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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10

2.7 Horizontal Output Transistors

Most deflection processors generate a base-drive pulse with a constant duty-cycle. This means that also the switch -on moment of the HOT will vary with the

load. This makes it extra difficult to optimize the base-drive because there is only a limited time interval where the HOT may be switch on and that interval is

shorter with high beam current load. On-time is typically between 50% and 55%, depending on the IC.

A more important rational is that a transformer is nice easy way of impedance matching the horizontal driver circuit to the few ohm input impedance of the

horizontal output transistor base which requires upwards of several amps for proper drive. A typical driver transformer may be in the 5-10:1 turns ratio

representing 25-100:1 impedance ratio.

Usually, the primary voltage is * constant*when the driver transistor is ON and thus the HOT is OFF. Then when the driver switches OFF, the stored magnetic

energy switches the HOT to ON. This is called non-simultaneous base drive, which is most common. The primary voltage that you see then is mostly a trans-

formed version of the secondary voltage, over the series base impedance. The voltage at HOT=ON is not forced from the primary side.

Horizontal output transistor specs and substitution:

Every line transistor has its own requirements for:

* Amount of base drive current, especially the Ib at end-of-scan.

* Waveform of base drive current (rising, steady, falling)

* Speed of reduction base drive current at switch-off.

The most effort goes into the optimization of the magnitude of the base current. The problem is : gain spread. There used to be other spread factors

influencing the dynamic transistor parameters but these have been mostly eliminated by better process control at P.S. . You have to find *one* optimum drive

so that neither the high-gain nor the low-gain type will dissipate too much.

With horizontal output transistor, it is *not* true that bigger is better . If you substitute a heavier transistor (more amps, more volts, more watts, faster

switching, whatever ) for a lighter one, then there is a very big chance that it will fail earlier, not later. The reason is that the drive conditions will now be wrong(

most likely underdrive) and the transistor will overheat from too high conduction losses . So do yourselves a favour and get a correct replacement type.

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