Vertical Output Amplifier

The Vertical Output Amplifier, also shown on Diagram 3, provides final amplification of the input signals for application to the deflection plates of the crt. Signals from the Delay Line are applied to a differential amplifier input stage composed of Q350 and Q360. The Delay Line is terminated in the proper impedance by resistors R338 and R348. Resistor R355 sets the gain of Q350 and Q360. Thermal compensation of the stage gain is provided by thermistor RT356, connected in series with R356 across R355. The RC networks connected across R355 provide both low- and high-frequency compensation of the stage.

The differential output is applied to output transistor pairs Q376-Q377 and Q386-Q387. These transistors form a common-emitter shunt-feedback amplifier stage, with R376, R377, R386, and R387 serving as feedback elements. Capacitors C377 and C387, connected across R377 and R387 respectively, provide increasing negative feedback as the signal frequency rises to limit the amplifier bandwidth at the upper frequency limit. Output voltage from the amplifier is divided between the two transistors of each half. The signal voltage applied to the crt vertical deflection plates is the sum of voltage drops across the pairs (Q376-Q377 and Q386-Q387). The deflection voltage is proportional to the signal current driving the bases of Q376 and 0386.

BEAM FIND switch S390 (Diagram 6) normally supplies -8.6 V directly to R390 to set the stage bias. When the BEAM FIND button is pressed in and held, the direct voltage is removed and the -8.6-V bias is provided via series resistor R391. The output voltage swing is thereby reduced to hold the vertical trace deflection to within the graticule area.

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