.
Connect a wire (resistor lead) from the junction of R18/19 to pin #3. Bend R16 (47k), trim and solder it upright across pins #1 and #2. See Pix. Solder a 10 k R22 to pin #1 and lay it over the top of the chip (see pix). Connect a 100 nF cap from pin#8 to ground.
Solder R20 (5m6) from pin #5 to ground. See pix. Solder the 470nF cap from pin#5 to ground as shown in pix. Connect D4, a 1N4148 diode from the free end of R22 to pin #5. The diode is under the 220 ohm resistor in picture #20. Observe the polarity of the diode. The banded end goes to pin #5. Connect a 220 ohm resistor (R25) from the junction of C33/R24 to pins #6 and #7. Use the resistor lead to bridge pins 6 & 7.
This completes the AGC amp.
END OF LESSON #018