Power supply for foreign TVs. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply for foreign TVs. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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Switching power supply - one of the nodes of imported TVs, which most often fails. Schematic diagrams, as a rule, are absent. In the published article, the power supply of several models of foreign TVs is considered, recommendations are given for its repair. It seems that the author's advice will be useful to radio amateurs and workers of repair television workshops.

Recently, when repairing TVs, cases of malfunction of switching power supplies, mainly assembled on four transistors (in the primary circuit), have become more frequent. These blocks differ little from each other, most often by the types of semiconductor devices used (they are similar and interchangeable in terms of characteristics). Absolutely similar power supplies are found in TVs PHILIPS - 2021, AKAI - CT-1407, AKAI - 2107, SHERION, CROWN - STA / 5176, ELEKTA - CTR-1498EMK.

Consider such a source used in the CROWN TV - CTV5176, the block diagram is shown in fig. 1. The mains voltage of 220 V through the power filter is supplied to the rectifier BR601, C601 - C604 and to the demagnetization loop L2001. On the collector of the key transistor Q604, the rectified voltage passes through the winding 1-5 of the pulse transformer T601.

(click to enlarge)

A blocking oscillator is made on the Q604 transistor - the positive feedback voltage is removed from the winding 7 - 8 of the transformer. The duration of the pulses generated by the blocking generator, i.e., the time the Q604 transistor is in a saturated state, is determined by the functioning of the pulse-width modulator (PWM).

A capacitor C604 is connected to the base of transistor Q607, which, during the closed state of the transistor, is charged by a voltage pulse of the winding 7 - 8 of the transformer through diode D604. When transistors Q602, Q603 are opened, the PWM capacitor C607 is connected to the emitter junction of the saturated transistor Q604, and the discharge current of the capacitor, flowing through the transistors and resistor R616, quickly closes transistor Q604. The bias voltage to the base of transistor Q604 is applied through resistors R603, R604. The C610R617 circuit limits the pulse surges at the collector of the Q604 transistor, thereby protecting it from breakdown.

To power the DC amplifier on transistor Q601, the alternating voltage from winding 9 - 10 is rectified by diode D603 and charges capacitor C606. The voltage at the emitter of transistor Q601 is stabilized by a parametric stabilizer on elements D601, R609, and the voltage to the base of the transistor is taken from the measuring resistive divider R606VR601R607. The latter depends on the voltage on the winding 9 - 10 of the transformer, i.e., the output voltage levels of the power supply + 110 and + 12 V. The voltage across the resistor R608 - the collector load of the transistor Q601 serves as an error voltage and controls the opening moment of the PWM on transistors Q602, Q603. The trimmer resistor VR601 sets the output voltage to + 110 V.

A sawtooth voltage is removed from the resistor R605 through the C605R611 circuit to the base of the transistor O602 of the PWM shaper. It also receives the error voltage from the collector of transistor Q601. Depending on the last PWM, it opens earlier or later, counting from the moment Q604 opens. Transistors Q602, Q603 are analogous to a trinistor. The principle of its operation is similar to the operation of the trinistor in the pulsed power supply module MPZ-3.

With an increase in the mains voltage or a decrease in the load, the voltage on the winding 9 - 10 of the transformer T601 increases. As a result, transistors Q602, Q603 open earlier, closing the output transistor Q604 at an earlier time. This reduces the energy stored in the T601 transformer, which compensates for the increase in mains voltage.

When the mains voltage decreases, the voltage on the winding 9 - 10 of the T601 transformer will accordingly be lower. At the collector of transistor Q601, the error voltage decreases, the PWM opens at a later time, and the amount of energy transferred to the secondary circuit increases to compensate for the decrease in mains voltage.

The secondary rectifiers of the block are made according to a half-wave circuit. Winding 4 - 2 transformers and elements D606, C612, L601 form a +12 V voltage source used to operate the remote control system and other low-current circuits. Winding 4 - 3 and elements D607, L602 are included in the +110 V voltage source that feeds the line scan output stage.

On transistors Q608, Q606, Q605, a unit for turning on and off the power of the horizontal scanning output stage is assembled. Thus, the TV system is turned on or off by the remote control system, that is, it is transferred to the operating or standby mode. In standby mode, transistor Q606 is closed and +110 V is not supplied to the horizontal output stage. In some TV models, relays are used for this purpose.

Typical malfunctions of such a power supply are similar to malfunctions of the MP3-3 module. For repair, the block board is removed from the TV case and placed so that there is free access to the elements. In parallel with the capacitor C604, a resistor with a resistance of 220 kOhm and a dissipation power of 0,5 W is connected. Through it, the capacitor will be discharged after the TV is turned off. Solder one of the conclusions of each of the elements L601, L602, D608, C617. In this case, the load circuits of the TV will be completely disconnected from the power supply. In parallel with the capacitor C615, a 220 V and 25 W incandescent lamp is connected, which will serve as the equivalent load of the power supply.

After repair, before connecting the power supply to the TV circuits, it is imperative to check the horizontal output transistor and the secondary circuits of the horizontal transformer. Voltage is often taken from the secondary windings of the latter, rectified and smoothed to power the TV nodes. One of the reasons for the failure of the power supply may be precisely these circuits.

When selecting transistors to replace failed ones, one should be guided by their characteristics indicated in Table. 1.

Power supply for foreign TVs

Transistors 2SC1815Y can be replaced with KT3102B, 2SB774T - with KT3107B, and 2SD820, BU11F - with KT872A. The latter is mounted on a heat sink with an insulating gasket. It is permissible to replace diodes with KD209B, KD226A, KD226B.

Horizontal output transistors 2SD2333, 2SD1876, 2SD1877, 2SD1554 and others with a built-in damper diode are replaced with KT872A according to the circuit in fig. 2. Attach it to the heat sink through an insulating gasket. You can also use transistors KT846V, KT838A, but there will be difficulties with their attachment to the heat sink.

Power supply for foreign TVs

In case of failure of the channel selector of an imported TV, it is possible to replace it with selectors SK-M-24 and SK-D-24. The correspondence of their conclusions, for example, with the conclusions of the TUGZ1-C07 selector, is shown in Table. 2.

Power supply for foreign TVs

The letter designations of the selector pins are found on the TV board, and the pin numbers are on the selector housing. Inside the TV, domestic selectors are fixed in any way and bring the antenna jacks to the TV case. All connections (wiring) are made, if possible, with short conductors.

In extreme cases, if it is impossible to repair the switching power supply of an imported TV, it can be recommended to replace it with a domestic MPZ-3, MP-42, or others. Perhaps the unit is completely suitable for replacement, depending on the brand of the imported TV. If the output voltages do not match those used on the TV, then you will have to switch the secondary windings of the pulse transformer as needed and adjust the sources to the voltages appropriate for the TV. During adjustment, the unit is loaded with an incandescent lamp of 40 W and 220 V connected to a winding with a voltage of 130 ... 150 V.

It should be remembered that there will be a slightly overvoltage on unloaded windings in idle mode. Unused windings are left unconnected. Don't forget to connect the degaussing loop as well.

The switching power supply is placed in a free space inside the TV and securely fastened.

Author: B. Kiselevich, Khatanga, Krasnoyarsk Territory

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