A device for testing horizontal scanning. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A device for testing horizontal scanning. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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When repairing a horizontal scan of TVs, it is quite common to be faced with the need to check the output transformer, deflecting coils and circuits connected to them. But since the line scan (the main consumer of energy on the TV) closely interacts with the power supply and protection nodes, if it is violated, the protection device is triggered and it turns out to be difficult to check its operation.

Sometimes, immediately after turning on the TV, powerful (so-called power) horizontal-scanning transistors or a power source instantly fail. In such an apparatus, it is generally impossible to check the output stage and its elements by conventional methods.

In these cases, it is recommended to use a simple method for testing horizontal scanning using a simple tester. Only the output stage is checked with the TV turned off. The device allows you to determine if the cascade is faulty, and to identify most defects in the output transformer and deflecting coils.

When checking from the tester, a supply voltage of 15 V is supplied to the output stage, which replaces the voltage of 120 ... 140 V, as well as pulses with a repetition rate of about 15625 Hz. They imitate the operation of the output transistor. Consequently, the test is performed at a reduced supply voltage, which does not interfere with the oscilloscope and current meter to check the main parameters of the cascade.

A schematic diagram of one of the possible variants of the tester is shown in fig. one.

(click to enlarge)

It consists of a voltage source of 15 V and a pulse generator with a duration of about 50 µs at the specified repetition rate. Through a key on a powerful field-effect transistor VT1, pulses are fed to the output horizontal transformer according to the circuit in Fig. 2.

Line scan tester

The pulse generator (see Fig. 1) is built on DD1 and DD2 microcircuits. Actually the generator is assembled on the elements DD1.1, DD1.2. Its operation, if necessary, can be blocked by the SA1 switch, which connects pin 1 of the DD1.1 element to a common wire. As a result of the passage of the generator pulses through the differentiating circuit C5R4, short pulses are obtained at the output of the element DD1.3, which trigger the single vibrator DD2. It, in turn, generates output pulses with a duration of about 50 μs. And since the repetition rate of short pulses is 15625 Hz, the duration of pauses between the output pulses reaches 14 μs. They enter the gate of the field-effect transistor VT1, operating in key mode, and open it. The drain and source of the transistor VT1 are connected respectively to the collector and emitter of the output (power) horizontal transistor (see Fig. 2). Moreover, the sweep transistor itself, if it is serviceable, does not need to be soldered, since it does not interfere with the operation of the tester.

The device also contains (see Fig. 1) a voltage regulator DA1 for 15 V, the output circuit of which includes a pointer (by the author) current meter PA1, consumed by the horizontal scanning output stage. The microcircuits of the tester itself are powered from the same stabilizer.

The details of the device are placed on a printed circuit board made of fiberglass (or on a breadboard). It is housed in a small plastic case. On its outer panel, sockets are fixed for connecting the oscilloscope and the device itself to horizontal scanning. You can not use a pointer current meter (then you don’t need resistors R7, R8), but place more sockets on the outer panel of the tester for connecting a separate milliammeter. In this case, fuse FU1 is best left to protect the device.

Before connecting the tester to the TV, you need to check if there is a short circuit in the horizontal power supply circuit (then you need to look for a defect in this circuit) and between the collector and emitter terminals of its output transistor. We repeat that if the transistor is broken, it is soldered. In the absence of a short circuit, the transistor is left in place.

The line-scan output stage is tested by measuring the current it consumes and by controlling the shape and duration of the reverse pulses that occur at the drain of the field-effect transistor VT1 during the operation of the tester with an oscilloscope. Obviously, with a supply voltage of 15 V, eight to nine times less than the real voltage, the amplitude of all measured pulses will be the same number of times less than in a working TV, but their shape will practically not change.

The consumed current should be in the range from 5 to 70 ... 80 mA (depending on the construction of the line scan of the TV). If the consumption is less, there is an open in the output stage. This can be either poor soldering, or a microcrack in the printed conductor, or a break in the primary winding of a line transformer (which is quite rare).

If the current exceeds 80 mA, there is a leakage in the cascade. It can be both direct and alternating current. To distinguish between them, the SA1 switch blocks the operation of the generator. In this case, the horizontal scanning circuits must consume a direct current of 5 ... 10 mA. If it exceeds these values, check the rectifier diode and the filter capacitor of the power supply, and also solder the horizontal output transistor. If the current is still high, turn off all the elements connected to the power circuit in turn.

After the fault is eliminated in the power circuits, the current is monitored when the tester generator is turned on. It must be within the limits indicated above. If it exceeds 80 mA, the most likely cause of AC leakage is a breakdown in the voltage multiplier. Leaks are also possible in the secondary circuits of a horizontal transformer or a breakdown between its windings. In imported TVs, first of all, you should check all the rectifier diodes and capacitors of the secondary power sources connected to the TDKS line transformer, and also make sure that there is no short circuit in any of these circuits when they are turned off one by one. Very often, a protective zener diode connected in parallel with a 12 V power supply becomes the cause of a short circuit. A TDKS malfunction is not such a frequent occurrence, and, most likely, a leak is detected precisely in the secondary circuits.

If the consumed current is normal, then reverse pulses are observed on the oscilloscope screen. The shape and the resulting duration of the pulses indicate whether there is the necessary timing in the circuits of the horizontal transformer and the deflecting coil and whether resonance has been achieved. The pulse duration should be in the range from 11 to 16 µs. It is set by the reactive elements of the output stage: mainly the inductance of the horizontal transformer and the deflection coil, as well as the capacitance of the flyback capacitors and the capacitor connected in series with the deflection coil. If the duration of the pulses does not correspond to the norm, the fault is sought in these circuits.

The tester can use any resistors and capacitors. Resistor R7, in the absence of an industrial one, is made from a piece of nichrome wire with a diameter of 0,2-0,4 mm. Resistor R6 is made up of two or three resistors connected in series.

The KTs405A diode bridge can be replaced with separate diodes, for example, KD212A, and the KR142EN8V microcircuit can be replaced with KR142EN8E or LM7815. It must be placed on a small heat sink, since in the process of testing a faulty TV, relatively large currents caused by leaks can flow through the stabilizer. The DD1 chip is interchangeable with a similar one from the K1561 series. But it is also possible from the K176 series, only then it will be necessary to add a separate stabilizer for it with a zener diode for a voltage of 10 ... 12 V. The KR1006VI1 microcircuit can be replaced with an imported analogue LM555. At position VT1, it is permissible to use transistors 2SK2038, 2SK792, KP809D.

Transformer T1 can be any with a voltage on the secondary winding of 16 ... 19 V. The author used a TPP252 transformer with windings connected in series 11-12, 13-14, 15-16, 19-20. Microammeter RA1 - M2001 or similar with a total deflection current of 50 μA.

Setting up a tester is not difficult. It consists in setting the readings of the PA1 milliammeter and adjusting the required frequency and duration of the output pulses of the tester. To calibrate the scale of the milliammeter between the sockets "+ PIP" and "Common." include a resistor with a resistance of 30 ohms and a trimmer resistor R8 set the readings of a milliammeter of 500 mA. If desired, the limits of 5 and 80 mA can be marked on the scale of the device with colored marks. Next, an oscilloscope is connected to pin 4 of the DD1 microcircuit and a trimming resistor R3 sets the pulse repetition rate to about 15625 Hz. After that, the oscilloscope is connected to pin 3 of the DD2 chip and make sure that it has rectangular pulses with a duration of about 50 μs. A slight deviation in the frequency and duration of the pulses from those indicated above is not significant. If necessary, the duration of the pulses can be changed by selecting a resistor R6 or capacitor C6.

For more reliable operation of the generator on DDI elements. 1, DD1.2 it is better to add one more element DD1.4 to it, which remained free in the microcircuit. It is turned on by combining the inputs, between the connection point of the output of the DDI.2 element and the capacitor C4 and the left (according to the diagram) output of the capacitor C5. To the connection point of the output of the new element DD 1.4 and the capacitor C5, the right (according to the diagram) output of the resistor R3 is connected, disconnecting it from the outputs 3, 5. 6 of the microcircuit.

Author: I.Korotkov, Bucha village, Kyiv region, Ukraine

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