Auto Trigger Circuit

When either AUTO or TV FIELD triggering is selected, the Auto Trigger circuit detects positive and negative peaks of the input trigger signal and produces output voltages that set the TRIGGER LEVEL control range to within the peak-to-peak amplitude of the triggering signal. The peak detectors are disabled when S611 is set to NORM, and fixed voltage levels are applied to both ends of TRIGGER LEVEL potentiometer R455.

In either AUTO or TV FIELD, the TRIGGER MODE switch (S611) opens the Auto Disable signal line to allow CR503 and CR504 to become reverse biased. This action isolates the voltage divider network (composed of R525, R527, R528, R526, and Q519) from the + inputs of U507A and U507B. The peak detectors (composed of 0503 for the positive peak and Q504 for the negative peak) become enabled when the Auto Disable signal is removed.

The trigger signal is applied to the bases of Q503 and Q504 via R444. Positive trigger signal peaks bias Q503 into conduction, causing its emitter voltage level to rise to the peak level of the trigger amplitude minus the base-to-emitter voltage drop.

Capacitor C503 charges up to the positive emitter voltage level. The charge is retained between trigger pulses due to the long RC time constant of R505 and C503. The comparator voltage is applied to U507A pin 3 which is a voltage follower and level shifter that sets the voltage at one end of the TRIGGER LEVEL potentiometer (R455). Transistor Q507 provides the feedback path for U507A and thermally compensates for Q503. The base-to-emitter drop of Q507 corrects for the dc offset introduced by Q503, and potentiometer R511 is adjusted to balance out dc offsets introduced from the trigger circuitry.

The negative peak detector operates in the same manner as the positive peak detector, with corresponding components performing the identical circuit function on the trigger-signal negative peaks.

When S611 is set to NORM, +8.6 V is applied through the switch to R525 and R517. Transistor Q519 is biased into saturation by the positive voltage, and both CR503 and CR504 become forward biased. This action reverse biases peak detector transistors Q503 and Q504 to prevent the trigger signal from affecting the TRIGGER LEVEL control range.

U460F. The fixed bias level on the bases of U460A and U460D is now higher than the bias on U460C and U460F so that U460A and U460D carry the signal current from the Comparator transitors. The collectors of U460A and U460D are cross connected to the collectors of U460F and U460C, so the resulting trigger signal output is inverted.

With CR503 and CR504 forward biased, the voltage divider network (R525, R527, R526, and R528) sets the input voltage to U507A pin 3 and U507B pin 5. A fixed positive output voltage from U507A pin 1 is applied to one end of R455, and a fixed negative output voltage from U507B pin 7 is applied to the other end of R455.

0 0

Post a comment