Fly-back

Fly-back . Rectifier that converts High Voltage pulses into direct current, which is then filtered or flattened by the capacitor formed in the TRC. The High Voltage can be developed directly into a single winding with many turns of wire, or a winding that generates a lower voltage and a diode-capacitor voltage multiplier.

It usually acts as a voltage divider that provides the focus and screen of the screen. The main causes of lack of focus, excessive brightness, or fluctuation of focus and / or brightness are found in potentiometers and a divider circuit.

Summary

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  • 1 Origin of the term “Flyback”
  • 2 Structure
  • 3 Problems
    • 1 Failures
  • 4 Sources

Origin of the term “Flyback”

In the USA (possibly throughout America ), the transformer that generates the high voltage in a Television , Monitor , or other equipment that uses TRC, is called “Flyback” or “Flyback Transformer”. Elsewhere in the world, or LOPT (Line Output Transformer) Transformer output lines or just LOP.

The term “Flyback” probably originates, because the high voltage pulse that charges the TRC capacitor is generated by the contraction of the magnetic field in the transformer core, during the delay period of the electron beam in the TRC, which “flies back” to the start of a new scan or scan line.

The flux in the core changes slowly during the sweep and is abruptly cut by changing polarity (HOT) and driving the damper diode during that “flyback” or delay period.

Many switched power supplies and DC-DC converters are also primarily “flyback type”, transferring power to their circuits during the same cycle period. But there is no TRC involved and its high frequency transformers are generally not called flyback transformers.

LOPT and LOT (Line Out Transformer) derive from the fact that it is wrapped in the sweep circuit and takes advantage of this for its performance.

Structure

  • Flybacks consist of a set of windings with a ferrite core. It can also contain high-voltage diodes and resistor dividers (often with adjustment potentiometers) for the focus and screen (G2).

-A typical flyback includes the following components:

  • Primary winding: an average of one hundred turns of wire (eg AWG # 26). This is what is connected in series with the B + to the horizontal output transistor on a TV or monitor.

Fly-back construction – Parts

  • High Voltage Winding: several thousand turns. This winding can be divided into several sections with high voltage rectifiers in series with each or it can be a single winding. An alternative is a winding that provides a lower voltage and that uses a voltage multiplier (diode-capacitor ladder) to achieve what is required by the TRC. Very fine wire is used (eg AWG # 40). The main thing is to reach the high voltage necessary to supply the TRC with a rectifier or multiplier.

Some TVs and monitors use a physically separate voltage multiplier (external to the flyback). In this case, the flyback’s high-voltage winding generates about 6 to 10 KVAC and the multiplier generally raises this X3 or X4 to 20 to 30 KVDC. The focus and screen divisor (G2) is generally part of the multiplier in these cases.

  • Resistive divider for focus and acceleration (G2). This will probably be powered from a single coil in the series (if you use them). Often included in the flyback are focus adjustments and image screens. The connections of this splitter can be connected to the pins at the base of the flyback or they can have their own separate connections, with cables that connect to the socket or the TRC board.
  • Auxiliary windings: from a pair of turns (for the TRC filament) to several hundred (for a reinforced source). They provide various voltages for the TV or monitor: the TRC filament, source for the analog and logic circuits, etc. The measurement of these windings will depend on the requirements of each case. They are connected to the solder pins at the base of the flyback.
  • Ferrite core: consisting of two U-shaped pieces fastened with clamps, screws or glued together. Between both pieces there are tenths of mm openings created by a pair of spacers.

Most modern flybacks have all windings on the same side of the core. The primary and auxiliary windings are wound separately, isolated under the high voltage winding. High voltage windings make up many layers and contain insulating material between them.

The other components are mounted in a separate part of the winding and the entire unit is filled with an Epoxy compound. Part of the core is generally accessible.

A flyback is not a common transformer. Its ferrite core has a gap (air gap). It stores energy in the core’s magnetic field during sweep with its up-ramp current. Energy is also coupled to certain secondaries during the sweep.

Power is sent almost exclusively to the secondary high-voltage (HV) winding when the primary current is cut off at the end of the sweep or scan (this is probably where the name flyback comes from, because it occurs during electron beam return).

The ratio of the number of turns for each case is calculated not only based on the expected voltages but also on the magnetic field at the time of the cut (determined by the design of the horizontal output circuit).

The gap or gap is critical to proper operation and is normally determined by some plastic spacer.

problems

Caution, Danger One of the first questions to consider before working in this area is the following:

The matt black paint that covers the TRC on its exterior is what is called ” AQUADAG ” and has conductive characteristics. You will see that it is connected to zero potential, that is, to GND .

On the other hand, the tube anode works with a voltage that is in the order of approximately 25000 Volts. It is considered that these two voltages (25000 Volts and 0 Volts) are separated by the glass of the ampoule, we will notice that we are in the presence of a capacitor of considerable dimensions. Despite the considerable time that can pass without the TV being used, this capacitor can store enough energy to electrocute a human being.

U measurement

The way to protect ourselves is as follows: “WITH THE TV OFF”, we take one of the cables from the multimeter, we place one end resting on the mesh that covers the aquadag and with the other end we touch under the rubber connector that comes from the Fly- back very carefully and holding this tip as far as possible. We will feel that the discharge occurs, we will disconnect the “pacifier” and for effective protection we will bypass this “pseudo-capacitor”
once again. Once we are clear on how to disconnect the anode, we continue.

In this space, we are not going to expand on explanations regarding how Fly-back’s are manufactured, since we understand that you, perhaps, are more interested in repairing this section of the TV than in debating whether they are third harmonica or fifth, or if the tuning factor is 3 or 12 times or what material the reels are made of. The most important thing to know is that it has a primary, a secondary and a tertiary winding, in addition to having a ferrite core, which we will describe below.

Failures

  • The Ferrite Core or armor of the Fly-back or Extra High Voltage Transformer sometimes tends to break when manipulated, which could make us think that it has been disabled, but a little Loctite type glue will suffice to solve this problem.
  • In other cases, a release of the glue that holds the core to the windings has been observed, producing a rather annoying whistling in situations of low audio volume. This can also be remedied with glue but this time of the Contact Cement type (Poxiran, Suprabond, etc). The primary winding generally has three connections, one is input and the other two are outputs.

The input is + B power from the power supply, the voltage of which will vary between 95 Volts and 135 Volts depending on the TV. One of the outputs is where, after a fuse resistor, rectifier and filter, approximately 180 Volts will be obtained to supply the RGB Amplifiers (See faults related to this output in the Tube section ). The last of the three is the one that will reach the collector of the Horizontal Output Transistor , it will be in charge of making the commutation in order to achieve induction in the secondary and tertiary windings.

Fly-back

  • When the coils of the primary winding are shorted together, they represent an over-consumption for the power supply that will make it stop. One of the ways to verify this symptom is to disconnect the terminal through which the + B voltage enters the Fly-back and connect there in the one that comes from the source, (not in the one that was left loose from the Fly-back) with respect to GND, a 220 Volt incandescent lamp for about 75 Watts (the approximate consumption of the TV) in order to replace the suspicious winding, for a constant and known load. If the lamp does not light, we must turn to review a possible failure in the Power Supply, instead if it lights, we will first measure that the source voltage is correct, to affirm then that we must proceed to replace the Fly-back.
  • A second test with the same lamp can be done by also disconnecting the output to the collector of the switching transistor. In other words, we will replace the winding with the filament of the lamp. In this way, we will check the operation of the Horizontal Oscillator, the horizontal driver and the switching transistor. If everything is in good working condition, the lamp will light.
  • The voltage supply to the primary is usually crossed by a fast diode and a fuse-type resistor of between 1 and 3.3 Ohms, which occasionally deteriorate.
  • As the set Fly-back – Transistor of switching , is a “tuned”, usually tends circuit happen that Irregularities due to malfunction or deterioration of the capacitors that are connected between the collector and emitter of the transistor mentioned. (Common failure on some Toshiba models ).

In the cases mentioned, when the lamp lights, it will not do so in all its intensity of brilliance, but will do so palely, enough for our guidance. These tests may be carried out on those circuits that have a Power Supply without feedback, that is, they do not depend on the operation of the Fly-back.

Regarding the secondary winding, we can say that it is a simple circuit with multiple outputs, which will be used in various sectors of the TV. Generally, outputs of 12 to 16 Volts are obtained for the tuner, audio, jungle, etc; 24 Volts for vertical circuits and in some cases 40 Volts to later pass them to 33 Volts for the tuner.

In all these outputs, do not try to measure with the multimeter directly on the Fly-back pin, but observe that each one has a fuse resistance, a diode and a corresponding electrolytic, so in this last one we must corroborate the correct output of tension. Among the other outputs of this winding we will find the one that will feed the filament (only through a fuse resistor) and one output that will serve as feedback for the horizontal oscillator located in the Jungle.

  • It may happen that we do not obtain any of the secondary voltages, so we must control the fuse resistors and diodes. In some cases, the deterioration of the electrolytes associated with these connections can cause us to lose these voltages.
  • It may also happen that, if one of the diodes is short-circuited, an overload occurs, which the Power Supply will detect and will stop. The tertiary winding is the one that will be in charge of generating the Extra High Voltage of 25000 Volts for the anode of the TRC at a current of the order of a few milliamps.

It has a pin that is on the lower side, along with those of the primary and secondary, screen printed as ABL, which connects to the brightness and contrast circuits as feedback. By means of this pin the control of what is known as “Beam Current” is made. We also find the set of the Screen (Grid 2) and Focus potentiometers connected to this winding.

  • It often happens that bad welds appear in the ABL connection or in its associated circuits towards the jungle.
  • This also happens in some cases where the potentiometer sets have a lower pin that connects to GND.
  • Any other defect observed in the tertiary will be decisive to replace the Fly-back, whether they are high-voltage losses to the exterior or problems with the Screen and Focus potentiometers. As much as we try to seal a high-voltage exhaust, it will always come up again.
  • Faults in the potentiometers will be very evident on the screen, with loss of focus or variations in the voltage of G2 randomly.

 

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