Section II. THEORY OF OPERATION
3-3. Switch Bracket Assembly
output for use by the receiver circuitry. The ac input is
applied, through power ON/OFF switch S4 and fuse F1,
to the primary of transformer T3. Two primary windings
a. The switch bracket assembly houses a variable
of T3 are connected in series for 230 Vac operation and
in parallel when operation from 115 Vac input is
attenuator and a front-end tuning circuit for the receiver.
required. The secondary of transformer T3 provides 24
V (rms) output which is full-wave rectified by a bridge
coupled through C3 to the attenuator circuit formed by
comprising diodes CR1 thru CR4. Filtering is provided
S1-a, S1-b and resistors R1 through R9. The amplitude
by C2, resulting in approximately 34 Vdc at the collector
of the incoming rf signal is proportional to the signal
of Q1. The dc voltage at this point is unregulated and
picked-up by the antenna and, under normal operation,
varies with the load current as well as the line voltage
remains constant. Depending upon the amplitude of the
rf input, appropriate attenuation is introduced to the
b. The 28 Vdc regulator circuit is formed by Q1,
signal so as to establish a suitable reference signal level
Q2, CR5, C1, R1 and R2. The base of Q1 is held at a
for the receiver.
b. The attenuator consists of two 20 dB pads and
constant potential of 30 V by zener diode CR5. The
voltage at the emitter of Q1 therefore equals the
one 40 dB pad. Rotary switch S1 permits selection of
breakdown voltage of CR5 less the base emitter voltage
one or more sections, permitting up to 80 dB of
drop of Q1.
This voltage is constant (regulated)
attenuation in 20 dB of attenuation in 20 dB steps.
regardless of the load current and the Q1 collect voltage.
c. Transistor Q2, together with resistor R2, provide
When RF ATTEN switch S1 is set to 0
the current limit function for the +28 V output. If a
dB position, the attenuator circuit is
malfunction within the receiver causes the current drain
on the +28 V bus to exceed 0.5 amps, the voltage drop
across R2 is sufficient to turn Q2 on. AS a result, the
c. Resistors R1, R2 and R3 form the first 20 dB
base drive current for Q1 is partially diverted to Q2 and
the current through Q1 is limited to a safe value.
pad while R4, R5 and R6 make up the second 20 dB
d. The regulated +28 V output from Q1 is used to
pad. The 40 dB pad is formed by R7, R8 and R9. The
operate relay K1, POWER lamp DS1, ALARM lamp
input impedance and output impedance of the switched
DS3, NORM lamp DS2, if amplifier assembly A2, and
attenuator is 50 ohms.
d. The output from the switched attenuator is
the audio amplifier assembly A2. In addition, the +28 V
output is fed to the +12 V regulator comprising Q3, Q4,
applied to a tuning circuit formed by T1, L2, T2 and C1.
CR6 and associated components. The +12 V regulator
Transformer T1 is a step up transformer which raised the
circuit operates as follows:
output impedance of switched attenuator from 50 ohms
(1) The emitter of Q4 is held at a voltage of 6.2
to 450 ohms. The secondary of transformer T1 feeds a
Vdc by zener diode CR6. The base of Q4 senses the
series tuned circuit comprising L1, T2 and variable
+12 V bus through divider R4 and R5.
capacitor C1. The tuned circuit output is taken from T2
(2) Any change in the +12 V bus causes a
which also serves to restore the 50 ohm impedance.
corresponding change in the collector current of Q4 and
Note that stepping up of impedance by T1 is performed
hence the base drive for Q3. The conduction of Q3 is
to permit use of a convenient value for tuning capacitor
therefore automatically controlled so as to produce
C1 (6.7 pF to 140 pF); if transformer T1 was not used for
constant + 12 Vdc output at its emitter.
impedance transformation, the value of C1 would be
(3) As the input to the +12 V regulator is the
e. Variable capacitor C1 is adjusted at the time of
output of the 28 V regulator, the over-current protection
provided by R2 and Q2 also safeguards the +12 V
installation to tune the receiver to its operating
frequency. The tuned circuit has a high Q, thus rejecting
all unwanted signals that may be present at its input.
3-5. RF PCB Assembly
3-4. Power Supply
a. The RF PCB performs the following functions:
a. The power supply operates from 115 Vac (or
(1) Accepts rf input from the tuning circuit on
230 Vac) input and provides +12 VDc and + 28 Vdc