Trigger Level Comparator

The Trigger Level Comparator circuit determines both the trigger level and slope at which a triggering signal is produced. Transistors U460E and U460B form a comparator circuit. It compares the trigger signal level applied to the base of U460E with the reference dc level set by the TRIGGER LEVEL potentiometer (R455) and applied to the base of U460B. Slope switching is accomplished by controlling the biasing of transistor pairs U460A-U460D and U460C-U460F.

When AUTO or TV FIELD triggering is selected, the Auto Trigger circuit maintains a dc level range at the base of U460B that is dependent upon the amplitude of the trigger input signal. In this instance, the Comparator (U460E and U460B) determines the point on the input trigger waveform at which the Schmitt Trigger circuit will produce an output.

When NORM triggering is selected, the TRIGGER LEVEL potentiometer (R455) is set manually to a dc level that will produce a trigger signal at the output of the Comparator. If the trigger signal amplitude at the base of U460E is below the reference level, the Schmitt Trigger circuit will never switch. If the trigger signal is above the reference level, the Schmitt Trigger circuit output will switch HI and remain HI until either the trigger signal is decreased or the reference dc level is increased.

The TRIGGER SLOPE switch (S464) controls the bias on U460C and U460F. When set to the positive slope position, the ground is removed from the bottom end of R464, and the forward bias is then determined by the voltage divider formed by R462 and R463. Both U460C and U460F are biased into conduction and carry the signal current from the Comparator transistors. Moving the SLOPE switch to the negative slope position grounds the bottom of R464 and reduces the bias level of U460C and

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