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Specifications
Frequency Range:
Band (Meters)
Range (mc)
160 1.5 to 2.5
80 3.2 to 4.2
40 6.8 to 7.8
20 14.0 to 15.0
15 20.8 to 21.8
11 26.5 to 27.5
10 28.0 to 29.0
10 29.0 to 30.0
Rack Mounting: Angle mounting kit available.
Number of Tubes: 22, including rectifiers.
AVC Time Constants: Rise Time - .01 second, Release Time - .1 second (fast), 1
second (slow)
AVC Characteristics: Audio rise less than 3 db for inputs of 5 to 200,000
uv.
Sensitivity: 1.0 microvolt for 6 db signal-to-noise ratio with 3 kc bandwidth.
Image and IF Rejections: Image rejection at center of each band is 50 db or
better. IF rejection at center
as each
band is 70 db or better.
Audio Characteristics:
Output - .75 watts with a 3.0 uv signal, 30% modulated.
Output impedance - 500 ohms, 4 ohms.
Response of audio circuits - +3 db 100 cps to 5000 cps.
Distortion--Less than 10%
Muting: Provisions for muting the receiver during key-down operation is provided. A muting voltage
of +20 volts must be supplied by
the transmitter.
Frequency Stability: 14 mc) - Temperature - Less than 1200 cycles drift from 0 to +/-60øC.
Warmup drift - Less than 300 cycles after
15 minute operation.
Line Voltage - Less than 100 cycles for +10% change.
Dial Accuracy - Within 300 cycles after calibration.
Size: 10-1/2" high x 17-1/4" wide x 15-1/2" deep.
Weight: 35 pounds.
75A-4 Circuitry
General
The 75A-4 amateur band receiver is a dual conversion receiver on all bands
except 160 meters where single conversion is employed. The dual conversion
scheme employs a fixed high frequency oscillator, crystal controlled, and a
variable first intermediate frequency. The signal from a type 70E-24 VFO is beat
against the variable IF to produce a fixed 455 KC second intermediate frequency.
One stage of RF amplification is employed in the receiver. The passband of the
fixed IF (455 KC) is shaped by a mechanical filter stage. A "Q"
multiplier stage provides a tunable notch to minimize heterodyne interference.
Two more 455 KC IF stages follow the "Q" multiplier. These feed
separate detectors for AM or CW-SSB reception. The output of the detectors feed
a combination AM and CW type adjustable noise limiter. Three stages of audio
amplification follow. The output stage feeds a headset jack and a 500-ohm load
or a 4-ohm speaker. A separate AVC amplifier and rectifier are used. Bias for
the audio output tube and the RF gain control system is obtained from a separate
rectifier. A built-in 100-KC calibration oscillator is included in the set. The
passband tuning feature is accomplished by gang tuning the BFO and variable
frequency oscillator by means of a metal belt. Tuning
All variable tuned stages including the RF stage, the first mixer, the
variable IF system and the variable frequency oscillator are operated by the
KILOCYCLES dial. This dial is coupled directly to the shaft of the variable
frequency oscillator. All other variable tuned circuits just mentioned are tuned
by a common platform to which powdered iron slugs are attached. The platform is
moved up and down at a linear rate by means of a mechanism which is coupled to
the VFO shaft by a system of split gears and metal belts. The receiver uses a
unique method of band switching in the RF stage in which only the 80-meter (T-2)
and the 160-meter (T-1) coils are tuned by the main tuning mechanism, and coils
for 40-10 meters are selected and connected across the 80-meter coil. Varying
the inductance of the 80-meter coil varies the total inductance, and therefore
the resonant frequency of the tuned circuit in use. RF Circuits
A simplified block diagram of the 75A-4 RF system is shown in figure 4-1. The
RF stage V-2 feeds the mixer at the carrier frequency of the incoming signal.
The first conversion circuit, consisting of a crystal controlled oscillator,
V-4 and a mixer tube, V-3, converts the incoming signal to the variable IF
frequency of 2. 5 to 1. 5 MC for all bands from 80 meters thru 10 meters. The
variable IF is mixed with a signal from the VFO, V-14 and V-15, in the second
mixer V-5 where it is converted to a fixed IF of 455 KC. See figure 5-7. A
discussion of the individual circuits in the RF portion of the receiver follows:
RF Stage
The RF stage uses a 6DC6 pentode. This tube was chosen because of its
low-noise, remote-cutoff characteristics. This tube allows greater grid voltage
swing without cross-modulation distortion. Individual variable slug-tuned coils
are switched into the grid circuit on 160 and 80 meters. On 40-10 meters the
coil in use is switched across the 80-meter coil, and varying the inductance of
the 80-meter coil tunes the coil for the band in use. The 80-meter trimmer
capacitors are not in the circuit on 40-10 meters.
One coil, T-7, is used for the 11 and 10-meter bands. Manual tracking is used
here employing the ANT TRIM capacitor. C-18. Separate antenna coils are employed
on the 10-11-meter coil, the 80-meter coil, and the 160-meter coil. The
160-meter band feeds on thru the first mixer, V-2, into the 2. 5 to 1. 5 MC
variable IF coils, which track with the receiver front end, and on into the
second mixer V-3. On 80, 40, 20, 15 and the highest frequency 10-meter band the
first mixer grid circuit is similar to the RF stage grid circuit with the higher
frequency coils being paralleled with the 80-meter coil to produce the tuning
for the band in use. On the low 10-meter band and the 11-meter band, capacitors
C-32 and C-31 are individually selected to pad the 10-meter coil to these bands.
The following table shows the tuning components used in the various bands for
each tube circuit.
Crystal Controlled Oscillator and First Mixer
The high frequency mixer stage employs a 12AT7 (V-4) in a crystal controlled
oscillator circuit to provide a heterodyning signal. In this oscillator circuit
the crystal is connected between the cathodes of a dual triode. One section (V-4
pins 6, 7, 8) is a cathode follower amplifier, the other section, a grounded
grid amplifier. Feedback voltage is coupled from the plate of one section, which
contains a tank circuit resonant at the crystal frequency, to the grid of the
other section. The crystal, inserted between the cathodes, acts as a filter. The
phase change through the loop is zero and oscillation takes place at the crystal
frequency. Crystal oscillator output voltage is coupled to the injection grid of
the 6BA7 first mixer. An individual crystal for each band is switched into the
crystal oscillator circuit except for 160 meters, where the high frequency
oscillator is not used. The crystal oscillator beats with the incoming carrier
to produce the first, or variable, intermediate frequency. In this stage,
because the crystal frequency is fixed and the incoming carrier frequency may be
anywhere in the range of the band in use, the difference frequency produced in
the mixer must be tuned by a variable IF system.
Variable IF
The variable IF covers the range 2. 5 to 1. 5 MC. The system consists of two
slug-tuned coils on the same frequency. The first of these coils is capacity
coupled to the second which in turn is connected to the grid of the second mixer
V-5. L-23, a 5. 7 MC trap is connected between the two coils to remove a
spurious response that occurs at 3. 533 MC. VFO and Second Mixer
A Type 70E-24 permeability-tuned precision variable frequency oscillator
provides the injection voltage to the second mixer V-5. The frequency range of
the VFO is 1955KC to 2955KC. This frequency is mixed with the variable IF in V-5
to produce the fixed 455 KC difference frequency which is the frequency of the
fixed IF amplifier. Mechanical Filter
The mechanical filter uses the principle of magnetostriction to convert
electrical energy to mechanical vibration. The magnetostriction transducer input
coil is resonated at 455 KC. A nickel wire within this coil vibrates
mechanically and transmits this mechanical energy to the first of a series of
nickel alloy discs. The mechanical vibration of this first disc is coupled to
succeeding discs by means of nickel-wire coupling elements. Biasing magnets at
either end of the mechanical filter polarize the filter elements to prevent
frequency doubling, in much the same manner as biasing magnets in a headphone
prevent the headphone diaphragm from bending in the same direction for both
halves of an AC cycle. The mechanical vibration of the last disc is coupled to a
magnetostriction transducer element identical to the one used at the input of
the filter. By a reverse principle of magnetostriction, the mechanical vibration
of the nickel-wire transducer core is converted to electrical energy.
Each of the discs employed in the mechanical filter has a mechanically
resonant Q exceeding 2,000. Six of these discs are over coupled to produce a
mechanically-shaped response curve with a flat top and straight, almost vertical
sides. Thus, the filter passes a band of frequencies very little wider than the
flat top of the selectivity curve. The mechanical filter furnished with the
75A-4 passes a band of frequencies approximately 3 KC wide and centered on 450
KC, providing an IF selectivity curve ideal for the reception of AM and
single-sideband signals. The 3-KC filter is supplied as part of the 75A-4;
however, a mechanical filter having similar selectivity characteristics but
having a band pass of 800 cycles is available for use in CW reception. A 6-KC
filter is available for double sideband reception of AM.
The mechanical filters used in the 75A-4 are the plug-in type that plug into
a 9 pin miniature tube socket. These are sealed units and must not be tampered
with. No external variable tuning is employed.
"Q" Multiplier
The "Q" multiplier, as used in the 75A-4, is employed only as a
rejection filter. In this capacity it performs the same function as the
rejection notch of the crystal filter in earlier receivers but does it much
better. It is capable of attenuating as much as 40 db, any single audio tone
(heterodyne) which may be present within the receiver's passband. The
"Q" multiplier consists of a cathode follower amplifier coupled to a
regenerative amplifier, the plate load of which is a bridge T-filter. The
regenerative amplifier is kept just below the oscillating point by R-36. At this
point the plate circuit has a very high Q and provides a very sharp null to
frequencies within the receiver passband. REJECTION TUNING capacitor C-72 can
shift the null around to any frequency within the passband. The "Q"
multiplier is removed from the circuit by turning the 455 KC IF
Amplifier
The 455 KC IF amplifier consists of V-6 which is associated with the
mechanical filter and V-8 and V-9; the latter two are conventional IF amplifiers
that contribute nothing to the passband wave shape, which is determined by the
mechanical filter. All are 6BA6 tubes. AVC is applied to the grids of all three
tubes. AM Detector
The AM detector is a conventional diode rectifier excited from IF transformer
T-3 and having R-56 and R-57 as its load. The audio from the detector is applied
to the noise limiter, V-12, when AM CW SSB selector switch S-3 is operated to AM
position. CW-SSB Detector
This detector, V-11 is designed especially for SSB reception. It is a mixer
type circuit that takes the output of the BFO and mixes it with the output of
the 455 KC IF. Tube elements 1, 2 and 3 perform as a cathode follower amplifier.
The remainder of the tube is a plate detector, the cathode of which is common
with the cathode follower amplifier. The detector greatly reduces the distortion
which is generated when a conventional diode detector is used for detecting SSB
signals. The audio from the detector is applied to the noise limiter V-12 when
S-3 is operated to the SSB position.
BFO
The BFO V-20, uses a 6BA6 in an electron coupled oscillator whose frequency
range is approximately 453-457 KC. The BFO is tuned by means of the knob on the
front panel labeled PASSBAND TUNING. The shaft of the PASSBAND TUNING Control is
attached by a metal belt to the frame of the variable frequency oscillator which
is mounted in a ball and bearing equipped cradle. As the BFO is tuned through
its range the VFO is also tuned a like amount. (The shaft of the VFO is
prevented from turning by the DIAL DRAG.) Because the actual intermediate
frequency is changing, the passband is being shifted and an unwanted signal can
be dropped off the edge of the passband while retaining the wanted signal in the
passband without changing its pitch.
Noise Limiter
Noise limiter V-12, a 6AL5 tube, couples the audio from the detectors to the
first audio stage. The function of the noise limiter is to minimize interference
by removing noise peaks which exceed the amplitude of the modulation. It is
effective on AM, CW and SSB.
Both diode sections of the 6AL5 are required in order to limit both the
positive and the negative peaks. During AM reception, a negative voltage is
derived across diode load resistors R-56 and R-57. NOISE LIMITER Control R-67 is
connected to this source of supply. As a result the cathodes of V-12 assume a
negative DC potential which is adjustable by means of the NOISE LIMITER Control,
and direct current flows through the diodes (the plates being positive with
respect to the cathodes). The AF signal voltage from the receiver is applied to
the anode (pin 7) of one section of V-12 through coupling capacitor C-93. This
AF signal modulates the DC flowing through this diode section and, as a result,
the AF signal appears across cathode resistor R-65. This resistor is common to
both diode circuits, therefore the AF signal is superimposed on the DC flowing
through the second section of V-12 and appears across load resistor R-63. From
this point, the signal is coupled through C-96 and AF GAIN Control R-62 to the
audio amplifiers.
Any negative impulse that drives the anode of the input diode (pin 7) more
negative than the cathode, will cut off the diode, and that impulse will be
limited to an amplitude equal to the threshold voltage (as set by the NOISE
LIMITER Control). Similarly, any positive impulse that overcomes the threshold
potential on the cathode of the second section (pin 5) will cut off that diode,
and the positive impulse will be limited. As the NOISE LIMITER Control is turned
toward 10, a less negative threshold voltage is applied to the diodes, and more
severe limiting results. The threshold voltage at any given setting of the NOISE
LIMITER Control varies with the average amplitude of the diode load signal,
therefore limiting action automatically adjusts itself. C-97 and R-66 decouple
the limiter circuit from the detector circuit.
During CW or SSB reception when the carrier is intermittent or absent, the
reference voltage is supplied by connecting the NOISE LIMITER Control R-67
through switch S-3 to a value of bias obtained from the receiver bias rectifier.
Because of the flat AVC characteristic of the receiver, frequent adjustment of
the NOISE LIMITER Control is unnecessary.
The noise limiter is made inoperative by applying a value of B+ to the diode
plates through R-64 and by grounding the cathodes of V-12 thus insuring that the
diode currents cannot be cut off on noise or high modulation peaks. This is done
by a switch associated with the NOISE LIMITER Control. C-98 provides a ground
path for the audio when S-4 is in the OFF position.
Audio Amplifiers
The audio section consists of two stages of voltage amplification (both
halves of the dual triode V-13, a 12AT7) and a 6AQ5 power amplifier. The AF GAIN
Control is located in the grid circuit of the first stage. Fixed bias from the
bias rectifier is applied to the grid of the power amplifier. The output
transformer secondary consists of a 500 ohm winding suitable for driving
auxiliary apparatus and a four-ohm winding for use with loudspeaker voice coil
and headphones. When a headphone is plugged into the headphone jack J-2, the
speaker connection is interrupted and a 10-ohm load resistor is connected in
parallel with the headphone to keep the output transformer properly loaded. RF
Gain Control System
The RF gain control system in the 75A-4 works in conjunction with the AVC
system. To control the sensitivity of the set, a source of fixed bias is added
to the AVC voltage which is then applied to the AVC controlled tubes. This
system maintains the gain distribution constant throughout all settings of the
gain control. A low impedance type AVC line is employed. In order to prevent the
RF GAIN Control from affecting the characteristics of the line due to loading an
RF gain gate is employed to decouple the RF GAIN Control from the AVC line. This
gate is in the form of 1/2 of a type 6AL5 twin diode, Y-19. The other half of
the tube is employed as a bias rectifier.
Bias from this bias rectifier is connected to one end of the RF GAIN Control.
The arm of the control is connected to the AVC line through the RF gain gate,
V-19 (pins 1 and 7). Advancing the control adds negative bias to the AVC bias
and reduces the gain of the tubes connected to the AVC line, namely, V-2, V-6,
V-8 and V-9.
"S" Meter
The "S" meter is connected in a bridge circuit between the screen
grids of V-6 and V-8 and the cathode of V-8. These are IF amplifier tubes that
are furnished with AVC voltage. A reference voltage is developed at the negative
terminal of the "S" meter by the cathode current flow of V-8. This
reference voltage is adjusted under no-signal conditions to a value equal to
that developed at the positive terminal of the "S" meter by the two IF
amplifier screen-grid voltages. The presence of a signal in the IF strip causes
an AVC voltage to be developed which reduces the screen grid current of the two
IF amplifiers, causing the screen-grid voltage on these tubes to increase. This
increase in voltage is applied to the positive terminal of the "S"
meter to produce an "S" meter reading proportional to the strength of
the incoming signal. R41 adjusts the "S" meter sensitivity. AVC
System
A low impedance AVC line is employed to minimize blocking on strong signals.
The RF amplifier V-2 and the 455 KC IF amplifiers V-6, V-8 and V-9 are all AVC
controlled. A stage of IF amplification V-21, separate from the signal IF
amplifier, is employed to amplify the IF signal for rectification for AVC
voltage. The IF amplified IF voltage is rectified by 1/2 of V-16, a twin diode.
The other half of V-16 is used as an AVC noise clipper. This tube clips sharp
noise impulses from the AVC voltage and thus prevents the noise from
desensitizing the AVC circuit. A small positive DC voltage is applied to the AVC
rectifier through R-86 to produce an AVC delay so that the AVC is ineffective on
weak signals. A network of load resistors is switched by switch S-5 to select
either fast or slow AVC characteristics. C-110 provides RF filtering for the AVC
detector output, R-89, R-91 and R-92 are the detector loads. R-90 and C 112
provide the AVC time constant with R-92 and R-91 modifying the time constant for
slow and fast AVC operation. AVC test point J-4 is provided for use in aligning
the AVC IF amplifier transformer T-4. Each controlled stage is decoupled from
the AVC line by suitable capacitors and resistors to prevent instability because
of feedback. Calibrator Circuit
The calibrator employs a 6BA6 tube in a crystal controlled oscillator
circuit. The fundamental frequency of the oscillator is 100 KC, therefore, a
harmonic appears at each 100 KC point over the entire range of the receiver when
the calibrator is turned on. C-1 is used to zero beat a calibrator harmonic with
a 1500 KC or 1600 KC broadcast station or with WWV at 2. 5, 15 or 30 MC. The
calibrator output is coupled to the receiver input by C-5. The calibrator is
turned on wherever S-6 is in the CAL position.
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