SERVICE

MODEL 8559A

Beamfinder

With 1 dB per division selected, the baseline is off-screen. Without a visible signal present, there is no displayed trace. This condition could be misinterpreted as a display malfunction. On an H P 180 series mainframe, a visible trace can be produced by pressing the BEAMFINDER switch on the mainframe. This causes the - 12.6V on the beamfinder line to be removed, turns Q19 off, and disables the current source, U2c. The vertical display then reverts to the 10 dB per division mode while the horizontal display sweep is narrowed and the trace is intensified by the mainframe. The H P 853A mainframe does not require a BEAMFINDER, therefore the mainframe always supplies - 12.6V to the beamfinder line.

-5.5V Temperature CompensatingSupply (B)

The - 5.5V Temperature Compensating Supply controls four current sources: U2c, U2d, 413, and Q15. The temperature sensing element, U2e, is connected as a diode and tracks the base-emitter temperature changes of the current-source transistors. Approximately -0.6V is provided by the voltage regulator (zener) diode, VR1, and transistor U2e.

Vertical Driver (E)

The Vertical Driver is a differential amplifier that consists of 42, 4 3 , 4 6 , 4 7 , and 414. Transistor Q15 is a temperature compensating current source (see Figure 8-71). The vertical signal from the preamplifier (0 to 800 mV) is converted to the push-pull signal needed to drive the vertical deflection plates. Dual transistor 414 is used as the input stage of the driver circuit. Its base voltage is adjusted from the front panel with the vertical position (VERT POSN) control A2AlR6. This establishes the input reference voltage.

I

LNORM I N P U T BYPASS + 1 2 V NORM - 1 2 V

OUTPUT TO

V E R T I C A L

DEFLECTIO N

P L A T E S

YNORM

I N P U T

Q 15 9 CURRENT SOURCE

015 Z 1 O m A

FIGURE &7l. VERTICAL DRIVER. SIMPLIFIED SCHEMATIC

The gain of the vertical driver is set by a voltage divider consisting of R39, R42, and vertical gain control (VERT GAIN) A2AlR7. This gain control adjusts the ratio of the voltage divider. Transistor pairs Q2/Q6 and Q3/Q7 are current-to-voltage amplifiers driven by the current from the collectors of Q14a and Q14b, respectively. Diodes CR5 through CR8 prevent the bases of 4 2 , 43, 46, and 4 7 from being driven negative more than 0.6V Resistors R44 and R52 decouple the capacitive load presented by the CRT plates from the emitter of 4 2 and 4 3 . Decoupling is necessary to prevent overshoot and ringing in the Vertical Driver.