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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The HOT has a storage time between 3us and 7us, thus the base - drive is switched off earlier, in a controlled way to properly remove the charge carriers

from the collector region in the HOT. The peak amplitude of base current and the way it is decreased determine the ultimate dissipation in the HOT and thus

subject of heavy optimization. This is hampered by the fact that there is much spread in HOT parameters.

Thus, the current in the flyback (ignoring the yoke components) is a nearly perfect sawtooth . The ramp portion is quite linear due to the essentially

constant +B across the flyback primary inductance. The current waveform can be easily viewed on an oscilloscope with a high frequency current probe.

The voltage across the C-E of the HOT is a half sinusoid pulse during the flyback (scan retrace) period and close to zero at all other times(-Vf of the

damper diode during the first half of scan; +VCE for the HOT during second half of scan).

Caution: without a proper high frequency high voltage probe, it is not possible or safe to observe this point on an oscilloscope with full +B. Excessive ringing

or other corruption would indicate a problem in the flyback,yoke, or elsewhere.

2.3 The deflection yoke connection

So, you ask : Why can t the yoke just be placed in series or parallel with the flyback primary?

There are several reasons including:

A The desired yoke current is not quite a sawtooth but includes two major corrections: S and E/W( described below). These cannot be applied easily with

such a configuration .

B The flyback also generates the HV and secondary output voltages and the primary current might then be affected by these and change as a function

of beam current(picture brightness) or audio level (although feeding the audio amplifiers from LOT windings is not common anymore).

2.4 S-correction circuit operation

The first correction to apply, in both directions, is S-cirrection. By simply putting a capacitor in series with each coil, the sawtooth waveform is modified

into a slightly sine-wave shape (the top and bottom are somewhat squashed). This reduces the scanning speed near the edges. Linearity over the two main

axis should now be good.

When we add in the joke components (only the horizontal deflection coil and S-correction capacitor or S-cap are actually shown above ) conditions are

only slightly more complex:

First , consider what would happen if instead of the S-cap, the yoke were connected to +B like the flyback. In this case, the total current would divide between

the flyback primary and the yoke . It would still be a sawtooth as described above . Of course , component values would need to be changed to provide the proper

resonant circuit behavior. .

That is called tuning of the flyback capacitor , to achieve the proper duration of the plyback pulse, matching the blanking time of the video signal, and to

achieve the proper peak flyback voltage, matching the Vce specification of V432 with a reserve of about 20%. That is two conditions, requiring two degrees of

design freedom . There are 3 freedoms : supply voltage, flyback capacitor and yoke inductance.

With the s-cap and yoke wired as shown as above, the inductance of the yoke and S-cap form a low pass filter such that voltage on the S-cap will be a

smoothed version of the pulses on the HOT collector (similar in effect to the +B feeding the flyback but not a constant value). The average value of the S-cap

voltage will be positive.

The S-capacitor together with the yoke inductance forms a resonant circuit whose frequency is tuned lower than the line frequency. It has the effect of

modifying the sawtooth current into a sine-wave shape. This is called S-correction . It reduces the scanning speed at the left and right edges of the screen.

Think of it this way : When the scan begins, the yoke current is at the maximum value it the direction to charge the S-cap. The voltage across the S-cap is

causing the current to decrease but the S-cap is also gaining charge so the rate of decrease is increasing. At the time the current passes through 0,the S-cap

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