Homemade simple video signal RF modulator

Recently, the video signal RF modulator in the video recorder was damaged, and it was difficult to buy spare parts. As a result, I could not watch the video programs. For this reason, I made a simple video signal RF modulator, which worked well. The modulator has a simple principle, is easy to make, and has no special requirements for components. It is very suitable for amateurs to make.

The circuit of the RF modulator is shown in Figure 1. R3, C2, R1, R2, R4, and BG1 form an emitter follower. The video signal is amplified appropriately. C1 is a coupling capacitor, R5, L2, L3, L4, C4, C6, and R6 form a π-type low-pass filter, and BG2, L1, and C7 form an image carrier oscillation circuit, whose frequency is determined by the selected TV reception channel. This machine uses the first channel of the VHF band. C1 and C9 are resonant capacitors. C9 has a very small capacity and is twisted with a 2-3cm long single-strand plastic insulated wire. By adjusting its twisting degree, the coupling degree of the signal can be changed. C20 is a high-frequency filter capacitor, L1 is an oscillation coil, and D1 is an isolation diode to improve the stability of the carrier oscillation frequency. The video signal input through C1 is filtered by a π-type low-pass filter and mixed with the carrier signal, and then sent to the TV antenna socket through capacitor C5 and a 75Ω feeder.

The accompanying audio signal of this machine is not adjusted at high frequency, but directly amplified. The sound is produced by the speaker. The circuit is shown in Figure 2. The audio amplifier uses the integrated block LA4112. The video signal and audio signal are obtained from the video and audio output jacks of the video recorder.

In this machine, transistor BG2 can be selected as 3DG56. BG1 can be selected as 3DG6, and β should be greater than 80. D1 uses 2CK9 with better performance. The speaker uses internal magnetic type. L1 is wound with 12 turns of enameled wire with a wire diameter of 0.51mm, and the inner diameter of the coil is 5cm. L2 and L3 are wound with 11 turns of enameled wire with a wire diameter of 0.51mm, and the inner diameter is 5cm.

After the installation is completed and carefully checked, the signal output cable can be led to the TV antenna socket, and the TV band can be adjusted to VHF channel 1. The power of the TV, modulator and VCR is turned on, and the video signal and audio signal output by the VCR are connected to the input socket of the modulator, and the image is adjusted by adjusting the TV channel knob.

When in use, the volume on the TV should be set to the lowest position. If there is no image signal during debugging, it may be that BG2 has not started to oscillate. If the image signal is very weak, first check whether the emitter voltage of BG1 is greater than 2V, and then make other checks; if the image is distorted in an S shape, it may be caused by the large resistance value of R7. As long as the sound of the accompanying sound is not distorted and the sound is loud, it will be fine.