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LA76931

64Pin LA76931 for Color Television Receiver / SERVICE MANUAL

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The scenario described below is only true in the steady state- the first few scans are different because the picture tube capacitance is still discharged.

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This represents a short-circuit at the secondary side of the flyback. It prevents proper demagnetizing , hence the core will go into saturation (unless special soft

start measures have been taken, like a V1 supply that comes up slowly). Generally, a hard start of the line deflection circuit represents a very heavy load on the

HOT. This will happen after a picture tube flashover or if the +B is connected suddenly (due to intermittent contact)and can mean instant death to the HOT due

secondary breakdown.

2.2 Basic deflection circuit operation

A very simplified circuit is shown below - many components needed to create a practical design have been omitted for clarity. First concentrate only the

portion of the schematic shown below to the left of the yoke components:

+B

The current in the flyback primary and collector of the hot are not equal. The horizontal

deflection yoke, damper diode , Hot collector, snubber HV capacitor(s),and flyback primary all

connect to the same point. We begin our adventure at the end of the scan-retrace-when the

flyback period begins:

Part of T471

Flyback

Primary

At the end of scan ,current is flowing through the flyback primary to the HOT, V432.

At the start of the flyback period, V432 turns off.(This must be done in a controlled manner-

not just a hard shutoff to minimize stress on the HOT-but that is another story). Since current Horizontal

in an inductor (the primary of the flyback has inductance) can not change instantaneously,

the current is diverted into the snubber capacitor,C435,C437.The inductance of

the flyback primary (T401) and C438,C436 forms a resonant circuit so that the

Drive

T431

Damper

Diode

Horizontal

Yoke

voltage climbs on C438,C436 as the current goes down. At its peak ,this voltage

will be 1000V to 1500V.

C438,C436 now begins to discharge in reverse through the primary of T401

(back into the +B supply-the filter capacitor will stabilize the +B output) until its

voltage (also C-E of V432) reaches o.

Drive

Stage

(not

shown)

V432

HOT

(inside

V432)

D1

S-Correction

capacitor

C441

If there were no damper diode(D1), this voltage would go negative and continue to oscillate

as a damped sinusoid due to the resonant circuit formed by T401 and C438,C436(and the other components)

However ,D1 turns on as the voltage negative and diverts the current through it clamping the voltage near0

(-Vf for the diodes).

Snubber

Capacitor

C438,C436

Note that the damper diode D1 is built into the HOT V432 .

Above steps have accomplished the flyback function of quicky and cleanly reversing the current in T401(and ,as we will see,the deflection yoke as well). The

full flyback (and yoke current) are now flowing through the forward biased damper diode,D1.

At the beginning of scan , the damper diode (forward biased) carries the bulk of the current from the yoke and flyback.. The nearly constant voltage of the +B

across T401 results in a linear ramp of current now through the damper diode since it is still negative and decreasing in magnitude.

At approximately mid-scan, the current passes through zero and changes polarity from minus to plus . As it does so, the damper diode cuts off and the HOT

picks up the current (with a voltage drop of +VCE ). Current is now flowing out of the +B supply.

The base-drive to the HOT must have been switched on before this point! Timing is not very critical as long as it happens between the end of the flyback and

zero crossing of the summed current.The location of the zero crossing depends on the secondary load, notably the beam current. Larger beam current requires

that the HOT be switched on earlier. The designer has to do some optimizing here...

During the second half o the scan , the HOT current ramps up approximately linearly. This is again due to the nearly constant voltage of +B across the

inductance of the flyback primary.

Near the end of scan , the HOT turns off and the cycle repeats.

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