75A-3 Receiver



The 75A-3 is a dual-conversion communications receiver covering all amateur bands in the range 160-10 meters. It is the only amateur-band receiver featuring the Collins Mechanical Filter. The 75A-3 also features excellent stability, sensitivity, and dial accuracy.

The Collins Mechanical Filter, a new and radically different means of achieving selectivity - in communications receivers, uses mechanically resonant elements to produce a selectivity curve having a flat "nose" for full sideband response and steep, almost vertical skirts for rejection of adjacent- channel interference.

Because the mechanical filter in the 75A-3 has nearly rectangular selectivity curve, the receiver is well suited for single-sideband suppressed-carrier reception. And with the 75A-3 it is possible to use single-sideband techniques on AM signals by tuning one sideband and the carrier to the exclusion of the other sideband. Because of the unusual tuning characteristics of the 75A-3 the owner should familiarize himself with the operational procedures described in Section III of this manual.

The mechanical filter supplied with the 75A-3 has a 3-kc bandpass for optimum performance in AM and single-sideband reception; however, an 800-cycle filter, an accessory unit, may be installed for use under conditions of heavy QRM and QRN. Both filters are plug-in units and mount side by side in the receiver. A two-position switch on the front panel of the receiver is used to select the desired filter.

The direct-reading tuning dial is calibrated in 1-kc divisions on 160, 80, 40, 20 and 15 meters and 2-kc divisions on 11 and 10 meters. A crystal-controlled front end and temperature-compensated, hermetically-sealed variable frequency oscillator contribute to the frequency stability of the 75A-3. Physical shock will not change the VFO frequency unless the shock is so severe that the dial setting is changed.

Included in the 75A-3 are a 4-position crystal filter, a phone noise limiter which automatically adjusts the clipping level according tothe strength of the incoming signal, a separate noise limiter for CW reception, delayed and amplified AVC, and provisions for external standby control and receiver muting. Two plug-in units available for use in the 75A-3 are the 148C-1 NBFM adaptor and the 8R-1 crystal calibrator.


 (1) SELECTIVITY.                        
   (a) With 3-kc filter  ........ approx. 3 kc at 6 db down and 6. 5 kc
                                  at 60 db down 
   (b) With crystal filter  ..... position 4: approx. 40 cps at 6 db down
                                  and 4 kc at 60 db down.
 (2) SENSITIVITY  ............... 2 uv for signal-to-noise ratio of 6 db.
 (3) STABILITY                                                            
   (a) Mechanical  .............. CW beat note willnot change if receiver
                                  is jarred unless dial setting is changed.
   (b) Electrical  .............. stable within a few cycles under normal
                                  operating conditions and after short
                                  warm-up period.
 (4) IMAGE REJECTION  ..........  at least 50 db.                           
 (5) AVC  ......................  constant audio output within 6 db for r-f
                                  input change of 5 uv to 0.5v.
 (1) COVERAGE  .................  1.5-2.5 mc; 3.2-4.2 mc; 6.8-7.8 mc
                                  14.0-15.0 mc; 20.8-21.8 mc; 26.0-28.0
                                  28.0-30.0 mc.
 (2) BANDSPREAD  ...............  linear; ten turns of vernier dial covers
                                  each range.
 (3) DIAL ACCURACY  ............  within 1 kc on 160-15 meters and 2 kc on
                                  11 and 10 meters.
 (4) "S" METER  ................  S9 corresponds to signal of about 100 uv.

d. POWER SOURCE .  .  .  .  .  .  115 v, 50-60 cps; power consumption approx.
                                  85 w.; 230-volt model available on special
                                  order or by use of a 230-volt power

e. ANTENNA INPUT   .  .  .  .  .  accommodates wide range of antenna
                                  impedances; designed for 50-150 ohms
                                  terminal impedance; coax connectors

f. AUDIO OUTPUT .  .  .  .  .  .  approx. 2.5 w

g. CABINET DIMENSIONS    .  .  .  21-1/8 in. wide; 12-1/2 in. high; 13-1/16
                                  in. deep; chassis fits standard relay rack;
                                  chassis depth 13-5/16 in.

h. FINISH . . . . . . . . .   . . dark grey wrinkle.


 (1) 800-CYCLE MECH FILTER .      mechanical filter with bandpass of approx.
                                  800 cycles; plugs in socket on receiver

 (2) SPEAKER . . . . . . . . .    external 10-in. speaker in matching cabinet;
                                  Speaker cabinet 15 in. wide, 11-1/8 in.
                                  high, 9-1/8 in. deep.

 (3) HEADPHONES .  . . . . . .    low-impedance type preferable.

 (4) ANTENNA . . . . . . . ..     see e.

 (5) CRYSTAL CALIBRATOR 8R-1.   . plugs into a socket on receiver chassis;
                                  provides 100-kc reference signals for

 (6) NBFM ADAPTOR 148C-1 . .  . . plugs into socket on receiver; contains
                                  limiter and discriminator.

Under the hood...

[Internal View]



The 75A-3 circuit consists of a crystal-controlled front end, a variable i-f system, a low-frequency converter stage and a fixed 455-kc i-f system (see block diagram, Fig. 4-1). In the fixed i-f system, a mechanical filter first provides the necessary selectivity-curve shaping. The 455-kc intermediate frequency is then amplified and fed to the detector and audio circuits. A 455-kc signal from the beat-frequency oscillator is injected at the detector stage for CW reception. The audio system uses separate noise limiters for CW and AM; the CW noise limiter is variable, and the clipping level of the AM noise limiter is automatically adjusted according to the strength of the incoming signal. The output stage feeds a 500-ohm load or a 4-ohm speaker.


All variable tuned circuits, including the first r-f stage,the variable i-f system and the variable frequency oscillator are operated by the main tuning control. The vernier tuning dial is coupled directly to the shaft of the variable-frequency oscillator. The other variable-tuned circuits, including the r-f stage and the variable i-f stages, are tuned by powdered iron slugs which are attached to a common platform. The platform is moved up and down by means of a cam-driven mechanism which is coupled to the VFO shaft by means of split gears and metal bands. The 75A-3 uses a unique method of band switching in the r-f stage in which only the 80 and 160 meter coils are tuned by means of the main tuning mechanism, and coils for 40-10 meters are connected across the respective 80 meter coils. Varying the inductance of an 80-meter coil varies the total inductance, and therefore the resonant frequency, of the tuned circuit for the band in use.


A simplified block diagram of the 75A-3 r-f system is shown in Figure 5-5. The first r-f stage feeds the mixer at the carrier frequency of the incoming signal. The coverage for each band is listed below:

     160 meters  -  1.5 -  2.5 mc
      80 meters  -  3.2 -  4.2 mc
      40 meters  -  6.8 -  7.8 mc
      20 meters  - 14.0 - 15.0 mc
      15 meters  - 20.8 - 21.8 mc
      11 meters  - 26.0 - 28.0 mc
      10 meters  - 28.0 - 30.0 mc

The first converter, consisting of a crystal-controlled oscillator circuit and a mixer tube, converts the incoming signal to the variable i-f frequency of 2.5 - 1.5 mc for 160-15 meters and 5.455 - 3.455 mc for 11 and 10 meters. From the variable i-f system the signal is fed to the second mixer where it is converted to a fixed i-f frequency of 455 kc. This signal is passed through the i-f amplifiers to the detector stage. A discussion of individual circuits in the r-f portion of the receiver follows:

a. RF STAGE. The RF stage uses a 6CB6 pentode. 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 connected across the 80-meter coil, and varying the inductance of the 80-meter coil tunes the coil for the band use. The 80-meter trimmer capacitors are not in the circuit on 40-10 meters.

On 160 meters the output of the r-f amplifier is fed directly to the variable i-f system. On 80-10 meters the r-f plate circuit tuning system is the same as that of the grid circuit.

b. CRYSTAL - CONTROLLED OSCILLATOR AND FIRST MIXER. The high-frequency converter stage employs a 12AT7 in a Butler crystal-oscillator circuit. In this circuit the crystal is connected between the cathodes of a dual triode. Feedback voltage is coupled from the plate circuit of one section, which contains a tank circuit resonant at the crystal frequency, to the grid of the other section. The 180-degree phase shift necessary for oscillation is provided by the second section of the tube, which acts as a cathode follower to couple feedback energy to the crystal. Crystal-oscillator output voltage is coupled to the injection grid of the 6BA7 first mixer. An individual crystal for each band is switched in to the crystal oscillator circuit except on 160 meters, where the high-frequency converter is not used. The crystal- oscillator frequency beat switch 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 i-f system. On 160 meters the band coverage corresponds to the lower frequency range of the variable i-f's.

c. VARIABLE IF. The variable i-f system covers two frequency ranges, 2.5 - 1.5 mc and 5.455 - 3.455 mc. The variable i-f system for the range in use consists of two slug-tuned circuits on the same frequency. The first of these two circuits is capacity coupled to the second, which in turn is coupled to the control grid of the second mixer.

d. VFO AND SECOND MIXER. A 70E-12 permeability-tuned precision variable frequency oscillator provides the injection voltage to the second mixer, a 6BA7. The frequency range of the variable frequency oscillator is 1.955 - 2.955 mc. On 160-15 meters the 2.5 - 1.5 mc variable i-f frequency is mixed with the oscillator frequency to produce a difference frequency of 455 kc. On 11 and 10 meters the second harmonic of the variable frequency oscillator, 3.910 - 5.910 mc, is mixed with the variable i-f frequency of 5.455 - 3.455 mc to produce the 455-kc fixed intermediate frequency.

e. CRYSTAL FILTER (see Figure 4-2). The output of the second mixer is fed to the crystal filter. A 455-kc crystal, Y-7, acts as a very high Q resonant circuit which will pass only a narrow band of frequencies peaked sharply at 455 kc.

In order to neutralize the effect of the capacity between the crystal holder plates, phasing capacitor C-58 feeds a small out-of-phase voltage to the crystal filter output circuit. The phasing capacitor serves another purpose- If the phasing capacitor is adjusted so that the crystal holder capacity is not quite balanced out, the crystal forms, in addition to its series resonant circuit, a parallel resonant circuit slightly higher or lower in frequency than 455 kc, depending upon the setting of the phasing capacitor. Signals at this parallel resonant frequency are prevented from passing through the crystal circuit. Thus, the phasing control can be used to "notch out" interfering heterodynes close to the operating frequency. C-58 is a two-section capacitor; as the capacity of one section increases,the capacity of the other section decreases. By this means a capacity balance is obtained so that adjusting the phasing capacitor does not detune L-24.

A five-position switch, S-2, inserts various resistances in series with the crystal. The series resistor "de-Q's" the crystal so that the value of the resistor determines the bandpass of the crystal filter circuit. In position "O" the crystal is shorted so that the input circuit feeds directly to the output circuit. In position "1" the crystal is in series with the parallel resonant i-f coil L-24 so that the crystal looks into a high impedance at 455 kc and is not fully effective. In positions 2 through 4, increasingly lower values of resistance are inserted in series with the crystal to increase the effective Q of the crystal circuit and thus provide greater degrees of selectivity.

f. MECHANICAL FILTER. The functional schematic diagram of the Collins Mechanical Filter is shown in figure 4-3. The mechanical filter uses the principle of magnetostriction to convert oscillating magnetic 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 a-c 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 impulses.

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 flattop 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 used in the 75A-3 passes a band of frequencies approximately 3 kc wide and centered on 455 kc, providing an i-f selectivity curve ideal for the reception of AM and single-sideband signals. The 3-kc filter is supplied as part of the 75A-3; however, a mechanical filter having similar selectivity characteristics but having a bandpass of 800 cycles is available for use in CW reception or in phone reception under conditions of extremely heavy QRM or QRN.

g. 455-KC IF. The rest of the i-f system consists of three 6BA6 455-kc i-f amplifiers. The first of these is broad- tuned and is coupled to the two succeeding amplifiers,V-6 and V-7 . The output of V-7 is coupled to the detector.

h. BFO. The BFO uses a 6BA6 in an electron-coupled oscillator circuit whose frequency range is approximately 453-457 kc. The BFO is tuned by means of a knob on the front panel.

i. "S" METER (see main schematic, figure 5-6). The "S" meter is connected in a bridge circuit between the screen grids of i-f amplifiers V-5 and V-6 and the cathode of i-f amplifier V-7. A reference voltage is developed at the negative pole of the "S" meter by the cathode current flow of V-7. This reference voltage is adjusted under no:signal conditions to a value equal to that developed on the positive pole of the "S" meter by the two i-f amplifier screen-grid voltages. The presence of a signal in the i-f strip causes an AVC voltage to be developed which reduces the screen-grid current of the two i-f amplifiers, causing the screen-grid voltage on these tubes to increase. This increase in voltage is applied to the positive pole of the "S" meter to produce an i'S" meter reading proportional to the strength of the incoming signal.


a. DETECTOR AND AVC (see figure 4-4). One section of V-8 is the detector. The other section is the AVC rectifier, which is coupled to the secondary of T-7 through C-80. The AVC control voltage is developed across R-32 and applied to the grid of the AVC amplifier, one section of V-9, through an audio filter network consisting of R-33 and C-81. The network consisting of R-35, C-82, and R-34 introduces degeneration to prevent the AVC amplifier from responding to low audio frequencies.

When no signal is entering the AVC rectifier, the AVC amplifier plate current is cut off by a bias of approximately 11 volts, which is obtained from bias load resistor R-36. Hence, the AVC amplifier does not function to reduce the sensitivity of the receiver until the voltage developed by the AVC rectifier reduces the AVC amplifier grid voltage to less than cutoff. Plate voltage for the AVC amplifier is developed across R-37 and R-38, two bias load resistors. The plate load across which the AVC voltage is developed is R-55.

The AVC circuit is disabled for CW. In CW position ofthe CW- AM-FM switch,the AVC amplifier plate circuit is broken,and a variable bias, adjusted by means of RF gain control R-57, is applied to the AVC line from the junction of R-57 and R-56, permitting manual volume control.

b . NOISE LIMITERS (see figure 4-5) . The 75A-3 employs separate noise limiters for AM and CW. The AM noise limiter automatically adjusts its clipping level according to the strength of the incoming signal. The clipping level of the CW noise limiter is adjusted by means of a knob on the front panel.

(1) AM NOISE LIMITER. The audio voltage developed at the junction of R-40 and R-39, the diode load resistors, is applied to the noise limiter plate. A filter network consisting of R-42 and C-84 filters the audio component from the voltage developed across the diode load resistors, and this voltage is applied to the noise limiter cathode. When a negative noise peak exceeds the noise limiter bias, the diode ceases to conduct so that the noise voltage is not developed across noise limiter load resistor R-41. Noise limiter bias, and hence the negative peak-clipping level, is proportional to the strength of the incoming signal.

Positive noise peaks are not clipped by the AM noise limiter, but by the diode detector. Because the diode load network has a relatively large time constant and will not follow sudden drops in applied voltage, noise peaks resulting from sharp decreases in the carrier envelope (which would appear as positive peaks at the detector output) are effectively clipped in the detector stage.

(2) CW NOISE LIMITER. The CW noise limiter is a shunt-type clipper employing 6AL5 dual diode, V-16. If clipping level control R-62 is set at minimum clipping, or B+, and a sine wave is applied to the input of the limiter (junction of pins 5 and 7) the section between pins 2 and 5 will at first draw a current until capacitors C-87 and C-86 are charged positive to exactly the peak audio input voltage. The signal at the junction of pins 5 and 7 will then swing between 0 and 2 times the peak applied audio voltage. No clipping will occur. Now if the arm of clipping level control R-62 is advanced toward ground,the diode section between pins land 7 will conduct when the positive signal voltage exceeds the voltage applied to the cathode. At this time the positive signal will be clipped, and capacitors C-87 and C-86 will be discharging by an amount equal to the difference in voltage between the d-c cathode voltage and the peak positive voltage of the applied audio. The negative half-cycle will then be clipped when the cathode of the section between pins 2 and 5 becomes negative with respect to the plate. When this occurs, capacitor C-87 and C-86 will again charge to the original voltage of the audio signal, preparing the section between pins 1 and 7 to again clip the positive peak.

c. AUDIO AMPLIFIERS. The first audio stage is one triode section of V-9. This stage provides grid swing to the output tube, V-11. Grid bias for V-11 is obtained from a fixed bias source, a point on the voltage divider in the negative lead of the Power Supply.

The output transformer secondary delivers approximately watts of audio to a 500-ohm or a 4-ohm load. The 4-ohm output may be connected to the voice coil of a speaker. The front- panel headphone jack is interlocked with the speaker output connection so that inserting a headphone jack disconnects the speaker. A 10-ohm load resistor is connected across the 4-ohm output when the headphone plug is inserted.

Standby and muting circuits are provided to disable the receiver audio section when an associated transmitter is operating (see main schematic, figure 5-6). The standby connection is provided to insert R-76, a 1500-ohm resistor, in series with the bias load resistors to increase the bias on the output tube and cut off the plate current. The muting circuit is provided for receiver silencing in break-in CW operation. A positive potential of 20 volts applied to pin 2 of the 6AL5 causes plate current flow in the diode, whose cathode is connected to the audio amplifier cathode (pin 8 of V-9). Current flows through the audio amplifier cathode resistor R-43 and increases the cathode bias sufficiently to cut off the plate current of the audio amplifier and silence the receiver.


The 148C-1 NBFM adaptor employs a 6AU6 limiter and a 6AL5 frequency discriminator. Because ofthe high value of grid load resistor,R-201, the limiter tube operates on a nonlinear portion of its characteristic curve and will not respond to changes in signal amplitude.

The discriminator circuit used in this adaptor relies on the phase difference between the primary and secondary potentials of doubled tuned, loosely coupled transformer, T- 201. At resonance the phase difference is 90 but varies above and below this value as the input frequency varies. A pair of coupling capacitors, C-204 and C-205, introduce to the secondary circuit a voltage in phase with the primary voltage. This voltage appears in the same phase and with equal amplitude at each plate of 6AL5 discriminator. The secondary voltage of T-201, however, changes in phase according to the magnitude and direction of frequency swing, and appears at the plates of the discriminator 180 out of phase.

Thus as the phase of the secondary voltage of T-201 changes with the change in frequency, the secondary voltage and the applied primary voltage add at one end of the secondary and subtract at the other. When the frequency swings in the other direction, the secondary voltage and the applied primary voltage shift in phase relationship to produce more r-f at the other end of the secondary. The 6AL5 rectifies the RF to produce an a-c audio voltage across load resistors R-206 and R-207.

The audio voltage is fed to the output pin of P-203, the adaptor plug, thru a de-emphasis network, R-208 and C-208. The adaptor unit is connected into the 75A-3 circuit when the CW-AM-FM switch is in FM position. Operating voltages for the adaptor are provided by the receiver power supply.


The calibrator unit uses a 6BA6 in a Pierce oscillator circuit. The oscillator provides usable harmonics each 100- kc to 30 mc.

Capacitor C-310 is provided for adjusting the oscillator to zero beat with a frequency standard, a 1500-kc or 1600-kc broadcast station, or WWV at 2.5, 15, or 30 mc (refer to section 3, part 7, for calibration and frequency measuring procedures).

Calibrator output is coupled to the grid of r-f amplifier tube V-1 by means of the capacity be-tween pins 3 and 4 of the crystal calibrator socket.

The calibrator receives its operating voltages from the receiver power supply. The oscillator is energized when the OUT-LIMITER-CAL. switch on the receiver front panel is in CAL. Position.

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This Home Page was created by wa3key, Tuesday, August 20, 1996
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