Auto Baseline Circuit

The Auto Baseline circuit (composed of U640A, Q605, and associated components) is enabled in both AUTO and TV FIELD triggering modes. This circuit provides a signal to the Sweep Generator circuit (Diagram 5) that initiates a sweep if a triggering signal is not received by the Schmitt Trigger circuit within a period of about 100 ms. A second output from the circuit illuminates the TRIG'D LED on the instrument front panel when the sweep is triggered.

When adequate triggering signals are being received, the output of Q493 is applied to pin 5 of monostable multivibrator U640A. The negative-going edge of the signal causes pin 6 of U640Ato switch HI. The HI forward biases CR615, and Q605 is then biased into conduction. With Q605 conducting, the Auto Baseline signal line is held LO to prevent the Sweep Generator circuit from free running.

The amount of time that pin 6 of U640A stays HI without receiving an input signal is determined by timing components R614 and C614. If a trigger signal is not received in about 100 ms, pin 6 of U640A will go LO and Q605 will be biased off. The Auto Baseline signal line then goes HI through pull-up resistor R610, and the Sweep Generator free runs to produce the baseline trace.

In NORM triggering mode, the Auto Disable signal (+8.6 V) is applied to the base circuit of Q605 via CR611 and R611. The signal holds Q605 forward biased and prevents the Sweep Generator from free running.

The other function of the Auto Baseline circuit is to illuminate the TRIG'D LED when the sweep is properly triggered. As long as U640A pin 6 remains HI (triggering signals occurring with the proper time), TRIG'D LED DS618 will be illuminated. The trigger mode in use does not affect the operation of the TR IG'D LED.

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