Ensuring reliable operation of imported TVs in standby mode. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Ensuring reliable operation of imported TVs in standby mode. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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In many single-board color TVs with a screen size of 14, 20 or 21 inches produced by companies such as LG, Akai, Samsung, Funai, JVC, Condor and some others, when switching the TV to standby mode, a horizontal line supply voltage switch of the TV is used. In this case, the absence of voltage at the output of the switch corresponds to the transition of the TV to standby mode.

This switch, as a rule, is made according to the circuit shown in the figure. It uses VT1 type 2SC1013, 2N5401 or A1013 as transistors; VT2 type 2SC2230; VT3 type 2SC3310 or BUT11AX. The switch is controlled by signals from the TV control processor sent to resistor R3. In this case, in the operating mode of the TV, the voltage of +5 V (log/T) at the output of the control processor corresponds to the on state of the TV’s horizontal scanning. In standby mode of the TV, this control voltage does not exceed 0,4 V (log."0").

Voltage log "1" through resistor R3 opens transistor VT2, which connects resistor R2 to the common wire of the TV. As a result, transistors VT1 and VT3 open and the input voltage of the switch from capacitor C2 is supplied to capacitor C3. The input voltage to the switch comes from the winding of transformer TV1 of the TV's switching power supply. The output voltage from the emitter of transistor VT1 is directly supplied to the horizontal output stage and TDKS. Resistor R5 accelerates the discharge of capacitor C3 and VT1 of the pn-p structure, which then leads to failure of the powerful transistor VT3. Sometimes only the power transistor VT3 fails.

The most unpleasant option: VT1 does not break through completely, but only loses its amplifying properties and does not open completely upon the log “1” signal from the control processor. This leads to incomplete opening of transistor VT1. In this case, the base current of VT3 turns out to be insufficient, as a result of which the voltage drop across this transistor reaches 10...20 V. The transistor intensively heats up, which can even lead to charring of the printed circuit board at the installation site of VT3.

When repairing such a TV, it is difficult to resist the temptation to replace the switch with an electromagnetic relay, the winding of which will be connected between the VT3 collector and the +12 V power supply. However, using such widespread electromagnetic relays as a switch as RES-9 or RES-42 allows you to restore functionality TV only for a short period - 1...7 days. After this, the relay contacts “sinter” and the switch stops working: its output voltage becomes equal to the input voltage. The use of more powerful electromagnetic relays in this case is impractical due to the high power consumed by their control windings and significant dimensions.

When repairing a switch, you have to face a situation where simply replacing failed transistors does not help. Some time after the repair, the TV breaks down again. In addition, the high-voltage imported transistors used in the switch are quite expensive and are not commercially available everywhere. Therefore, to restore the functionality of the switch, you can use domestically produced transistors: VT1 type KT605BM, VT2 type KT814 only with the letter G, VT3 type KT872A. In addition, it is advisable to supplement the switch circuit with a resistor R4 type C2-23-0,125 W with a nominal value of 470 Ohms and a zener diode VD2 type KS620A or similar (shown in the dotted line in the figure). In this case, transistor VT3 and zener diode VD2 should be installed through insulating gaskets on the radiator of the horizontal output transistor.

As a rule, this L-shaped plate radiator has a very large area, so there are no problems with placing additional elements on it. The transistor and zener diode must be located away from the installation location of the horizontal output transistor on the radiator, so as not to worsen its thermal conditions. It is advisable to install the VT3 transistor on this radiator, even if an imported device is used as it. In this case, the terminals of the transistor should be connected to the printed circuit board of the TV with wires with a cross-section of at least 0,25...0,35 mm2.

After such modification of the switch, in a situation where power supply voltage surges occur, the voltage on transistors VT1 and VT3 will be limited by the stabilization voltage VD2, i.e. 120 V. At the same time, during voltage surges in the supply network, a slight increase in the voltage at the output of the switch to 30...40 V is possible. However, this small voltage does not lead to the start of horizontal scanning. Moreover, the flow of current through the zener diode VD2 and resistor R5 leads to a decrease in the voltage on capacitor C2 and thereby protects it from breakdown.

Author: A.Yu.Saulov, Kiev

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