Testing horizontal scanning at low supply voltage. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The difficulties that arise when troubleshooting a TV, especially in a line scanner, are familiar to many radio amateurs and repairmen. To solve them, the author of the article published here suggests using a simple tester. It allows you to check the operation of not only the horizontal output stage of televisions and monitors, but also switching power supplies, as well as inductive elements included in such devices.

When repairing TVs, especially modern ones, there are often malfunctions, the search and elimination of which causes certain difficulties not only for radio amateurs, but also for TV technicians. A significant proportion of them is associated with horizontal scanning defects. This problem has become truly relevant with the appearance on the domestic market, and hence in repair shops, of televisions with digital control and signal processing, since the process of troubleshooting in them is associated with the specifics of their work. This is described in detail in the book by P. F. Gavrilov and A. Ya. Dedov "Repair of digital TVs" (M .: Radioton, 1999). The fact is that the slightest deviation in the operating modes of the horizontal scanning units of such TVs causes blocking of both its processors and the power supply, and therefore, difficulties arise with their launch for traditional verification.

In most cases, the problems that arise can be solved by the so-called load testing of the horizontal scanning output stage. The proposed check can not only significantly reduce the troubleshooting time, but, most importantly, clearly answer the question of whether this cascade is faulty or not. Testing is carried out with the TV turned off. It reveals most of the defects of horizontal transformers and deflecting systems. This testing method can be used (according to the author) to test TVs of both domestic and foreign production, both modern and the oldest, as well as scanners for computer monitors and switching power supplies with a corresponding change in the signal parameters of the testing device - load tester .

The essence of the load testing method is that a low supply voltage (about 15 V) is applied to the horizontal scanning output stage, which is significantly less than the nominal one and replaces the power source of the device. The pulses at the output of the tester connected to it, following with a frequency, for example, 15625 Hz for a TV, imitate the operation of the output stage transistor. At the same time, oscillations are generated in the horizontal transformer and the deflecting coil, which quite accurately reflect its operation, only the amplitude of the currents and voltages arising in it is approximately 10 times less than the operating amplitude.

Using such a tester, as well as a milliammeter and an oscilloscope, they check the operation of the output stage. Practice shows that it is advisable to always carry out the specified check when troubleshooting in horizontal scanning circuits.

The schematic diagram of the load tester is shown in fig. 1. Its field-effect transistor VT1 plays the role of a power switch, connected in the required polarity to the horizontal scanning output stage transistor. The gate of the field-effect transistor receives pulses from a master oscillator assembled on a DD1 chip. The pulse duration is regulated by a variable resistor R4, and the repetition rate - by a variable resistor R1. Toggle switch SA1 is designed to switch test modes: "Test." or "Percall" (this mode will be discussed later).

(click to enlarge)

In the test mode, the generator frequency is set equal to the operating frequency of the pulse converter of the device under study. For a horizontal TV it is 15625 Hz, and for a VGA monitor it can be 31,5 kHz or higher. In the "ringing" mode, the generator frequency is about 1 kHz. The pulse duration and frequency for the TV are chosen so that the open state of the field-effect transistor is 50, and the closed state is 14 μs.

The field-effect transistor is shunted by a protective diode VD1, which increases the reliability of the tester. It is a fast acting 350V voltage limiter that protects the transistor from high voltage spikes during testing. You can, of course, refuse to use it, but then this will reduce the reliability of the device.

Structurally, the tester is made in the form of a board with a separate power supply. The tester is assembled on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2.

The device uses variable resistors SP4-1 or any other, suitable in size, fixed resistors MLT, OMLT, S2-ZZN, etc. Capacitors C2, C6 - any oxide with a minimum leakage current, the rest - K10-17 or KM. Capacitor C5 is soldered between the power leads of the DD1 chip, either from the side of the printed conductors or from the side of the parts, placing it above it. Flexible contacts from connectors 15...20 mm long are used as output terminals ("Output" and "Common").

The adjustment comes down to setting the frequency and pulse duration marks corresponding to the testing modes on the scales of variable resistors.

The load tester is "hung" on the board of the device under test - two flexible leads ("Output" and "Common") of the board are soldered to the soldering points of the collector and emitter of the output transistor (respectively) of the line scan under test as seen on the 1st p. covers. In this case, you must not forget to apply the supply voltage (+ Upit \u15d 3 V) to its output stage. The scheme for connecting the tester and measuring instruments to the horizontal scanning cascade using an imported TV as an example is shown in fig. XNUMX.

The power supply unit of the tester can be any 15 V DC voltage source capable of providing current up to 500 mA.

Let's move on to the line scan itself. First, they check (with an ohmmeter) the transistor of the output stage for a breakdown. If it is broken, then it should be unsoldered before starting testing. In good condition, the transistor does not affect the instrument readings.

By connecting the tester (according to the diagram in Fig. 3), they measure the current consumed by the output stage. If the milliammeter shows a value in the range of 10 ... 70 mA, then this is normal for most output stages. A value less than 10 mA indicates the presence of an open in the circuits, and more than 70 mA (especially more than 100 mA) - an increased current consumption by the output stage, line transformer or other circuits that load the main power supply of the device. At the same time, turning on the TV, if you do not understand the cause of the phenomenon, most likely can cause either the operation of the protection of the power supply, or the failure of the output transistor. In this case, it is necessary to find out why the consumed current has increased.

(click to enlarge)

Reduced consumption is usually associated with breaks in the elements and circuits of the output stage or consumers of energy converted by a line transformer, for example, in a vertical scan. With increased consumption, you must first determine what kind of current it is caused - AC or DC. To do this, they are measured in two modes: variable - when the connected tester is operating, constant - when its output transistor is off (closed). You can get the second mode in a variety of ways. For example, just unsolder the "Exit" output from the line scan (which is what the author did). However, for the same purpose, you can set the slider of the resistor R4 to the uppermost (according to the diagram) position or provide a switch that short-circuits this resistor.

Consumers of increased direct current are leaking capacitors, punched semiconductor elements or an interwinding short circuit in the output line transformer (TVS). Increased AC consumption is most often caused by an interturn short circuit in the fuel assembly, the deflecting system or other reactive elements, as well as leaks in the secondary circuits of the fuel assembly.

In order to find short circuits or leaks in the secondary circuits of fuel assemblies, a DC voltmeter can be used when measuring rectified voltages. It should be remembered that the load tester only simulates the operation of the horizontal scanning output stage at a supply voltage that is much lower than the nominal one. In this case, all secondary rectified and pulsed voltages will have values ​​that are approximately an order of magnitude lower than the nominal ones.

If the measured pulse or DC voltage is significantly lower, then you need to check the elements in the circuits: the filter capacitor or rectifier diode, as well as the vertical scan chip (if it is powered by TVS).

However, it is impossible to focus only on current consumption to make a final decision about the malfunction or serviceability of horizontal scanning. More precisely, low current consumption does not always indicate the health of horizontal scanning. So, a number of defects were revealed, when during testing the consumed current remains within the normal range. For example, in the SONY-KV-2170 TV, when the winding of the diode-cascade horizontal transformer (TDKS) is closed to a voltage of 24 V (vertical scan power supply), the current consumption from 18 mA increases to only 26 mA, and the short circuit of the filament winding on the same TDKS causes an increase current up to 130 mA. This is probably due to the different arrangement of the coils on the TDKS magnetic circuit and different inductive couplings with the main winding. In addition, for example, in the PHILIPS TV - 21PT136A, the horizontal scan current consumption was 74 mA, and turning off all loads reduced it to only 70 mA. This again did not allow us to unambiguously judge the state of the cascade.

A more accurate conclusion about the malfunction allows the oscillogram of the reverse pulses on the collector of the key transistor. The oscilloscope can also measure the duration of these pulses, which depends on the operation of the output stage circuits, mainly the flyback transformer, the flyback capacitors, the deflection coil, and the feed-through capacitors in the deflection coil circuit. The duration of the pulse indicates whether the line transformer and deflection coil circuits have the necessary timing and whether resonance has been reached.

With a good horizontal scan, pulses of the correct shape are observed without parasitic resonances and bursts, as in Fig. 4a. If their duration is in the range of 11,3 ... 15,9 µs, it is safe to say that the output stage generates normal reverse pulses.

Broken diodes, turn-to-turn short circuits necessarily distort the waveform. When closing in the load circuits, the oscillogram looks like in Fig. 4b. During the breakdown of rectifier diodes, the oscillogram looks like in Fig. 4, in or d.

When the results of load testing indicate that there is a problem with the horizontal output stage, the repairer will of course want to check its components, including the flyback transformer and deflection coil. But if there is only a slight deviation from the norm in load and pulse duration, then with these main components, most likely, everything is in order. In this case, there is no need to waste time testing them. It is better to continue measuring with the TV turned on and find the source of the problem. That will be much faster.

You should be careful not to touch the scanning elements with your hands during testing, since when the load tester is operating, quite high voltages still arise on the collector of the output transistor, the terminals of the horizontal transformer and the multiplier.

There are malfunctions in which the duration of the pulses may be on the border of acceptable values ​​or even change. This may indicate either a weak shunting of the transformer windings, or a break in one of the loads.

Checking in the considered way can be of great help when replacing horizontal transformers and deflecting systems, when it is not possible to find the original part and you have to be content with analogues.

The load testing method can detect such rare malfunctions as flickering circuits. They are mainly associated with defects in the elements, which appear sporadically. One of these defects is the fraying of the insulation of turns of overheated, poorly stretched or loose windings of pulse transformers according to technological requirements. Uneven heating of the windings and their expansion, taking into account vibration in a magnetic field, create conditions for local destruction of the insulation and the occurrence of flickering turn-to-turn short circuits. Then the power transistors fail as if suddenly and for no reason.

These defects require special diagnostic methods and it is with the use of the active mode of the transformer.

Now let's move on to checking the inductive elements with a load tester in the "Continuity test" mode, which was mentioned at the beginning.

There are many methods for resonant testing of transformers using 3H generators. The reliability of such verification methods is such that when trying to check a transformer by examining the shape of a sinusoid or the resonant frequency of a winding, one often has to regret wasted time.

After all, the resonant frequency of the transformer depends on the number of turns, the diameter of the wire, the properties of the material of the magnetic circuit, the width of the gap. Many years ago, by closing part of the turns of the coil of a magnetic antenna (similarly in a transformer), the resonance was shifted higher in frequency without much damage to resonance operation. Therefore, coil closures do not affect the absence of resonance, but only increase its frequency, reducing the quality factor. The shape of a sinusoid on a winding with closed turns may not even be distorted. And there may be several resonances.

One of the reliable ways to test inductive elements should be called continuity or quality factor assessment. When performing continuity, parallel to the winding of the inductive element (line transformer, deflecting system, etc.), a capacitor with a capacity of, for example, 0,1 μF is connected and pulses are supplied from the generator with a duration of about 10 μs and a frequency of 1 ... 2 kHz. For this purpose, it is just possible to use the master oscillator of the load tester by setting the SA1 switch to the "Continuity" position and adjusting the frequency with the variable resistor R1.

In the parallel oscillatory circuit formed by the capacitance of the capacitor and the inductance of the transformer winding, oscillations damped after several cycles appear (they say: "the circuit rings"). The decay rate depends on the quality factor of the coil. If there is a short-circuited coil, then the oscillations will continue for no more than three periods. With a working coil, the circuit will ring 10 or more times.

The continuity of the horizontal transformer can be performed without even unsoldering it from the TV board. It is only necessary to turn off the horizontal scanning power circuit. If the tested transformer is in good condition, then the oscillogram shown in Fig. 5.

If the oscillations decay much faster, for example, as in Fig. 6, then it is necessary to turn off the load circuits of the secondary windings in turn until long-term oscillations appear. Otherwise, it is necessary to unsolder the transformer from the board and finally verify the results of the survey. It should be borne in mind that even due to one closed turn, all the coils in the transformer will not ring.

You can also find closed turns in deflecting systems and transformers of switching power supplies.

And finally, it is necessary to say a little about checking the TDKS. The features of their verification are due to the fact that the high voltage multiplier is mounted in the transformer along with the windings. The high-voltage diodes of the multiplier can be broken, broken, have a leak, as a result of which the anode and focusing voltages can be underestimated or absent altogether, and the load testing of the cascade does not clearly distinguish between the field of troubleshooting (winding, magnetic circuit or multiplier). But there are ways to restore the TDKS if it has a broken filtering high-voltage capacitor. And picking up and replacing a magnetic circuit from another transformer is not particularly difficult.

By applying pulses to the primary winding of the TDKS, similar to the pulses of the horizontal scanning output stage, you can conduct dynamic testing, check how the applied pulses are rectified and multiplied. A faulty diode, winding or magnetic circuit of a horizontal transformer will lead to a decrease in the output voltage of the TDKS. Dynamic testing is performed by the same tester as load testing. It is only necessary to adjust the supply voltage supplied to the primary winding of the transformer so that the pulse amplitude at the drain of the key transistor of the tester is approximately 25 V. The output voltage at the anode of the kinescope is measured relative to the aquadag. It must be over 600 V.

The values ​​of the measured voltage for a serviceable TDKS must correspond to those indicated in the table.

(click to enlarge)

So, for example, if in a normally working TV the amplitude of the pulses at the collector of the horizontal output transistor is 900 V, and the voltage at the anode of the kinescope is 25 kV, then when checking the TDKS according to the above method, a voltage of about 695 V should be present at the output of the multiplier (in the table these values ​​are in bold).

The considered principle of checking horizontal scanning is the basis for the operation of many branded devices. However, at a price they are not available to ordinary radio amateurs and private repairmen. And the simple tester described here can completely replace such devices.

Author: D.Malorod, Kovrov, Vladimir region

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