HEWLETT PACKARD (HP) 120B
Service Manual

Section IV Paragraphs 4-9 to 4-15

Model 120B tween the two triodes. Potentiometers R21 (VERNIER control) and R20 (CAL) adjust the resistance between the plates and therefore determine the gain of the amplifier. The VERTICAL VERNIER control provides a ten-to-one variation in gain between ranges of the VERTICAL SENSITIVITY switch. The second differential amplifier, V2, has potentiometer R32 (vertical POSITION control) between its cathodes. This potentiometer controls the current distribution between the two triodes and thus determines the vertical position of the c r t display. The third differential amplifler, V3, is the output stage. Synchronization signals a r e taken from the plates of V3 for application to the TRIGGER SOURCE switch. Since the horizontal sweep circuits trigger only on the negative slope of the signal, provision i s made to take the proper synchronizing signal from either plate of V3. 4-14. One section of the BEAM FINDER switch i s in the cathode circuit of V3. When pressed, the switch increases the common cathode resistance and so r e duces the gain of the stage that no amount of unbalance ahead of V3 can deflect the sweep off the crt. 4-15. The calibrator circuit consists of neon lamps V4, V5, and associated circuit parts. When VERTICAL SENSITIVITY switch S2 i s placed in the CAL position, V4 ionizes and capacitor C14 begins charging. As the voltage across capacitor C14 rises, the voltage a t the junction of V4 and R52 r i s e s proportionally. When the voltage a t this junction reaches the firing potential of V5, V5 ionizes. Since the conducting potential of V5 is less the firing potential, the voltage a t the junction of V4 and V5 drops, and V4 deionizes. C14 discharges through R53, and the potential across V4 increases. When the potential across

4-9. CIRCUIT A N A L Y S I S .
4-10. VERTICAL AMPLIFIER.

4-11. For a c coupling, capacitor C1 is placed in s e r i e s with the signal path by means of the AC-DC switch. (See figure 5-6. ) The input signal is then applied to VERTICAL SENSITIVITY switch S2. When the switch is in the CAL position, the input of the amplifier i s directly connected to the output of the calibrator (V4, V5, and associated circuit parts). When the VERTICAL SENSITIVITY switch is in other positions, precision frequency-adjusted attenuators a r e inserted. These attenuators give a ten-to-one attenuation between adjacent positions. The input signal is then applied to the control grid of amplifier VIA. 4-12. Balanced input to differential amplifier V1B is obtained by removing the ground jumper across r e sistor R1 (ground strap on front panel) and applying a balanced signal input. Balanced input i s useful in applications where it is desired to simultaneously amplify the out-of-phase (differential) signal and attenuate the in-phase (common mode) signals, such a s hum, noise, etc. This rejection is an inherent property of differential amplifiers. The common-mode signals a r e attenuated by 40 db (100:1), whereas any differential input is amplified. 4-13. The vertical amplifier consists of three s e t s of balanced differential amplifiers (Vl, V2, and V3) in cascade. The three stages a r e neutralized by plate-to-grid c r o s s neutralization. The f i r s t stage, V1, has balance and gain adjustments. The balance adjustment (BAL) is potentiometer R16 in the cathode circuit, which adjusts the current distribution be-

9 [TRIGGER
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LEVEL

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GATE-OUT CATHODE FOLLOWER V204

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UNBLANKING GATE TO HIGH-VOLTAGE POWER SUPPLY

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NETWORK TRIGGER v202

SWITCH DIODE

FROM ITRIGGERSOURCE ISYNC -+ SW

TRIGGER GENERATOR v201

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SAWTOOTH INTEGRATOR V205A

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A
CLAMP V203A

STOP

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SAWTOOTH CATHODE

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:-HOLD- O F F CATHODE FOLLOWER V203B
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FOLLOWER V205B

ISWEEP MAGNIFIER]
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A
TO HORIZONTAL AMPLIFIER
83-U-267

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Figure 4-2.

Sweep Generator, Block Diagram 01260-1