A device for extending the life of a kinescope. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The device proposed for repetition for extending the life of the kinescope was developed for the Sadko-Ts280D TV, but can also be used in TVs of other brands with a corresponding change in its connection. The main reason for the aging of the kinescope during its long operation is the decrease in cathode emission. The owner of the TV has the opportunity to actively intervene in this process and significantly extend the life of the kinescope. Although such a natural process occurs under any conditions, its rate can be changed several times depending on the operating conditions of the cathode.

A lot of devices have already been proposed to slow down the reduction of emissions. The device described below differs from them in more stable parameters, a wide interval of possible delays, reliable protection of the kinescope from overvoltage and the device itself from an output short circuit. The device is connected to the network with the TV power button. The kinescope filament voltage gradually increases from zero to the nominal value and is rigidly stabilized at the reached level. At the moment it is reached, the relay is activated, its contacts turn on the TV and an accelerating voltage is supplied to the anode of the kinescope.

The schematic diagram of the device is shown in fig. 1. Its basis is a linearly increasing voltage generator (LVN) on transistors VT2 and VT3. The transistors are connected according to the source follower circuit with servo feedback. At the moment the power is turned on, the voltage at the capacitor C1 is zero, and at the source of the transistor VT3 (i.e. at the output of the repeater) it is about 0,3 V. The last capacitor C5 starts charging through the resistor R1, and the voltage across it and at the output of the repeater grows . Since the repeater gain is close to unity, the voltage across resistor R5 is kept approximately constant. Remains constant and the charging current of the capacitor C1. Consequently, the voltage on it grows linearly (nonlinearity does not exceed ± 1,5%).

Fig. 1

The advantage of the generator can be called small for such a long delay time (about 50 s) ratings of the R5C1 circuit elements. This is due to the fact that the voltage difference at the terminals of the resistor R5, due to which the capacitor C1 is charging, does not exceed 0,3 V and the charging current is also very small (about 0,3 μA). At the same time, the voltage up to which the capacitor C1 is charged can significantly exceed this difference and is limited only by the supply voltage. The temperature stability of the repeater gain is very high, which is explained by the mutual compensation of the temperature coefficients of the field (VT3) and bipolar (VT2) transistors, which have opposite signs, and deep negative feedback.

The increase in voltage at the output of the repeater occurs to the level specified by the voltage limiter on the transistor VT1, zener diode VD2 and resistor R1. The limitation level Ulimit is: Ulimit = Ust + Ube = 4,7 + 0,6 = 5,3 V, where Ust is the stabilization voltage of the zener diode VD2, Ube is the base-emitter voltage at which the transistor VT1 opens. At the moment when the voltage at the output of the repeater reaches the value Ulim, the previously closed transistor VT1 opens, relay K1 is activated and its contacts turn on the TV.

The TV turn-on delay time can be changed over a wide range by selecting R5C1 circuit elements. A 280LK61Ts-5 kinescope with a low-inertia cathode is installed on the Sadko-Ts1D TV. Its warm-up time does not exceed 12 s, so the delay time (about 50 s), which is provided with the indicated circuit ratings, is quite sufficient.

More intense than the capacitor C1 enters the input of the filament current regulator, assembled on the op-amp DA1 and transistors VT4, VT6. Connecting the input (output 3) of the op-amp to capacitor C1 does not affect the operation of the LVN generator, since the op-amp K544UD1A has a very large input impedance. The operation of the stabilizer is reduced to automatically maintaining the voltage at the inverse input of the OU (U2) equal to the voltage at the direct input (U3). Due to the large voltage transfer coefficient in the feedback loop of the stabilizer and the accuracy of the operation of the op-amp, the voltages U2 and U3 differ by no more than a few millivolts.

Fig. 2

Fig. 3

The voltage on the cathode heater of the kinescope Un, is proportional to the voltage U2, and is equal to Un = U2((R7+R9)/R6)+1). It is set with a trimmer resistor R7. The diode VD3 protects the emitter junction of the transistor VT4 from breakdown with a negative output voltage of the op-amp.

Experience in the operation of zener diodes shows that an internal break is possible in them. In the event of a break in the Zener diode VD2, the voltage at the output of the device will increase to 11 V and the kinescope may fail. To prevent this from happening, the device has protection elements R10, VT5. When a break occurs in the zener diode VD2, the voltage at the emitter of the transistor VT1 increases sharply, the voltage across the capacitor C1 and at the output of the op-amp begins to grow, the collector current of the transistor VT4 also increases, which leads to an increase in the emitter current of the transistor VT6. The voltage drop across the resistor R10 increases, and at some point the transistor VT5 opens, shunting the emitter junction of the transistor VT6. The growth of its collector current stops. With the resistance of the resistor R10 indicated on the circuit, the filament voltage is limited to 6,8 V. The filament current in this case reaches 0,75 A (at a nominal value of 0,7 A), which is quite acceptable. These elements simultaneously protect the transistor VT6 from a short circuit at the output. The graph of voltage change at the output of the device with increasing load current is shown in fig. 2.

The schematic diagram of the power supply unit of the device is shown in fig. 3. The numbering of the elements on it continues the numbering of the device parts.

The power transformer T1 is made on a low-dissipation magnetic circuit (with an extended jumper) SHU 13x26-40. Winding I contains 3000 turns of wire PEV-2 0,21; II and III - 230 turns of wire PEV-2 0,12 each; IV - 360 turns of wire PEV-2 0,16 and V- 205 turns of wire PEV-02 0,62. You can also use the magnetic cores OL, PL and SL.

In most similar devices, the authors chose the relay according to the input parameters (voltage and trip current) and recommend the RES-9 and RES-22 relays. But they are completely unsuitable in terms of output parameters - voltage, current and type of load. The RES-9 relay switches an active load at an alternating voltage of not more than 115 V and a current of up to 0,1 A, and the RES-22 relay also switches an active load at an alternating voltage of up to 250 V and a current of up to 0,1 A. However, the TV is a load with a significant reactive component and a current of at least 0,5 A. Therefore, you need to choose a relay designed to switch an inductive load at a voltage of at least 250 V and a current of at least 0,5 A. Suitable relays MKU-48, RKS-3, etc. The device uses the RPT-100 relay. In terms of output parameters and dimensions, it fits with a margin. It is designed to work with an alternating supply voltage of 220V, but, of course, it can also work with a constant voltage. To increase sensitivity, three of the four pins have been removed. The relay is mounted with the anchor up. To eliminate sticking of the armature, a film of non-magnetic material 0,01 mm thick is glued between it and the magnetic circuit. After such an alteration, the relay operates at a voltage of about 30 V and a current of 15 mA.

In the device, capacitor C1 is MBGO, C2 is any, C3-C5 is K50-6, C6 is K50-35. Transistor VT6 is mounted on a radiator with a surface area of ​​150 sq.cm, and rectifiers VD4 and VD6 - on a radiator with an area of ​​60 sq.cm. The VD2 zener diode is also equipped with a 15x20x0,5 mm aluminum heatsink. Resistors - MLT, except for R10, which is made in the form of a frameless spiral of nichrome wire with a diameter of 0,4 mm and a length of 20 cm.

Preliminary adjustment of the device begins before installation in a TV using a load equivalent - a wire resistor with a resistance of 9 ohms for a dissipation power of at least 5 watts. Turning on the power of the device and the stopwatch at the same time, measure the time after which the relay K1 will operate. If it differs from the desired, you need to achieve it by selecting a resistor R5 or capacitor C1. A trimming resistor R7 sets a voltage of 6,3 V on the equivalent load (its change limits are -5,9 ... 6,7 V).

In some devices, the possibility of a significant increase in the heating voltage is provided. This is done in order to restore the cathode emission for some time by stepwise increasing it to 11...13 V. However, this method allows you to extend the life of the kinescope after each stage by two to three months, but not more than a year, after which the cathode irreversibly loses emission. Experts do not recommend this, therefore, in the proposed device, the heating voltage increase mode is not provided.

After preliminary adjustment, you need to disconnect the device from the network, fix it in the intended place inside the TV case and according to the diagram in Fig. 4 connect to his circuits. To do this, you need to unsolder the end of the wire coming from the mains switch S1 of the TV to connector X17 (A12), from pin 1 of the connector and solder it to pin 2 of the device. Then, remove the wire connecting pins 3 of the A8 board to pin 4 of connector X4 (A7), and connect pin 3 of the A8 board with pin 4 of the device, and pin 2 of the board with pin 3 of the device. It is impossible to unsolder the wire connecting pin 2 of the A8 board with pin 3 of the X4 (A7) connector, since a bias voltage of +65 V is supplied to the cacao filament through it. Next, turn on the power, let the device and the kinescope cathode warm up for 10 ... 12 minutes, measure the filament voltage and, if necessary, set its value to 6,3V. This completes the setup. It remains only to turn off the power, connect with a wire pin 1 of the device to pin 1 of connector X17 (A12) of the TV and close its back cover.

Fig. 4

Author: M. Dorofeev, Moscow; Publication: cxem.net

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