Analysis and debugging method of multi-stage tuned amplifier

Relevant key device information applied in this article: 2g210 2g910 The figure below is the mid-amplifier circuit of a 4D4 TV, in which 201, 202, 203, and 204 are absorption loops, which interfere with the image and sound frequencies of adjacent high and low channels respectively. For absorption, the absorption frequencies are 38.5 MHz, 29 MHz, 30.5 MHz, and 39.5 MHz respectively. This circuit is composed of a three-stage single-loop tuning amplifier. The resonant frequencies of the first to third stages are: 34.5 MHz, 33 MHz, and 36 MHz respectively. Therefore, it is a three-stage staggered tuning amplifier circuit. This circuit uses the mismatch method. To prevent self-excitation, a damping resistor (2 kΩ, 3.3 kΩ, 22 kΩ) is connected in parallel to each stage of the tuning loop. In order to avoid AC coupling between stages through the internal resistance of the power supply, multiple filter circuits are used: Cφ1Rφ1 -Cφ4Rφ4 and Cφ5Lφ1, Cφ6Rφ20 adjustment methods: 1. Adjust the DC operating points of each level according to the regulations (omitted) 2. Adjust the frequency characteristics of the intermediate frequency amplifier circuit. Here is a simple multimeter debugging method, suitable for amateurs to repair TVs. This method uses the AGC voltage as an indication of the output voltage of the intermediate amplifier circuit. When adjusting, it is assumed that the tuner is working normally and can receive TV station signals. The complete steps are as follows: (1) Disconnect the ground terminal of the absorption loop (201, 202, 203, 204) first, and then connect them one by one after adjusting the absorption loop. , (2) Receive TV signals, monitor the AGC voltage with a multimeter, adjust the magnetic core of mid-week 2B1 to maximize the multimeter reading. At this time, 2B1 resonates at 34.5 MHz; then adjust mid-week 2B1 to maximize the multimeter reading, and then adjust Rotate the magnetic core 1-2 times in the direction of increasing inductance, so that the multimeter reading decreases slightly, but not very obviously. This will make 2B2 resonate around 33 MHz. Recently, adjust the mid-circle 2B3 to maximize the multimeter reading. Rotate the magnetic core 1-2 turns in the direction of decreasing inductance, so that the multimeter reading decreases slightly, but not significantly, so that the resonance of 2B3 is around 36. (3) Adjust the inductor 206 to make the multimeter read the maximum, and then readjust it according to the process of step (2). During the above debugging process, the problem of sound interference may occur, but you can ignore it for the time being. This phenomenon will be eliminated when the absorption circuit is adjusted. Three points should be paid attention to when using this method: 1. When adjusting, you must clearly understand the core position and inductance of the mid-circle The relationship between the increase and decrease of the inductance. For the 10A type mid-circuit, when the magnetic cap is close to the "king" magnetic core, the inductance increases. On the contrary, the inductance decreases. 2. If the signal is weak, it is appropriate to put the AGC in the middle of monitoring; If the signal is strong, it is appropriate to monitor the high-amplitude AGC. 3. The multimeter range should be appropriately selected or the voltage compensation method should be used to display the changing voltage. For example, when the AGC voltage changes from 0-0.2 volts, then switch to the 1 volt level for monitoring, so that the pointer swing range is more obvious. This method is not only suitable for adjusting the TV mid-amplifier, but can also be used to adjust other intermediate-frequency circuits. At this time, the mid-range output can be connected to a two-pole detection circuit, the detected DC voltage can be monitored with a multimeter, and the signal source can be used to input the signal to the mid-amplifier. Circuit and rounding method can be carried out by referring to the aforementioned step (2).