Diode Gates

The Diode Gates, consisting of eight diodes, act as switches that are controlled by the Channel Switching Logic circuitry. The Q- and Q-outputs of U317A (pins 5 and 6 respectively) control forward biasing of the diodes to turn the gates on and off.

CHANNEL 1 DISPLAY ONLY. To display only the Channel 1 signal, the CH 1 Enable signal (U317A pin 5) is HI and the CH 2 Enable signal (U317A pin 6) is LO.

With CH 1 Enable HI, gate diodes CR187 and CR177 are reverse biased (see Figure 3-3). Series gate diodes CR 188 and CR 178 are forward biased, and the Channel 1 vertical signal is allowed to pass to the Delay Line Driver. A LO CH 2 Enable signal applied to the Channel 2 gate diodes forward biases CR287 and CR277, and the Channel 2 vertical-signal current is shunted away from series diodes CR288 and CR278. The Channel 2 series diodes are reverse biased, and the Channel 2 signal current is prevented from reaching the Delay Line Driver.

CHANNEL 2 DISPLAY ONLY. When CH 2 VERTICAL MODE is selected, the CH 1 Enable signal goes LO and the CH 2 Enable signal goes HI. The Channel 1 signal is blocked, and the Channel 2 signal reaches the Delay Line Driver.

ADD DISPLAY. Both Diode Gates are biased on to pass the Channel 1 and Channel 2 vertical signals. The channel signal currents are summed at the input to the Delay Line Driver. The Add Enable signal supplies the extra current required to keep both Diode Gates forward biased and to maintain the proper dc level at the base of the Delay Line Driver input transistors (Q331 and Q341).

ALTERNATE AND CHOPPED DISPLAY. The Diode Gates are switched on and off by the Channel Enable signals from the Channel Switching Logic circuit. When ALT VERTICAL MODE is selected, the Diode Gates are switched at the end of each trace. For CHOP VERTICAL MODE, the gates are switched at a rate of about 250 kHz.

X-Y DISPLAY. Setting the SEC/DIV switch to the X-Y position activates the X-Y display feature. The Channel 1 Diode Gate is held off, and the Channel 2 Diode Gate is biased on. The Channel 2 signal is passed to the Delay Line Driver and ultimately to the crt to provide the Y-Axis display deflection. The X-Axis deflection signal is supplied to the XY Amplifier (Diagram 7) from the Channel 1 signal via the Internal Trigger Amplifier (Diagram 4).

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