Collins 75a-1 manual

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. 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. 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 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 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. 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 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 kc in the 80, 40, 20 and 15 meter bands and kc in the 11 and 10 meter bands.

The frequency coverages of the variable i-f are:. In order to produce heterodynes suitable for amplification by the variable frequency i-f stage i. 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:. In each case, the high frequency oscillator harmonic output is higher in frequency than the received signal by 2. Refer to figure In order to get a 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. These requirements are met by the use of a Collins 70E-7 precision oscillator.

The fundamental output frequency range of this oscillator is 2. The second harmonic therefore, would be 4. This 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. This feature is useful in cw work, for reading frequency.

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 kc beat note, the output of a variable oscillator is beat against the output of the variable i-f stage. The 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. The crystal filter in the 75A receiver functions as follows. The 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.

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. 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.

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.

Numbers Stations. Shortwave Listening. Radio Clubs. International Broadcasters. Scanner Listening. Web-Controlled Radios. See Comments. Readout: Analog Lin. Physical: Uses 70E-7 sealed VFO. IF , kHz. Accessories: C 1. Another theory suggests that a small run of less than 75As were made in Ranges: 3. Only the band in use is illuminated. Dial accuracy is to 1 kHz 2 kHz above 26 MHz. Virtually no backlash. James gray wrinkle finish.

Requires a speaker. Uses 70E sealed VFO. Requires speaker. The 75A-3 saw the introduction of the famous Collins Mechanical Filter.