LA76931 Ver la hoja de datos (PDF) - Unspecified

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LA76931

64Pin LA76931 for Color Television Receiver / SERVICE MANUAL

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3

CHAPTER ONE Power Supply

TV require a variety of voltage ( at various power levels) to work normally. The function of the voltage power supply is to

take the AC line input and produce various DC voltages. In all case , the power to the horizontal output transistor (HOT)

of the horizontal deflection system is obtained directly from the DC voltage power supply. In the power design of LA 76931

series , there is a separate switch mode power supply that provides all of the DC voltage.

There will be always be :

A. A power switch . It enable to turn on or off the main power.

B. A set of rectifiers - in a bridge configuration - to turn the AC into DC. Small ceramic capacitors are place across the

diodes to reduce RF interference.

C. One large filter capacitor to smooth the unregulated DC. In countries with 220 -240VAC power , it will typically be around

400V DC.

D. In TV , it need switching typical power to provide stable DC .

E. A degauss control circuit including a Posistor ( a combination of heater disk and Positive Temperature Coefficient (PTC)

thermistor in a single package). When power is turned on ,a relatively high current is applied to the degauss coil wrapped

around the periphery of the CRT . The PTC thermister heats up, increase in resistance , and smoothly decrease to nearly

zero over a couple of seconds.

F. A standby power supply for the microcontroller and remote sensor. It derives from the DC voltage power supply.

Always use an isolation transformer when working on a TV because this is especially important-for your safety - when dealing with the

non- isolated line operated Power supply .

1.1 Typical power rectifier

The partial schematicbelow is foundedin the LA76931 series TV, some partsare not shownincluding the powerswitch, bypass capacitors

across the rectifie diodes (C503-C506)and the RFI line filter(L501,L502).

VD 503

RM 11C

AC 220-240V IN

VD 504

RM 11C

Fu501

T2.5A 250V

VD 505

RM 11C

VD 506

RM 11C

R502

3R 9

+

+

DC 300V OUT

-

-

C507

C518

150u/400V 1nF/1kv

The line fuse F501 is typically 2.5A, usually a normal fast blow type . Even so, it may not blow as a result of faults down

the line - the fusable resistor or regulator may fail first. The main bridge rectifier is composed of 4 discrete diodes(VD503-VD506)

but may also be a single unite. Failures - usually shorted diodes-are common. The main filter capacitor (C507) is very important .

A typical TV continue to work at normal line voltage without any noticeable degra-dation in performance( hum bars, hum in sound

or shutdown) even if this capacitor is reduced in value by 75%. Its value is therefore not critical.

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