Assembling an FM Radio Kit with CXA1619BS
In the initial part of this series, I had mentioned that IC CXA1619BS with 30 pins is being used in this Do It Yourself FM Radio Kit. Though there a few online videos about this FM board, I could not get detailed information of pin connections and hence this effort for better learning. Now I am taking a closer look at the circuit. We can identify the pin sequence by looking at small circular depression near the notch on one end of the integrated circuit. This particular version is for through the hole mounting. Pin near the marker is pin number 1, which is on the upper part of this image. Corresponding pin on the lower side is pin number 30. Position of the tuning or gang capacitor has been marked at the top of the image. This makes it easy identify the orientation of the image. Pin 15 and 16 have been marked on the opposite side. Other pins can be easily counted from these reference pin numbers.
In IC CXA1619BS, pins 1, 2 as well as 29 and 30 are connected to the ground. CXA1619BM which is a surface mount device, has only 28 pins and hence pins 1 and 28 are connected to the ground. Image of the reverse side of the board shows the pins 1, 2, 30 and 29 soldered together to the ground strip of the printed circuit board. Now it is easier to identify the pin connections on both sides of the board as we have a reference point. Pin configuration can be verified with reference to the datasheet from the manufacturer. The red power supply wire is connected to pin 27 which is the Vcc of the integrated circuit. This connection is seen well in the smaller image.
White wire on this FM board goes to pin 5. Pin 5 is marked as VOL in the data sheet. Application circuit in the data sheet of CXA1619BS, tells us to connect pin 5 to central pin of the volume control potentiometer. Blue wire below the white wire in the image is connected to the ground strip of the PCB. That goes to a side pin of the volume control potentiometer.
Antenna wire is also a blue wire. It is connected to the coil inductor with 5 turns. There are two other coil inductors on the board with lesser number of turns. External antenna wire can be connected to this blue wire for better reception of FM Radio signals. Pin 14 is marked as FE GND in the data sheet. That is the functional earth ground. Pin 12 is marked as NC, meaning no connection, in the data sheet. It is also connected to the ground strip on the FM board.
Pin 19 is also shown as NC in the data sheet and connected to the ground strip. Pin 21 is IF ground. Pin 28 marked as AF OUT in the data sheet gives the audio output of the IC. It is connected to the yellow wire for the loudspeaker, through a 220 microfarad capacitor. Pin 28 is also connected to the ground through a CEV 104 capacitor. Initial 10 stands for first two digits. 4 stands for the zeros after that. That is a 100,000 picofarad or 0.1 microfarad capacitor which shunts the radiofrequency components coming to the AF output pin to ground. This prevents RF from reaching the loudspeaker. The other connection to the loudspeaker is the nearby green wire. It is also connected to the ground stip of the PCB. I took several hours of effort to figure out these connections. I had to repeatedly look at both sides of the FM board, enlarged views and the data sheets. Stay tuned for further updates.