Judge Whether the Row Output Transformer Is Short Circuited by Using the Coil Short Circuit Tester

The methods of judging whether the line output transformer (hereinafter referred to as "line transformer") is short circuited have been introduced in many articles. For example, "voltage method", "current method" and "substitution method" are also useful for special circuit detection, each with its own advantages.

The article "ringing method and practical ringing waveform excitation circuit", published by Gao Rongwu in the 8th Journal of home appliance maintenance technology in 2000, introduces the "ringing method" to judge whether the line output transformer or deflection coil is short circuited. After the author's test, it is indeed a very effective method, but it only needs an oscilloscope. It is inconvenient to carry it during door-to-door maintenance, which makes many friends who don't have an oscilloscope envy! In fact, as long as the original circuit is slightly changed, it can accurately judge whether the line output transformer is short circuited without an oscilloscope? The circuit is shown in Figure 1.

Fig. 1 circuit diagram of coil short circuit testerThe integrated circuit 555 forms a multivibrator circuit with peripheral components, and the oscillation frequency is: F = 1.443 / TRL 2 (R2 VR)] C2.

In the figure, I 'is the primary winding of the row output transformer, which forms a series resonant circuit with capacitor C1, and the oscillating square wave pulse is output by pin of 555. Adjust the potentiometer VR, and the square wave frequency output by pin changes. When this frequency is equal to the resonant frequency f = 1 / J 10000 of L and C series circuits, the voltage at both ends of I 'is the highest. If the line output transformer L is short circuited, its inductance will be greatly reduced, so the voltage at both ends will be greatly reduced, and the resonant frequency will be greatly increased, which exceeds the frequency range of the circuit and will not resonate. Therefore, as long as the voltage at both ends of L is measured, it can be judged whether the row output transformer is short circuited.

Figure 2 shows the waveform of measuring the voltage at both ends of Toshiba 1yb4039ad line output transformer with the excitation frequency. Resonance occurs at about 9.6lhz. At this time, the voltage at both ends of L measured by multimeter is 89v (measured with dw926b 700V AC gear. Note that the readings are different at different gears). If you increase or decrease VR, the voltage will decrease. When the trial conductor is short circuited around the magnetic core for one turn, the universal representation number is reduced to 2V, and whether there is a short circuit is adjusted. Only individual deflection coils fail to resonate, which may be due to the difference of inductance. However, the universal representation number is generally more than 30V, which is still different from the short circuit. Readers can test by themselves and accumulate experience.

In order to facilitate carrying during door-to-door maintenance, the circuit can be installed in the shell of a mobile phone charger, the power line is led out separately, and the signal output end and ground wire end are welded with fresh fish clips to connect with the "line transformer". When detecting whether the "traveling transformer" is short circuited in the circuit, because the resistance value of the kinescope filament is very small, it is similar to short circuiting the secondary of the "traveling transformer", pull out the video amplifier first; At the same time, disconnect the deflection coil before measurement to avoid misjudgment.

This paper illustrates the application of "coil short circuit tester"Fault phenomenon: Hitachi cpr2038 (np82c movement) "three noes"

Maintenance: at the moment of startup, there is a "squeak..." sound from large to small, followed by "three noes", which may be the action of the protection circuit. The resistance of c-pole of the tube to the ground is more than 15k0, and there is no breakdown. Connect the 100W bulb as a dummy load, and the measured power supply voltage is 139v (this voltage is still normal due to the lack of line synchronization pulse). When a 7200pf / 2KV capacitor is connected in parallel with pole C of the tube to the ground, the "squeaking..." sound is prolonged for several seconds, but it is still protected. It should not be the reverse capacitor fault.

&nbs p; In order to determine whether the line output stage is short circuited or the protection circuit acts incorrectly, an ammeter is connected in series at the output end of 4-b. after starting up, the line current suddenly rises to 1.2A, which will automatically shut down, confirming that there is a serious short circuit. In the line to line output circuit test, first pull out the deflection coil and the visual amplifier board. The square wave output end of the "coil short circuit tester" is connected to the C pole of the line tube (the bulb dummy load should be removed) and the ground wire is connected.

Adjust the potentiometer VR, measure that the terminal voltage of the line tube C changes between about 7V and 8V, and there is no resonant voltage. It is judged that it is a "line transformer" short circuit. The "traveling transformer" was removed for separate test, but there was 88v resonant voltage, which was obviously normal. The fault point is the short circuit of line output secondary load? Check that all secondary loads are normal! In desperation, first connect the T B and C pins of the "line transformer" to the circuit, connect the "coil short circuit tester", and then connect the other pins one by one. Finally, it is found that after grounding the soft wire wound on the magnetic core of the line output transformer (this wire is grounded in the original circuit, which may be anti-interference), the original 88v resonant voltage immediately drops to 2V. The fault reason is that the coil of "traveling transformer" is short circuited with the magnetic core. After disconnecting the grounding conductor of the magnetic core, everything is normal during the test run.

Summary: in this example, it is unexpectedly found that the method of connecting (or disconnecting) the line load one by one is more effective than removing the separate detection when detecting the line output transformer, which also shows that the maintenance method should change with the "machine". For the model with line output protection circuit action, the "method of measuring line current after parallel reverse capacitance" shall be adopted to determine which type of protection? Then carry out targeted maintenance. It is best not to disconnect the protection circuit at will and try the machine.