(a ground from Tune Control 1A4) is not present, transistors 1A5Q4 and 1A5Q6 conduct. This applies
grounds to the common bases of transistors 1A5Q7 and Q8 and 1A5Q11 and Q12, respectively, and
prevents any error signals from amplifiers 1A5AR1-2 and 1A5AR2-2 from passing those points. How-
ever if a SERVO ENABLE (ground) signal is present, transistors 1A5Q4 and 1A5Q6 are turned OFF,
enabling both the A and B error channels. As the variable coil 1A8A5L1 is tuned toward the correct
position, the error signals applied to the servo amplifier will decrease until there is insufficient amplitude
to turn on differential amplifier 1A5AR1-2 or 1A5AR2-2. When this occurs, both servo outputs (SERVO
IN A and SERVO IN B) are grounded through transistors 1A5Q10 and 1A5Q14, respectively, and torque
motor 1A8A3B1 stops.
2-105. Tune Detector Module Assembly 1A10 contains the phase and level detector circuits which compare
the phase of RF signals from the cathode and plate circuits of the tubes (lA14V1 and 1Al4V2) in the
final RF stage of the power amplifier and provide dc signals to the servo amplifier and tune control
PCB'S for tuning. There are three separate passive networks in the phase and level detector circuitry. The
following discussion breaks out each network separately for better understanding.
2-107. The function of the RF input voltage sampling network is to produce a dc output voltage pro-
portional to the RF drive voltage at the cathode of power amplifier tubes 1A14V1 and 1Al4V2. The
RF drive is sampled through a capacitive divider network which consists of capacitors 1A14A2C3 and C4.
The output of the divider is applied through pin 8 of Tune Detector Module Assembly 1A10 to the
cathode of diode 1A10CR4. Here the signal is peak detected by diode 1Al0CR4 and capacitor 1Al0C11,
then filtered by capacitor 1Al0C12 and attenuated by resistor 1Al0R18 to produce the CATHODE
LEVEL signal. The CATHODE LEVEL signal is sent to Tune Control PWB Assembly 1A4 and to the
monitoring circuit on Meter PWB Assembly 1A1.
2-109. The function of the rf output voltage sampling network is to produce a dc voltage proportional
to the rf voltage at the plates of power amplifier tubes 1A14V1 and 1A14V2. The rf output voltage
is picked off of Plate Assembly 1A9, just after the junction of inductors 1A9L2 and L3, by a capacitive
divider network which consists of 1A9C4 and C5. The output of the divider is applied through pin 6 of
Tune Detector Module Assembly 1A10 and capacitor 1A10C13 to the cathode of diode 1A10CR1. Here
the signal is peak detected by diode 1A10CR1 and capacitor 1Al0C1, then filtered by capacitor 1Al0C2
and attenuated by resistors 1A10R3 and R5 to produce the PLATE LEVEL signal. The PLATE LEVEL
signal is sent to Tune Control PWB Assembly 1A4.
2-111. The function of the phase detector network is to produce two dc outputs (SERVO IN A and
SERVO IN B) each of which is directly proportional to one of the sampled rf inputs. These two signals
are applied to the input of Servo Amplifier PWB Assembly 1A5, and are equal when variable inductor
1A8A5L1 in the final tank circuit is correctly positioned and the power amplifier is working into an
essentially resistive load. Figure 3-74 shows the complete phase detector. Inductor 1A10L3 and
resistor 1Al0R2, in the CATHODE SAMPLE input, are used to compensate the input impedance of
capacitor lA10C10 and resistor 1A10R15 by completing an all-pass circuit. Similarly, capacitor
1 Al0C3 and resistor 1Al0R1, in the PLATE SAMPLE input, form another all-pass circuit with
inductor 1Al0L1, transformer 1A10T1, and resistor 1A10R7. This is necessary so that the amplitude and
phase differences between the CATHODE and PLATE voltage samples can be maintained accurately as
they go through voltage dividers to the input of the detector.