Figure 2-15. Power amplifier, simplified schematic diagram.

TM 11-6660-204-25

Figure 2-15. Power amplifier, simplified schematic diagram.

a rectifier and the other half ( V407B ) functions

through dropping resistor R414. The screen-grid

as an amplifier control. In operation when a signal

is bypassed to circuit ground by capacitor C407.

is received from the power amplifiers, the pen

Cathode bias is produced by the voltage drop

rises from the chart when it travels to its new

across cathode resistor R416, which is unbypassed

position. The solenoid, however, does not lift the

for degenerative purposes.

pen but merely releases a spring that performs

c. The ac error signal is applied from the plate

the lifting. An energized solenoid compresses the

of tube V310B, through coupling capacitor C422,

spring and allows the pen to lower.

across grid resistor R440, to the control grids of

b. The amplified ac error signal from the plates

tubes V405 and V406 and through grid-current

of tubes V405 and V406 is fed through coupling

limiting resistor R415. The output power is ap-

capacitor C410 to pen-lifter potentiometer R215.

plied to the control winding of balancing motor

A portion of this ac error voltage, determined by

B501 and C409 which is a parallel-tuned circuit.

the position of the PEN LIFTER potentiometer

arm, is rectified by tube V407A and dc voltage is

d. When a signal is applied to the grid of tubes

developed across resistors R419 and R420. The

V405 and V406, an amplified drop with the proper

voltage developed across resistor R420 and fil-

phase is developed across the tuned circuit of the

tered by capacitor C403 is applied to the grid of

motor winding (B501) to drive the motor. A por-

control tube V407B. The entire rectifier circuit is

tion of this plate signal voltage is also applied,

returned to a negative 30-volt bias supply which

through capacitor C410, to pen-lifter potentiome-

is obtained from the junction of bleeder resistors

ter R215 to operate the pen-lifter circuit. Jacks

R436 and R437, across the negative regulated 175-

J407 and J309 can be used for checking grid and

volt supply. The negative 30-volt bias holds pen-

plate voltages.

lifter control tube V407B at cutoff during no-sig-

nal conditions. Potentiometer R215 is used to ad-

2-16. Pen-Lifter Circuit

just pen-lifter operation for various noise levels

(figs. 2-16 and FO-6)

and is set low so the internal noise of the ampli-

fier circuit does not overcome the fixed bias of

a. The pen-lifter circuit uses both sections of

tube V407B, but is set sufficiently high to obtain

miniature twin-triode tube V407. One half of the

reliable operation for input signals.

tube (V407A) is connected as a diode and used as

2-18