75A-1
THE COLLINS 75A-1 RECEIVER
In recent years, the number of licensed amateurs has been increasing at an accelerated rate. The recent war has introduced a great many people to our great hobby. With the advent of undreamed numbers of amateur stations on the most popular amateur bands, it is apparent that a receiver capable of extreme selectivity and with a high degree of accuracy and stability will be necessary to maintain a high percentage of 100% QSO’s.
With this in mind, the Collins engineers designed a receiver especially for the amateur which solves the reception problems of the modern amateur better than any other receiver. In addition to superb selectivity and stability, the 75A receiver has a sensitivity which satisfies the most critical of DX hounds.
The AVC, image rejection, and cross modulation characteristics are in line with modern commercial practices. Embodying many new electrical and mechanical features never before used in an amateur receiver, it has been described as “The first really new amateur receiver since the advent of the superheterodyne circuit.” This is the receiver you amateurs have been waiting for.
DESCRIPTION
FREQUENCY COVERAGE – The Amateur Bands are covered as follows:
80 meters – 3.2 – 4.2 mc 15 meters – 20.8 – 21.8 mc
40 meters – 6.8 – 7.8 mc 11 meters – 26.0 – 28.0 mc
20 meters – 14.0 -15.0 mc 10 meters – 28.0 – 30.0 mc
BANDSPREAD – An entirely new system of permeability tuning provides linear calibration on all bands. Ten turns of the vernier tuning dial cover the ranges shown above. Each division of the vernier dial (which has 100 divisions) represents 1 KC on 80, 40, 20 and 15 meters, and 2 KC on the 11 and 10 meter bands.
ACCURACY AND STABILITY – Extreme stability and precise calibration assure visual tuning accurate to within 1 KC (one dial division) at 21 mc or 2 KC (one dial division) at 27 and 30 mc. This accuracy and stability is accomplished by the use of: (1) quartz crystals in the first conversion circuit, (2) the inherent accuracy and stability of the VFO in the second conversion circuit, and (3) linearity and absence of backlash ln the tuning mechanism. The stability is such that on CW reception extreme variation in the supply voltage causes a change of only a few cycles in the 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 This outstanding stability is available as soon as the receiver is turned on.
IMAGE AND IF REJECTION – The modern circuit design of the 75A has inherently high rejection to spurious frequencies Image rejection is a minimum of 50 db even on 10 meters. IF rejection is 70 db minimum.
SENSITIVITY AND SIGNAL TO NOISE RATIO – A one microvolt r-f input signal provides 1 watt audio output with approximately 6 db of signal to noise ratio at 300 ohms antenna impedance with a bandwidth of 4 kc.
SELECTIVITY – The crystal filter controls provide a bandwidth that is variable in 5 steps from 4 kc to 200 cycles at 2 times down (6 db down from the peak of the resonant frequency). There is no loss in gain caused by use of the crystal filter with the exception of the extremely sharp position which gives about 6 db loss.
AUTOMATIC NOISE LIMITER – The 75A receiver contains a new series type noise limiter developed during the war. It automatically adjusts itself to all signal levels.
AUTOMATIC VOLUME CONTROL – Constant output within 5 db is obtained for a change in r-f input from 5 microvolts to 0.5 volt. AVC is applied to the r-f stage 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. Two external adjustments are provided, one for zero adjustment, and one for adjusting the sensitivity to compensate for variations in antenna installations.
AUDIO OUTPUT – 2.5 watts of power are available.
TERMINAL IMPEDANCES
INPUT – The antenna input circuit is designed for a nominal 300 ohms impedance but will accommodate a wide variety of antennas both balanced and unbalanced without serious loss.
OUTPUT – 500 ohm and 4 ohm terminals are provided as well as a low impedance output for headphones.
CONTROLS – The following controls are on the front panel to provide complete operation of the receiver:
Tuning Control – ON-OFF Standby Switch Band – Switch – Crystal Selectivity Switch RF – Gain Control – Crystal Phasing Control – Audio Gain Control – AVC-Manual-CW Switch – CW Pitch Control – Noise Limiter Switch –
CIRCUIT – The double conversion circuit of the 75A employs fourteen tubes, including rectifier. The use of double conversion avoids the compromise always made in conventional receivers, i.e., a high IF is desirable for image rejection and a low IF is best for selectivity. The 75A uses two intermediate frequencies and has both features. Because of the high frequency of the first IF, only one stage of r-f amplification is needed to give extremely high image rejection. Additional stages are unnecessary and unwarranted. Following the r-f stage the incoming signal is mixed with the output of a crystal oscillator to produce the first IF. The first IF is amplified and mixed with the output of the variable frequency oscillator to produce the 500 KC second IF. The crystal filter is incorporated in the 500 KC second IF circuit. The audio is then removed from the carrier, passed through the automatic noise limiter, amplified, and fed to loudspeaker or headphones. BFO output is applied to the second detector. AVC voltage is obtained from the same point and fed to the controlled stages.
Permeability tuning is employed in the radio frequency, first and second mixer, and first IF stages. Gang tuning is accomplished by the use of a variable platform to which the powdered iron cores of the coils in the above stages are attached. The variable frequency oscilLator is also permeability tuned and ganged to the above assembly, however, a precision lead screw acts upon the tuning core of the oscillator core to vary the frequency of the oscillator. Permeability tuned transformers are also employed in the second IF stages. A separate tube is used to rectify the carrier voltage for AVC bias. Two audio stages provide ample amplification and power output for normal amateur requirements.
POWER SOURCE – The power supply is self contained and well filtered. It requires a 115 volt 50/60 cps source. Power consumption is 80 watts.
DIMENSIONS AND CABINET DESIGN – 21-1/8″ wide, 12-1/4″ high, 13-7/8″ deep overall. The chassis is mounted on a standard 19″ panel and can be removed from the cabinet and mounted in a standard relay rack.
WEIGHT – 57 lbs.
FINISH – St. James Grey wrinkle.
ACCESSORIES
MODEL 270G-1 SPEAKER – An external speaker is available, mounted in a matching cabinet The speaker cabinet measures 13″ wide, 10-19/32″ high, 6-5/8″ deep and the speaker and cabinet weigh 9 lbs.
HEADPHONES – Any good headphone may be used.
ANTENNA – Any good antenna may be used; however, a balanced antenna, well in the clear, connected to the receiver terminals through a 300 ohm transmission line is recommended.
Under the hood…
75A-1 CIRCUITRY
MECHANICAL
GENERAL – The 75A1 receiver is constructed in two major units, the receiver unit and the speaker unit. The receiver is constructed on an aluminum chassis. The receiver and speaker cabinets both 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 recessed and trimmed for neat appearance. Both the receiver and the speaker cabinets are finished in a hard St. James Grey wrinkle finish.
TUNING – The first completely permeability tuned amateur receiver to reach the market, the 75A, contains many new tuning principles and ideas. 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 which tune the r-f, first mixer, first i-f and second mixer stages are all mounted on a moveable 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 dial pointer is cord driven. The BFO coil is placed for most efficient operation and a long shaft is used to connect the tuning control with the panel knob. All other stages are fixed tuned with iron cores and variable ceramic capacitors.
BAND SWITCH – Band switching of r-f stages is accomplished by means of a multiple section switch gang. Each switch section with its accompanying components is completely shielded from the others. In addition to r-f circuits, the band switch selects crystals and dial illumination lamps for the various bands.
ELECTRICAL THEORY
GENERAL – Refer to block diagrams of the receiver, figures 1-1 and 2-1. The general plan of the 75A receiver is a result of efforts to give to the amateur a receiver which has a stability and calibration accuracy never before obtainable in any amateur receiver. In addition, the receiver features an image rejection ratio, selectivity ana sensitivity not found in many receivers of modern design. Improved AVC and noise limiter circuits are incorporated to complete the long list of desirable features of the equipment. How these features are obtained is explained in subsequent paragraphs.
CIRCUIT – As shown in the block diagram, figure 1-1, the receiver has one stage of pre-selection. A high gain 6AK5 tube is used here because of its excellent electrical characteristics and desirable physical features. Following the r-f stage is the first mixer of the double detection system. The signal grid of this tube, a 6SA7, is tuned to the received frequency, the injection grid receives voltage from the high frequency oscillator circuits at a frequency within a band 2.5 to 1.5 mc or 5.5 to 3.5 mc removed from the received frequency. This oscillator voltage is supplied by a 6AK5 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 i-f channel for the first intermediate frequency. A type 6SK7 tube is used in the variable frequency i-f stage. The second mixer is a type 6L7 tube, the signal grid of which is tuned to the frequency of the variable i-f. To produce the second i-f frequency of 500 kc (fixed), the output of a precision variable frequency oscillator is fed into the injection grid of the second mixer tube. This oscillator employs a 6SJ7 tube in a highly stabilized, temperature compensated circuit. The output of the second mixer tube is amplified by a 500 kc i-f channel composed of two 6SG7 tubes. A 6H6 as a detector and noise limiter follows the i-f channel. The audio thus produced is amplified by a 6SJ7 voltage amplifier and a 6V6 power amplifier. AVC bias is produced by a 6SJ7 in a controlled rectifier circuit. A type 6SJ7 tube is used in a BFO circuit coupled to the detector input for cw reception.
TUNING – Tuning of the r-f stage, the first mixer, the variable i-f stage, the second mixer, and the VFO is accomplished by changing the inductance of the tuned circuits by means of a powdered iron core 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 ceramic capacitor.
A unique method of band change is employed in the 75A receiver. In the r-f and first mixer stages, the inductance of only one set of coils, the 80 meter, is varied by the tuning slugs. To change bands, the 80 meter coils are paralleled with a tuned circuit having characteristics which will combine with the 80 meter coils to produce a tuned circuit 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 varied by the tuning apparatus. Refer to the complete schematic, figure 5-4. The two frequency ranges of the variable i-f channel are produced in like manner.
The tuning ranges of the coils in both the r-f portion and the variable i-f portion are 1000 kc in the 80, 40, 20 and 15 meter bands and 2000 kc in the 11 and 10 meter bands.
The frequency coverages of the r-f stages are:
80 meters = 3.2 to 4.2 mc 15 meters = 20.8 to 21.8 mc
40 meters = 6.8 to 7.8 mc 11 meters = 26.0 to 28.0 mc
20 meters = 14.0 to 15.0 mc 10 meters = 28.0 to 30.0 mc
The frequency coverages of the variable i-f are:
80, 40, 20, 15 meter bands = 2.5 to 1.5 mc, 11 and 10 meter bands = 5.5 to 3.5 mc
In order to produce heterodynes suitable for amplification by the variable frequency i-f stage i.e. 2.5 to 1.5 mc or 5.5 to 3.5 mc, fixed frequency high frequency oscillator outputs are necessary. These are obtained by the use of a crystal oscillator and six crystals (one for each band). Since it is impractical to get crystals with fundamental frequencies as high as is necessary for the higher frequency bands, low frequency crystals and harmonic operation is employed.
Crystal frequencies vs harmonic output frequencies are shown below:
CRYSTAL OUTPUT
BAND FREQ (kc) MULTIPLIER FREQ (kc)
80 5700 1 5700
40 9300 1 9300
20 8250 2 16500
15 11650 2 23300
11 10500 3 31500
10 11166 3 33500
In each case, the high frequency oscillator harmonic output is higher in frequency than the received signal by 2.5 to 1.5 mc or 5.5 to 3.5 mc depending upon which band is being used.
Refer to figure 2-1. In order to get a 500 kc heterodyne for the second, or fixed, i-f amplifier stages, it is necessary to introduce another signal to beat against the variable i-f. Since the output of the variable i-f does change from 2.5 to 1.5 mc or 5.5 to 3.5 mc, the output frequency of this new signal must also be variable in the ranges 2.0 to 3.0 mc and 4.0 to 6.0 mc. These requirements are met by the use of a Collins 70E-7 precision oscillator. The fundamental output frequency range of this oscillator is 2.0 to 3.0 mc. The second harmonic therefore, would be 4.0 to 6.0 mc, the 4.0 to 6.0 mc output being used when the variable i-f is 5.5 to 3.5 mc (when tuning in the 11 and 10 meter bands).
This 500 kc difference frequency is amplified by two stages of fixed i-f amplification, the output of which is detected and sent through the noise limiter and audio amplifiers.
The beat frequency oscillator employed in this receiver is highly stabilized and the dial used in varying the frequency is calibrated +1 and -1kc. This feature is useful in cw work, for reading frequency. If the dial is set at +1 kc, add 1 kc to the vernier dial reading for the exact frequency of the received station or if the dial is set at -1 kc, subtract 1 kc.
Summarizing the above description of the tuning scheme of the 75A receiver, it can be seen that the received signal beating against the output of a crystal oscillator produces an intermediate frequency which varies across the band. Therefore, a variable i-f amplifier is used, following the first mixer tube, which covers the frequency range of the beat note of this intermediate frequency. Now in order to get a 500 kc beat note, the output of a variable oscillator is beat against the output of the variable i-f stage. The 500 kc heterodyne thus produced is amplified by a fixed tuned amplifier. The unequalled stability of the receiver is obtained because of the inherent stability of the quartz crystals in the first oscillator and the highly stabilized output of the 70E-7 variable frequency oscillator which operates in a frequency range more readily controlled.
Linear tuning is accomplished by the use of a cam wound oscillator coil which has the coil turns spaced non-linear in such a manner that linear movement of the tuning slug within the coil produces a linear frequency output of the oscillator. In addition, a mechanical frequency correcting mechanism is attached to the tuning slugs all coils which are tuned by movement of the tuning dial are wound in similar fashion.
The high degree of selectivity obtainable with this receiver is due to an efficient crystal filter circuit in addition to the many tuned circuits.
CRYSTAL FILTER – Refer to figure 5-4. The crystal filter in the 75A receiver functions as follows. The 500 kc i-f channel input transformer T1 has a tuned primary which is tuned to the intermediate frequency. The secondary on this transformer is a low impedance coil, the center tap of which is grounded. One stator of phasing capacitor C71 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 coil) to complete the bridge circuit. This is done by means of the selectivity control resistors R21, R22, and R23 or through i-f coil T2. The bridge circuit is necessary to balance out the capacity of the filter crystal holder plates to prevent the signal from by-passing the crystal. If the point of attachment of the rotor of C71 and the output plate of the crystal was returned directly to ground, the Q of the crystal would be at its highest point and the selectivity would be so great as to be almost unusable, therefore, resistors R21, R22, and R23 are placed in series with the crystal circuit to vary the Q. When the SELECTIVITY switch S11 is in the “O” 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
T2 (a parallel tuned circuit having high impedance). When the SELECTIVITY control is at “2”, the Q of the crystal circuit is improved because of the lower value of series resistance and so on through position 3 and 4 until at position 4 the series resistance is at the lowest usable value and the crystal Q is highest with a resultant high degree of selectivity.
Because the phasing capacity is across T2, detuning of T2 would normally occur then changing the setting of the phasing condenser. To neutralize this effect an additional set of stator plates has been placed on the phasing capacitor which compensates for this detuning effect.
NOISE LIMITER – Refer to figure 2-2. One half of V-8, a type 6H6 tube, is used as a noise limiter. The circuit employed here is a new circuit developed for military use. In this circuit the negative half of the audio wave is automatically clipped at approximately 35% modulation by virtue of the heavy value of AC load impedance ln the detector circuit. This eliminates the noise peaks from the negative half of the audio wave. However the noise peaks still appear on the positive half of the audio wave so the automatic noise limiter is inserted in the circuit to remove these. This limiter is a series type limiter in that it is placed between the detector and the first audio stage. In operation, the plate of the noise limiter tube has a voltage, taken from the detector load resistor, placed upon it. Since this voltage is positive with respect to the limiter tube cathode, current flows through this portion of the tube. This current is modulated at the cathode by audio from the detector through capacitor C78, thus the audio appears at the plate of the limiter tube from where it is fed to the grid of the audio amplifier tubes. Since the positive audio peaks appear as positive impulses across the detector diode load, the audio impulses through C78 are positive or in opposition to the negative potential on the cathode of the noise limiter tube. Whenever these positive audio impulses get high enough in amplitude to cancel this negative cathode potential, the tube ceases drawing plate current and the audio is interrupted. The value of plate voltage applied to the limiter plate can be set by varying the sizes of the circuit components therefore the cut-off point can be set at any degree of modulation desired. In the 75A receiver this is set at approx. 35% modulation. As a result, any degree of modulation above approx. 35% is clipped on both the positive and negative halves of the audio; therefore, any noise impulses which are greater than 35% modulation are also clipped off. Since the operating voltage for the limiter is taken from the detector load resistor, the clipping level is always at approx. 35% regardless of how weak or how strong the signal becomes. In-as-much as regular speech frequencies do not suffer in intelligibility to any great extent under such circumstances, this system makes an efficient noise limiter.
A filter composed of R39, C80B, and R40 is inserted in the noise limiter plate return to prevent any of the audio from the diode load resistor reaching the noise limiter output directly rather than through the noise limiter tube. A switch, S14, has been placed in the circuit to by-pass the noise limiter where operating conditions do not require its use.
AUTOMATIC VOLUME CONTROL – In order that the receiver may operate at peak efficiency on weak signals, a system of delayed AVC is employed. Refer to figure 2-3. Notice that the cathode of V9 is connected to ground through a voltage divider consisting of R43 and R44. B+ voltage is introduced to the cathode of V9 through R43. This places the delay voltage on the cathode of V9. The plate of V9 is coupled to the i-f amplifier through C74 and therefore, as soon as the received signal becomes strong enough to overcome the positive bias on the cathode of V9, rectification of the signal takes place and AVC control voltage appears across the load resistor R36. At the same time, rectification of the i-f signal is taking place in the detector circuit, and since the grid of the AVC tube, V9, is connected to the positive side of the detector load resistor, the delay voltage is being canceled by the positive detector voltage. Therefore, the delay voltage is canceled out allowing the full AVC voltage to be realized, and at the same time an accelerating effect is produced allowing the S-meter to begin functioning sooner than in other delay circuits.
AVC voltage developed across load resistor R36 is fed to the controlled stages through filter resistor R35. Filter resistor R35 and filter capacitor C80A remove AC components from the AVC line. The r-f amplifier, variable i-f, and the first and second fixed i-f