Simplified Circuit Theory - Hammarlund SP-600-JX-21 Service Instructions Manual

Section IV
Paragraphs
4-22
to
4-28
T.O. 31R2-4-101-2
4-22. SIMPLIFIED CIRCUIT THEORY.
4-23. GENERAL. A schematic analysis of the receiver
circuits is given in paragraphs 4-24 through 4-60. The
radio receiver provides continuous coverage over a fre-
quency range of 0.54 mc to 54.0 mc in six bands. The
"BAND CHANGE" control (17, figure 1-2) positions a
rotary turret assembly which inserts the proper r-f tuner
subassemblies for each band into the r-f amplifier circuits
and the circuit of the variable-frequency heterodyne os-
cillator. All simplified schematic circuit diagrams show
the circuit elements used on the 0.54-to-I.35-mc band.
The antenna input circuit of the receiver is designed to
match a 95-ohm transmission line. A balanced doublet
or a single-wire antenna installation may be used.
4-24. R-F AMPLIFIERS. (See figure 4-3.) Two double-
tuned r-f amplifier stages utilizing 5749/6BA6W pentode
tubes comprise the input stages of the receiver. These
r-f amplifiers, whose bias can be controlled either manu-
ally by the "RF GAIN" control or automatically from
the a-v-c circuit, increase the amplitude of the r-f signals
supplied from the antenna system, provide for an in-
creased signal-to-noise ratio, and provide a high order
of image and adjacent-channel signal rejection.
4-25. Balanced antenna input transformer Ll, reso-
nated by sections CIA and CIB of the "TUNING" con-
trol, is an impedance matching device which applies the
r-f signals through coupling capacitor C18 to the control
grid of first r-f amplifier tube VI. Capacitor C2 is used
to trim the tank circuit at the upper end of the band,
and the slug in the secondary of Ll is used for tracking
at the low end. The first r-f amplifier has a shunt-fed
plate circuit in which the +265-volt potential is applied
through "SEND/REC" switch S9, dropping resistor R6,
decoupling network C22-R5, and r-f choke L7 to the
plate of tube Vl. The screen is decoupled from the
supply voltage by means of resistor R4 and capacitor
C2l. Resistor R3 is the screen dropping resistor; ca-
pacitor C20 is the screen bypass. R-f plate current flows
through coupling capacitor C24 and limiting resistor R7
into the primary of interstage r-f transformer L8. The
bottom end of transformer L8 is held at r-f ground po-
tential by means of capacitor C27. The signals induced
in the secondary of transformer L8 are applied through
parasitic suppressor Rll and coupling capacitor C25 to
the control grid of second r-f amplifier tube V2. The
selectivity of the stage, as well as its overall gain, is
maintained relatively constant over the tuning range of
the band by the complex interstage coupling network.
The bias for the stage is determined by the setting of
"RF GAIN" control R93. For manual volume-control
operation, the "RF GAIN" control is placed at the set-
ting that provides maximum signal input without over-
loading and optimum signal-to-noise conditions. For
automatic operation, the "RF GAIN" control is usually
set to its maximum clockwise position and the a-v-c po-
tential, applied through decoupling network R2-C19 and
grid-return resistor Rl to the control grid, is used to
control the gain of the stage. The "RF GAIN" control is
always operative, even in automatic operation, and may
18
be used to control the overall r-f gain of the stage. For
direction-finding applications where greater sensitivity
and greater selectivity are required, the bias lead is
removed from the "NORMAL" link and placed on the
"DF" link. In the "DF" position, the a-v-c potential is
less than that in the "NORMAL" position and the tun-
ing response is made sharper, since the a-v-c potential
does not have so great a tendency to broaden the tuning
indication.
4-26. The second r-f amplifier is identical to the first
r-f amplifier except for the input circuit which is part of
the interstage coupling network. The "SEND/REC"
switch is used to disable the receiver by removing plate
and screen potentials from both r-f amplifiers whenever
the switch is placed in the "SEND" position. The output
of the second r-f amplifier is applied to the signal grid
of first mixer tube V5.
4-27. SINGLE-CONVERSION CIRCUITS. (See figures
4-4 and 4-5.) When the "BAND CHANGE" control
is set to one of the three lower frequency bands, single
conversion is used in the receiver. The frequency con-
version circuits utilized on these three bands are the
first mixer, the variable-frequency heterodyne oscillator
or the fixed-frequency heterodyne oscillator, and the
gate circuit. A conversion switch ganged to the "BAND
CHANGE" control disables the second-conversion cir-
cuits. In the single-conversion mode of operation, the
heterodyne oscillator circuits operate at a frequency 455
kc above the frequency of the incoming signal and the
output of the first mixer stage is a 455-kc i-f signal. This
signal is applied to the crystal filter circuit through the
path provided by the gate circuit. The first mixer and
heterodyne oscillator circuits are also used for double
conversion; however, to provide adequate image rejec-
tion, the heterodyne oscillators operate 3955 kc above
the frequency of the incoming signal and a 3955-kc i-f
signal results. The conversion switch disables the gate
circuit, when the "BAND CHANGE" control is set to
one of the three, upper bands, and applies power to
the second-conversion circuits to automatically change .
from single conversion to double conversion for all
signal frequencies above 7.4 me.
4-28. The first mixer consists of a pentagrid tube and
associated circuitry. Two inputs are applied to the stage.
One input is obtained from the r-f amplifiers and is
applied
to
the signal grid of pentagrid mixer tube V5.
The other is obtained from the first heterodyne oscil-
lator (i.e., from either fixed-frequency oscillator tube
V3 or variable-frequency oscillator tube V4) and is ap-
plied to the injection grid of the pentagrid mixer. The
plate circuit of the mixer contains two parallel reso-
nant networks which are connected in series. One of
these networks is resonant at 455 kc and presents negligi-
ble impedance at 3955 kc; the other is resonant at 3955
kc and presents negligible impedance at 455 ke. Thus
only the 455-kc conversion frequency is developed in
the plate circuit when operating on the three lower
bands, and only the 3955-kc conversion frequency is
obtained when operating on the three higher bands. The
network comprised of capacitor C69 and slug-tuned in-