75A-2
THE COLLINS 75A-2 RECEIVER
The Collins Model 75A-2 Receiver is designed for the amateur bands in the frequency range of 1500 kc to 30 mc. The receiver provides facilities for the reception of CW, MCW, and AM PHONE reception.
Two octal sockets have been provided for internal plug-in attachment of a Narrow Band Frequency Modulation Detector unit and a Crystal Calibrator unit which provides reference frequencies every 100 kc. Controls for these accessories are provided on the front panel and are wired ready to use.
The receiver uses the double-conversion superheterodyne principal to obtain high image rejection. Stability is obtained by the use of quartz crystals in the high frequency oscillator stage and a Collins Type 70E-12 sealed VFO in the low frequency oscillator circuit.
Additional features of the receiver are separate noise limiters for PHONE and CW, amplified AVC, crystal filter, direct reading dial with frequency readings accurate to within 1 kc up to 21.8 mc and 2 kc from 26 to 30 mc. Provision has been made to connect a blocking bias to the receiver to mute the receiver audio when the key of an associated transmitter is closed.
DESCRIPTION & SPECIFICATIONS
FREQUENCY COVERAGE – The amateur bands are covered as follows:
160 meters – 1.5 – 2.5 mc 15 meters – 20.8 – 21.8 mc
80 meters – 3.2 – 4.2 mc 11 meters – 26.0 – 28.0 mc
40 meters – 6.8 – 7.8 mc 10 meters – 28.0 – 30.0 mc
20 meters -14.0 -15.0 mc
The above table shows the tuning ranges within which the amateur bands fall. The exact frequencies of the amateur bands are given in the latest amateur radio handbooks.
BANDSPREAD – The permeability tuning system employed in the 75A has been engineered to give linear tuning on each band. Ten turns of the vernier tuning dial cover each of the individual ranges shown above. Each division of the vernier tuning dial (which has 100 divisions) represents 1 kc on the 160, 80, 40, 20, and 15 meter bands, and 2 kc on the 11 and 10 meter bands.
ACCURACY AND STABILITY – Visual tuning accuracy to within 1 kc from 1.5 mc to 21.8 mc and 2 kc from 26 mc to 30 mc provided the vernier dial corrector (zero set control) is exactly calibrated at the centers of each tuning range. Extreme variation in plate supply voltage causes a change of only a few cycles in the CW note. Furthermore, the CW note is absolutely independent of all except the tuning controls Physical shock will not disturb the frequency unless the shock is severe enough to change the dial settings. The stability is available after a very short warm up.
IMAGE AND I-F REJECTION – The circuit design of the 75A receiver has inherently high rejection to spurious frequencies. Image rejection is a minimum of 50 db. I-F rejection is 50 db minimum.
SENSITIVITY AND SIGNAL TO NOISE RATIO – A 10 db signal to noise ratio and one watt of audio output is obtained with signal inputs of 2 microvolts or less.
SELECTIVITY – The crystal filter controls provide a bandwidth that is variable ln 5 steps from approximately 4 kc to 200 cycles at 2 times down (6 db down from the peak of the resonant frequency). There is only slight loss in gain caused by use of the crystal filter with the exception of the extremely sharp position which gives about 6 db loss. The fixed I-F selectivity provides a bandwidth of approximately 13 kc at 1000 times down (60 db down from the peak of
the resonant frequency).
PHASING – The crystal filter includes a phasing control which provides a rejection notch for suppressing heterodynes. The range of rejection of this control has been extended downward to 250 cps or lower.
AUTOMATIC NOISE LIMITER – The 75A receiver contains a series type noise limiter which automatically adjusts its limiting threshold to all carrier levels.
CW NOISE LIMITER – A shunt type noise limiter with front panel control of limiting level is provided for CW operation.
AUTOMATIC VOLUME CONTROL – Delayed, amplified AVC gives constant output within 6 db for a change in r-f input from 5 microvolts to 0.5 volt. AVC is applied to the r-f stages and three i-f stages. The proper amount of AVC delay is employed for maximum sensitivity on weak signals.
SIGNAL STRENGTH METER – The S meter is calibrated from 1 to 9 in steps of approximately 6 db each, and for 20, 40, and 60 db above S9. Zero adjustment is provided. A reading of S9 is obtained with an input of approximately 100 microvolts. The AVC amplifier tube works into an unusually low value of load impedance which permits quick recovery from noise pulses or strong signals from the associated transmitter, thus allowing. fast break-in when the receiver is used to monitor operation of the transmitter.
AUDIO OUTPUT – 2.5 watts of audio power are available.
TERMINAL IMPEDANCES
INPUT – The antenna input circuit is designed for a nominal 50 to 150 ohms impedance but will accommodate a wide variety of antenna impedances, both balanced and unbalanced, without serious loss. Mounting holes for an Army type SO 239 coaxial connector are provided to allow convenient connection to coaxial transmission lines, such as RG-8/U (52 ohms) and RG-11/U (73 ohms).
OUTPUT – A 500 ohm output and two 4 ohm outputs (one of which is interlocked with the panel headphone jack) is available on a rear terminal board. The panel headphone jack is a four ohm termination so that any value of headphone impedance will function satisfactorily.
CONTROLS – The following controls are on the front panel of the receiver:
Tuning Control – RF Gain Control – Band Switch – Audio Gain Control – CW Pitch Control – Crystal Phasing Control – Antenna Trim Control – CW-AM-FM Switch – Off-Standby-On Switch – Noise Limiter-Calibrate Switch – Crystal Selectivity Switch – Zero Set for Tuning Control – Headphone Jack – CW Limiter Control
CIRCUIT – Dual Conversion superheterodyne. One r-f amplifier stage, 1st mixer stage, crystal controlled h-f oscillator, variable i-f filter, 2nd mixer, three fixed i-f amplifier stages, detector/AVC Rectifier stage, two audio amplifier stages, AVC amplifier/noise limiter stage, CW noise limiter, variable frequency oscillator, beat frequency oscillator, and power supply. All circuits concerned with the tuning process are permeability tuned and ganged to one control.
POWER SOURCE – Power supply self-contained. Requires 115 volt 50/60 cps source. Power consumption about 85 watts.
DIMENSIONS – CABINET – 21-1/8″ wide, 12-1/2″ high, 13-1/6″ deep. The receiver chassis is mounted on a standard 10-1/2″ x 19″ panel and can be removed from the cabinet and mounted in a standard relay rack. Depth behind the panel is 13-5/16″.
WEIGHT – 50 lbs.
FINISH – St. James Gray wrinkle.
ACCESSORIES
SPEAKER TYPE 270G-2 – An external 10 inch speaker, not furnished, is available, mounted in a matching cabinet. The speaker cabinet measures 15″ wide, 11-1/8″ high, and 9-1/8″ deep overall. Weight 15 lbs.
HEADPHONES – Any good headphones may be used. The 4 ohm receiver output impedance provides sufficient signal level for low or high impedance headphones.
ANTENNA -Any good antenna may be used; however, the receiver input circuit is designed For antenna impedances in the order of 50 to 150 ohms. In most cases, the transmitting antenna will also be the best choice for receiving. Connections on the rear permit the use of both balanced and unbalanced lines. Mounting holes have been provided for installing a Coaxial connector. This allows advantage to be taken of the low noise pickup of coaxial transmission lines.
CRYSTAL CALIBRATOR – The type 8R-1 Crystal Calibrator is available on order. The 100 kc crystal oscillator in this unit provides reference frequencies every 100 kc. This unit plugs into a socket within the receiver. Operating voltages and controls are provided in the receiver.
NBFM ADAPTOR, TYPE 148C-1 – This unit is also available on order. With it, narrow band FM signals can be detected and fed through the receiver audio circuits. This unit also plugs into a socket within the receiver. Operating voltages and controls are provided in the receiver.
Under the hood…
75A-2 CIRCUITRY
MECHANICAL
GENERAL
The 75A2 receiver is constructed in two major units. the receiver unit and the speaker unit. The receiver is constructed on an aluminum chassis. Both the receiver and speaker cabinets are constructed of heavy gauge steel. The receiver cabinet has a hinged cover utilizing inside hinges. Ventilation openings are punched in the sides and rear of the cabinet. The front panel is flush and trimmed for neat appearance. Both the receiver and the speaker cabinets are finished in a hard St. James gray wrinkle finish.
TUNING
The vernier tuning dial is directly coupled to the lead screw of the variable frequency oscillator thus eliminating any possibility of back lash. The iron cores that tune the RF, first mixer, first IF and second mixer stages are all mounted on a movable platform. This platform is geared and belted to the VFO shaft by means of split gears and metal belts thus giving ganged tuning. The slide rule guide pointer is cable driven. The BFO coil is placed for most efficient operation and a long shaft is used to connect the tuning capacitor with the panel knob. All other stages are fixed-tuned with iron cores.
BAND SWITCHING
Band switching of RF stages is accomplished by means of a multiple section switch gang. In addition to RF circuits, the band switch selects high frequency oscillator crystals.
ELECTRICAL THEORY
CIRCUIT – As shown in the block diagram, figure 1-1, the receiver has one stage of pre-selection. A high gain 6CB6 tube is used here because of its excellent electrical characteristics anddesirable physical features. Following the RF stage is the first mixer of the double detection system. The signal grid of the tube, a 6BA7, is tuned to the received frequency, the injection grid receives voltage from the fixed high frequency oscillator circuits at a frequency within a band of either 2.5 to 1.5 megacycle or 5.455 to 3.455 megacycles removed from the received frequency. This oscillator voltage is supplied by a 12AT7 crystal oscillator tube. Since the high frequency oscillator frequency is fixed (by the quartz crystals) the output frequency of the first mixer tube varies. This necessitates a variable IF channel for the first intermediate frequency. Two tuned circuits are used in the variable frequency IF stage. The second mixer is a type 6BA7 tube, the injection grid of which is tuned to the frequency of the variable IF. To produce the second IF of 455 kc (fixed), the output of a precision variable frequency oscillator is fed into the signal grid of the second mixer tube. This oscillator employs a 6BA6 tube in a highly stabilized temperature compensated circuit followed by a 6BA6 isolation stage. The output of the second mixer tube is amplified by a 455 kc IF channel composed of three 6BA6 tubes. A 6AL5 tube as a detector and AVC rectifier follows the IF channel. The audio produced by the detector is amplified by 1/2 of a 12AX7 voltage amplifier and a 6AQ5 power amplifier. AVC bias is produced by 1/2 of 12AX7 tube in an AVC amplifier circuit. A type 6BA6 tube is used in a BFO circuit coupled to the detector input for CW reception. Single conversion is employed for the 160 meter band wherein the signal is amplified by V-1 and fed directly to the grid of the second mixer through the variable IF filter.
TUNING – Tuning of the RF stage, the first mixer, the variable IF stage, the second mixer and the VFO is accomplished by changing the inductance of the tuned circuits by means of powdered iron cores varied within the magnetic field of the coils involved. The tuning cores of all of the above stages are ganged together and are varied as one unit. The inductance of each coil is trimmed with a similar iron core whereas the capacitance trimming of each coil is done with a variable ceramic capacitor.
An unusual method of band change is employed in the 75A receiver for all bands other than the 160 meter band. In the RF and first mixer stages, the inductance of only one set of coils, the 80 meter set, is directly varied by the tuning cores. To change bands, the 80 meter coils are paralleled with tuned circuits having characteristics which will combine with the 80 meter coils to produce tuned circuits suitable for the new frequency range. Five sets of tuned circuits are used, one set for each band. In each case, however, the 80 meter coil is the only coil in which the inductance is directly varied by the tuning apparatus. Refer to the complete schematic, figure 5-5. The 160 meter band has its own separate antenna coil. The first mixer and crystal oscillator are not used in 160 meter operation. The high frequency range of the variable IF channel is produced by paralleling the tuned i-f coils with additional fixed tuned circuits.
The tuning ranges of the coils in both the RF portions and the variable IF portions are 1000 kc in the 160, 80, 40, 20 and 15 meter bands and 2000 kc in the 11 and 10 meter bands. The frequency coverages of the RF stages are:
160 meters = 2.5 to 1.5 mc 15 meters = 20.8 to 21.8 mc
80 meters = 3.2 to 4.2 mc 11 meters = 26.0 to 28.0 mc
40 meters = 6.8 to 7.8 mc 10 meters = 28.0 to 30.0 mc
20 meters = 14.0 to 15.0 mc
The frequency coverage of the variable i-f stage is: 160, 80, 40, 20, 15 meter bands = 2.5 to 1.5 mc; 11 and 10 meter bands = 5.455 to 3.455 mc. In order to produce heterodynes suitable for amplification by the variable frequency i-f stage i.e., 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles, six high frequency oscillator outputs are necessary. These are obtained by the use of a crystal oscillator and six crystals (one for each band except 160 meters).
In each case, the high frequency oscillator output is higher in frequency than the received signal by 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles depending upon which band is being used.
Refer to figure 4-2. In order to get a 455 kc heterodyne for the second, or fixed, IF amplifier stages, it is necessary to introduce another signal to beat against the variable IF. Since the output of the variable IF changes from 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles, the output frequency of this new signal must also be variable and in the ranges 2.955 to 1.955 megacycle and 5.910 to 3.910 megacycles . These requirements are met by the use of a Collins 70E-12 precision oscillator which has a fundamental output frequency range of 2.955 to 1.955 megacycles. The second harmonic of the oscillator is 5.910 to 3.910 megacycles; the second harmonic output is used when the variable IF is 5.455 to 3.455 megacycles (when tuning in the 11 and 10 meter bands). The output of the variable i-f and the VFO are mixed in V-4 and the resultant 455 kc output is fed to the first 455 kc amplifier V-5.
The 455 kc intermediate frequency is amplified by a three stage amplifier, the output of which is rectified and sent through the noise limiter and audio amplifiers.
The beat frequency oscillator employs a 6BA6 in a highly stabilized circuit. The dial used in varying the VFO frequency is calibrated +1 and -1 kc; a feature useful in CW work for reading frequency. With the receiver tuned to zero beat, if the dial is set at +1 kc, add 1 kc to the vernier dial reading at zero beat for the exact frequency of the received station or if the dial is set at -1 kc, subtract 1 kc. The BF0 PITCH control allows approximately +/-2000 cps change from zero beat.
Summarizing the above description of the tuning scheme of the 75A receiver; the received signal beats against the output of a crystal oscillator and produces an intermediate frequency which varies across the band. This variable intermediate frequency is mixed with a variabLe oscillator output to produce a fixed 455 kc i-f signal. The 455 kc signal is rectified and the resulting audio is fed through an automatic noise limiter to the audio stages. Linear tuning is accomplished by the use of a cam wound coil, in the VFO, which has the coil turns spaced non-linearly in such a manner that linear movement of the tuning plug within the coil produces a linear frequency output of the oscillator. In addition, a mechanical frequency correcting mechanism is attached to the oscillator tuning slug. All coils which are tuned by movement of the tuning dial are wound similar to the oscillator coil.
CRYSTAL FILTER – Refer to figure 5-5. The crystal filter in the 75A receiver functions as follows: The 455 kc IF channel input transformer T-3 has a tuned primary which is tuned to the intermediate frequency. The secondary on the transformer is a low impedance coil, the center tap of which is grounded. One stator of phasing capacitor C-58, is attached to one end of this secondary winding while one side of the filter crystal is attached to the other end. A bridge circuit is formed by attaching the rotor of the phasing control to the opposite side of the crystal. This point of attachment must return to ground (or center tap of the secondary of T-3) to complete the bridge of the circuit. This is done through the SELECTIVITY control resistors R-18, R-19 and R-20 or through IF coil L-24. The bridge circuit is necessary to balance out the capacity of the filter crystal holder plates to prevent the signal from bypassing the crystal. If the point of attachment of the rotor of C-58 and the output plate of the crystal was returned directly to ground, the Q of the crystal would be too high, therefore, resistors R-18, R-19, and R-20 are placed in series with the crystal circuit t vary the Q. When the SELECTIVITY switch S- 2 is in the zero position, the crystal is short circuited and the selectivity is determined by the receiver circuits only. When the SELECTIVITY control is in position 1, the crystal Q is at its lowest point because of the return circuit through L-24 (a parallel tuned circuit having high impedance). When the SELECTIVITY control is in position 2, the Q of the crystal circuit is improved because of the lower value of series resistance and so on through positions 3 nd 4 until at position 4 the series resistance is at the lowest useful value and the crystal Q is highest with a resultant high degree of selectivity.
Because the phasing capacity is across L-24, detuning of L- 24 would normally occur when changing the setting of the phasing condenser. To neutralize this effect, an additional set of stator plates has been placed on the phasing capacitor to compensate for this detuning.
NOISE LIMITER – A series type noise limiter is used in the 75A receiver for phone reception. This limiter employs 1/2 (pins 1 and 7) of the type 6AL5 dual diode tube V-10. Refer to figure 4-3. Due to AC loading of the second detector, heavy noise impulses are automatically clipped from the positive audio peaks in the detector. The noise appearing on the negative side of the audio cycle is clipped by the noise limiter. In operation, a negative voltage produced by rectification of the carrier, is developed across capacitor C-84. This voltage cannot change rapidly due to the size of C-84 and R-42 through which C-84 is charged. This negative potential is placed upon the cathode of the noise limiter tube through R-41. The cathode is then negative in respect to the plate of the noise limiter tube and plate current flows. This plate current is modulated by the receiver audio. The modulated plate current produces audio on the noise limiter cathode (to which the grid of the audio amplifier section of V-9 is connected). The noise limiter diode will conduct as long as the cathode is negative in respect to the plate, however, when a heavy noise impulse is received, the plate is being driven negative faster than the cathode can follow (due to the time constant of R-42 and C-84). If the plate is driven more negative than the cathode, the tube will cease to conduct and no audio will reach the grid of the following audio tube. The audio cannot reach the cathode of the limiter tube directly from the bottom of the detector transformer because of the filtering action of R-42 and C-84. The percentage of modulation, at which the limiter clips, can be adjusted by changing the values of R-39 and R-40. Increasing R-39 and decreasing R-40 while keeping the sum of their resistances at approximately 100,000 ohms will raise the percentage of modulation at which limiting starts. In this receiver, limiting starts at approximately 35% modulation with sine wave input. Distortion will be evident on heavily amplitude modulated signals, particularly if clipping is used at the transmitter. Switch S-4 bypasses the audio signal around the noise limiter when receiving conditions do not require its use.
CW NOISE LIMITER – A separate noise limiter is used during CW reception. This limiter, a shunt type is bridged across the audio line to the 6AQ5 grid. This limiter short circuits the audio line on noise impulses above the level chosen by the operator. The value of limiting is adjustable by R-62, the CW LIMITER control. Refer to figure 4-3. A dual diode tube is used in this limiter. The adjusting bias applied to pin #1 is obtained from the main power supply. Capacitors C-86 and C-87 accumulate a charge so that clipping will occur equally on both the positive and negative portions of the audio cycle. This limiter is turned on automatically when placing switch S-3 in the CW position. If limiting is not wanted, the CW LIMITER control should be rotated to the counterclockwise position.
AUTOMATIC VOLUME CONTROL – The problem of blocking due to strong signals or heavy static is reduced by the use of an amplified AVC system and a low impedance AVC line. Refer to figure 4-40 The second triode section of V-8 is used as an AVC rectifier to produce the control voltage for the AVC section of amplifier tube V-9. The AVC voltage applied to the grids of the controlled tubes is produced by the voltage drop across resistor R-55 when plate current flows through the AVC amplifier tube V-90 Plate voltage for V-9 is obtained from the voltage drop across resistors R-36, R 37 and R-38 which are in series with the center tap of the power transformer to ground. V-9 will not draw plate current, however, with no signal input to the receiver because of approximately 11 volts of bias placed upon its grid by the voltage drop through R-36. This bias voltage for V-9 is taken from the end of R-32 through which the rectified carrier flows in opposition to the bias voltage. Thus, when the rectified carrier becomes strong enough to overcome the bias voltage on V-9, V-9 will draw plate current and produce a voltage drop across R-55 thereby producing AVC voltage in proportion to the strength of the received signal. The bias on the grid of V-9 is high enough to produce adequate delay in the generation of AVC voltage to allow the receiver to function with full sensitivity on weak signals. Resistor R-33 and capacitor C-81 form the time constant in the AVC circuit. R-34 and C-82 and R-35 are used in a degenerative circuit to prevent the AVC amplifier tube from responding to low audio frequency. The AVC is turned off by opening the plate circuit of the AVC amplifier tube V-9. Tubes controlled by the AVC bias included V-1, the RF amplifier, V-5, V-6 and V 7, the 455 kc IF amplifier tubes.
AUDIO AMPLIFIER – Two stages of audio amplification are employed in the 75A receiver. The first stage utilizes the second triode section of V-9 in a resistance coupled amplifier arrangement. A type 6AQ5 miniature pentode power amplifier tube is used in the audio output stage. This stage is biased with fixed bias obtained from the voltage drop produced across R-38 in the center tap lead of the high voltage transformer secondary. The 500 ohm secondary of the audio output transformer is tapped at 4 ohms to excite the voice coil winding of a speaker directly. Both the 500 ohm and the 4 ohm outputs are terminated on the rear of the chassis on terminal strip E-3. Headphone connections are also made to the 4 ohm tap. When the headphones are plugged into the headphone jack J-l, the speaker is disconnected and a 10 ohm loading resistor is connected across the 4 ohm winding in parallel with the headphones to load the 6AQ5.
148C-1 NARROW BAND FREQUENCY MODULATION ADAPTOR – The Model 148C-1 NBFM
adaptor employs a type 6AU6 tube as a limiter and a type 6AL5 tube as a frequency discriminator. The limiter tube provides constant input to the discriminator tube due to the high value of grid load resistance (R201). The discriminator circuit used in this adaptor relies on the phase difference be tween primary and secondary in coupled circuits. A 90 degree phase difference exists between the primary and secondary potentials of a double tuned, loosely coupled transformer when the resonant frequency is applied, and this phase angle varies as the applied frequency varies. The potentials at either end of the secondary winding with respect to a center tap on that winding are 180 degrees out of phase. When the center tap of the secondary is connected to one end of the primary, the potentials between the other end of the primary and each end of the secondary will reach maxima, one above and the other below the center frequency. At the center frequency, the resultant difference of potential between the two is zero. These potential differences vary at audio frequency rate when a frequency modulated signal is applied to the discriminator input. The audio frequency voltage is taken from the diode load resistors and sent through a de-emphasis network, R208 and C208, to pin number 2 of the power plug P203. The unit is ready to operate at all times by merely throwing the CW-AM-FM control on the 75A-2 Receiver to the FM position which disconnects the AM detector and substitutes the FM adaptor. The regular receiver audio circuits are used for FM reproduction. Operating voltages are provided by the receiver.
8R-1 CALIBRATOR UNIT – The 8R-1 Calibrator Unit uses a type 6BA6 tube in a Pierce circuit. A 100 kc crystal is used to give check harmonics at every 100 kc spot on the receiver dial. Capacitor C-301 is provided for zero beating the calibrator output with a known frequency standard such as a broadcast station in the tuning range of the 160 meter band or WWV at 2.5, 15 and 30 mc. The calibrator receives its operating voltages from the 75A-2 Receiver power supply and is turned on when the LIMITER control on the 75A-2 Receiver is placed in the CAL position. The output of the calibrator unit is coupled to the grid of the r-f amplifier tube V-1 through the capacity between pins 3 and 4 of crystal calibrator socket E-5.