b. Temperature Controller Circuit (fig. 2-12). U1 is an
(2) The varactor bias voltage (J1 terminals 12
and 13) is set in Unit 1 during gain-bandwidth alignment.
integrated circuit zero-voltage switch. Q1 is a thyristor
The presence of the voltage, not its value, is the
that, when triggered on, conducts current for the heater
important factor. Because the bias circuit cannot tolerate
circuits. The function of U1 is to trigger on Q1 when heat
any appreciable dc current (10 microamp, maximum) no
is needed. The pulse output to Q1 is at U1 terminal 4.
impedance measurements can be made. Current will
This pulse is produced when the ac voltage crosses zero
cause irreparable damage to the varactor diode mounted
voltage line. The thyristor then conducts until the voltage
again returns to the zero line (a half cycle). The process
device that provides parametric amplification. Bias
is repeated until the heat produced is at the preset level.
voltage can be measured on Unit 1 at 1J6, SECOND
The thermistor RT1 (a negative temperature coefficient
STAGE VARACTOR VOLTAGE TEST.
resistor) is connected across card terminals C and A and
(3) The temperature of Module 2A2 is
U1 terminals 7 and 13. On U1, terminal 7 is the common
maintained by controlling power to three 100-W heaters
terminal. The pulsing of Q1 occurs when the resistance
located in heat sinks adjacent to AR1. The heaters are
of RT1 is greater than the resistance of R2 plus R3.
wired in parallel with a thermostat in series with the
Resistor R2 is variable and is preset to provide the
heaters. The thermostat is normally closed but will open
required temperature. As the heat sink on which RT1 is
if the temperature reaches 150F. This is a safety device
mounted becomes warmer, the resistance of RT1
to prevent overheating.
Capacitor C1 across the
decreases and when it matches that of R1 + R2 U1 does
thermostat terminals is used to suppress transients when
not produce a pulse and the thyristor is not pulsed on;
the thermostat switches. Thermistor RT1 senses a
hence, no current flows through the heaters. R2 is a
change in temperature of the heat sink on which it is
series limiting resistor used to protect U1. C1 is an input
located and is the controlling device for temperature
capacitor and C2 is an output capacitor, both for U1.
controller A1. Temperature controller A1, in response to
Because of the nature of U1 and Q1, impedance
AT1 resistance variations, switches on and off power
measurements are not practical. Voltage measurements
(120 12 Vac) to the heaters to maintain the
(120 12 Vac) can be made be- tween card terminals L
temperature at 140 4F. Primary power to the heaters
and C (power input) and J and L (load-heater circuit).
enters through J1 terminals 1 and 3. This power comes
Under normal operating conditions, the J-L voltage is
from the output side of Unit 1 line filters 1 FL2 and 1 FL4
intermittent. One temperature controller is used in
and is present whenever MAIN POWER circuit breaker
Module 2A2 and three, in Module 2A1. The circuits are
identical; the settings of the variable resistor R2 differ in
the 120-V power line is mounted on the panel of Module
each to provide a different control temperature.