Homemade Manual Tuning Cable TV Channel Adder

At present, there are quite a lot of cable TV programs launched by various cable TV stations, but because the maximum cutoff frequency of the high-frequency head of the old color TV is too low, it is not possible to receive all cable TV programs. Replacing the high-frequency head can receive all TV programs, but the high-frequency heads of some old color TVs are difficult to exchange with the most widely used TDQ-3 full-channel high-frequency heads. Therefore, a manual tuning cable TV amplifier is made to solve this problem. It is also very suitable for black and white TVs and monitors without RF input, only audio and video input, to watch all programs transmitted by cable TV.

The specific circuit block diagram is shown in the attached figure.

1. Working Principle

The RF signal sent by the cable TV enters the TDQ-3 high-frequency head, and after the selection of the band switch K and the manual tuning of the tuning potentiometer W, a certain channel program is obtained, and then converted into a 38MHz intermediate frequency and input into the LA7533 intermediate amplifier board. After being processed by the intermediate amplifier board, the audio and video signals of the channel program are output. One of the audio and video signals is directly sent to the audio and video output sockets for the monitor to watch.

The other audio and video signals are sent to the RF modulator for RF modulation, and a full television signal with a fixed RF (carrier) frequency is output for reception by the RF input terminal of the TV.

2. Component Selection

The high-frequency head of the amplifier is TDQ-3 full-channel high-frequency head, and its frequency range is 87-860MHz. It is better to use a more precise multi-turn wirewound potentiometer for tuning potentiometer W, which is conducive to tuning accuracy. The band conversion switch K uses a knob switch with three output terminals (it can be replaced by a Hongyun fan speed control switch). The choice of the intermediate amplifier board can be any board as long as it has audio and video output.

Here we use the LA7533 amplifier board made in Thailand, which has a good effect. You can choose the RF modulator yourself. I use the RF modulator that comes with the BBK VCD. Its power supply voltage is 5V and the output frequency is 200.8MHz, which means 10 channels are output. All the above power supplies use regulated power supplies to ensure the viewing effect. In addition, if the cable TV signal is too strong or too weak, you can connect a resistor of appropriate resistance in series on each foot of AFC and AGC to achieve the best image and sound quality.