Change the frequency of the campus radio

The frequency change of campus radio in some universities is a headache for many teachers and students. The original 72MHz is changed to 52.5MHz. The radios bought by teachers and students cannot receive the 52.5MHz frequency. For those who do not know the technology of radio frequency change, they have to spend money to change a radio or find someone to change it. Now we introduce the method of frequency change.

There are now 6 brands of campus radios. Looking at the components used inside, they are all assembled with the single-chip TDA7021 integrated circuit, so changing the frequency is very simple.

1. TDA7021 external circuit

The actual circuit diagram of TDA7021T is shown in the figure.

In the figure, earphones are used instead of antennas. They send high-frequency signals to the integrated circuit through C12. The variable circuit composed of C4 and L1 selects the radio signal frequency. BG is used as a low-frequency amplifier to have sufficient output power. L2 and L3 are high-frequency chokes, which not only prevent the high-frequency signal from passing through, but also allow the audio signal to reach the earphones smoothly.

Component selection: BG can use C9014, β is about 120, L2, L3 can be wound 30 turns on the waste medium-circle coil I-shaped magnetic core with φ0.51mm enameled wire. If you find it troublesome, you can also buy commercially available 1μH color-coded inductors. C3, C6, C9 are monolithic capacitors. C14, C15 are electrolytic capacitors, and the others are ceramic capacitors. The oscillation coil can be wound 5 turns on a 5mm diameter round rod with φ0.51mm enameled wire, and then removed to form a hollow coil, with an inductance of about 0.08μH. E1, E2 are 32Ω ordinary stereo headphones, and the power supply is 2 No. 5 batteries installed in the battery box.

2. Frequency Change

After circuit analysis and circuit assembly, we can see that C4, C5 and L1 determine the choice of receiving frequency. Campus radio can also choose FM and campus broadcast, that is, connect a switch at the input of pin 4. Select the inductor coil respectively. When disassembling the machine, you can see that there are two inductors with different numbers of turns connected to the switch, which are responsible for the frequency reception of FM and campus broadcast respectively. This is what we want to change.

Method 1: Forced modification (single-link adjustable capacitor, the lowest receiving frequency is 70MHz): Use φ0.5mm～φ0.65mm enameled wire to wind 13-14 turns on a 5mm diameter round rod. Remove the paint on the leads at both ends of the coil with a knife, disassemble the radio and solder the coil with more turns near the switch with a soldering iron, and then solder the inductor just made on it. If your radio has a frequency display, take a look, the frequency display can be adjusted down to below 50MHz.

Method 2: Easy modification (dual adjustable capacitor, the lowest receiving frequency is 58MHz): There are 2 types of radios connected with a dual capacitor, which is very large. There are 4 round holes on the back, 2 of which have no copper sheets, and the other 2 can be adjusted with a flat screwdriver. Note that the first hole with a copper sheet should be rotated counterclockwise with a flat screwdriver to move the copper sheet on the left (which can be seen) to the lower right by 30 degrees, that is, rotate 180 degrees counterclockwise, so that the receiving frequency drops from 58.MHz to about 51.0MHz, including 52.5MHz, right?