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LA76931

64Pin LA76931 for Color Television Receiver / SERVICE MANUAL

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4

1.2 Self - excitation oscillation Circuit

The DC electricity (300V) is divided into two bypasses . One arrives the collector of V513

through pin3 and pin7 of T511, the other arrives the base of V513 through startup resistance ( R520,

R521,R522), limiting resistor (R524).

The startup current cross the B-E of V513 and make it working in the amplificatory condition.

The current (Ic ~~ Ib)flow from pin3 to pin7 of T511 showing right.

The current induct the voltage between the pin1 and the pin2 of T511(pin1 +,pin2 -),and the

voltage can charge the capacitor C514 through R519. The charge current input the base of V513

through R524 , increase the current between B-E of V513 . The current Ic will increase, it causes

the inductive voltage increases. The process is shown below:

+ 300V

+ T511

V513 Ib

V513 Ic

T511( pin 3 - pin7) current

3

R520

R525

Through R519,C514,R524

T511(pin1- pin2) Ug

R521 R522

C516

7

This is a positive feedback process . The transistor(V513) can come up to

the completely saturationin the condition of Ib > Ic/

VD 514

Ic can not increase very quickly to the maximum because of the function

C514

of induction in pin3 and pin7 of T511. It will undergo a linear increasing

process . In this process , the current variational ratio supports the

constantly charge voltage. This is the flat-top period of self - excitation

R519

pulse.

300V -

-

R524

Ib Ic

Ie

+ V513

C517

1

Ug

2

The current, which the voltage inducted from pin1,pin2 of T511

Diagram 1

charge the capacitor (C514) , decreases gradually while time going.

The current in the base of V513 (Ib) also decreases simultaneously.

When Ib equals ¦IÂc , V513 run into the amplification. This make the

current between pin3 and pin7 of T51 1 to decease. The voltage(Ug)

between pin1 and pin2 inducted by this current will change its

polarity ( pin1 is ¡-¡°p±in2 is¡+°¡).±The current produced by Ug charge

C514 reversely through C517,R524,L511 and discharge C514. The diverse current flow back pin1 of T511

via R519. In this process, Ug does not put any current in the base of V513, on the contrary, decreases

the startup current of V513 rapidly. V513have to shut down .

This means finishing the behind porch of pulse. Although V513 completely shut down, the current across

pin3 and pin7 still exists and decreases rapidly through C516 and R525. Ug charge

C514 reversely . The shut-up condition of V513 is still maintained. This is called flat-bottom period or

pulse interval. Obviously ,when the reverse charge current is less than startup current, V513 will turn on

again and finish another self-excitation oscillation.

The self-excitation oscillation circuit repeat again and again and produce switchmode signal about 40Khz.

We can see that the switchmode pulse on base of V513 derive from itself including V513,T511. It is not

generated by special circuit. So we call it self-exiciation.

1.3 Regulative control circuit

The turn-on and turn-off of V513 lead to the current between pin3 and pin7 of T511 existent or not,

increasing or decreasing . The variational current produces all kind of pulse voltage in all coils l of T511

including positive feedback coil (pin1-pin2) . The inductive voltage from pin11 to pin12 is rectified by

VD 551 , filtered by C561 and formed the main output DC voltage (+B).

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