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LA76931 Datasheet PDF - ETC

Part Number LA76931
Description USOC SERIES SERVICE MANUAL
Manufacturers ETC 
Logo ETC Logo 



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LA76931 datasheet, circuit
USOC(LA76931) SERIES
SERVICE MANUAL

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LA76931 equivalent
CHAPTER ONE Power Supply
3
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 variousDC 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 circuitincluding a Posistor( a combination of heater disk and Positive Temperature Coefficient (PTC)
thermistor in a single package). When power is turned on ,arelatively high currentis 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 usean isolation transformerwhen working on a TV because this is especially important-for your safety- when dealing with the
non- isolated lineoperated 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 rectifiediodes (C503-C506)and theRFI 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
150u/400V
C518
1nF/1kv
-
-
The line fuse F501 istypically 2.5A, usuallya normal fastblow type .Even so, itmay not blow as a resultof faults down
the line -the fusable resistoror regulator may fail first. The main bridgerectifier is composedof 4 discretediodes(VD503-VD506)
but may also be a singleunite. Failures -usually shorted diodes-arecommon. The main filter capacitor(C507) is veryimportant .
A typicalTV continue to work atnormal line voltagewithout any noticeabledegra-dation in performance(hum bars, humin sound
or shutdown) evenif this capacitoris reduced in value by 75%.Its value is therefore not critical.

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Information Total 30 Pages
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