The "front-end" circuit is shown in Figure 18A. The tuning capacitors placed across the input and output circuits of the mechanical filters are determined by the choice of filter. See filter specification sheet.
Type F455C filters were used in this receiver.
M—0-1 d-c milliammeter, 1-inch diameter
SiA-B—DPDT rotary switch. Centralab PA-2001 decks, with Centralab PA-302 index assembly
SiA-B-C-D—4 pole, 3 position switch. Centralab PA-1013
T—1500 kc. i-f transformer. J. W. Miller Co. 5I2-W1. Remove turns from primary and secondary to permit resonance at 1695 kc. l-Ti—455 kc. i-f transformer. J. W. Miller Co. I2-CI, or equivalent I—455 kc. diode transformer. J. W. Miller Co. 12-C2, or equivalent I—455 kc. BFO transformer. J. W. Miller Co. 112-C5, or equivalent —150-0-150 volts at 100 ma. UTC 5-5J :—10 K pri., 4 ohm sec., Stancor A-3879 •—6.3 volts, 3 amp., Stancor P-6466
reading on the 0-1 d-c milliammeter connected between the plates of the 12AU7 triodes.
The selenium rectifier power supply delivers 140 volts at the receiver current drain of 65 milliamperes. The low plate voltage and small power consumption of the receiver result in a very low degree of thermal drift. If desired, the selenium supply may be replaced with a more conventional supply using a 5Y3-GT rectifier tube. Plate voltage to the receiver should be held below 160 volts. Plate voltage to the 6J6 oscillator section and the 12AU7 S-meter tube is regulated by an OB2 gas regulator.
Plate voltage is removed from the r-f and audio stages when the receiver is in "standby" condition. All oscillators are left running continuously to reduce initial warmup drift.
Receiver The receiver is built upon a plated Layout steel chassis measuring 8" x 15" x 2i/2". Panel height is 8". To achieve maximum rigidity, the panel is braced to the chassis with two steel angle brackets visible in figure 19. The tuning dial is centered on the panel, and the three gang tuning capacitor is behind the dial, on the center-line of the chassis. Directly to the left of the tuning capacitor is a 5Vi" x 5" cut-out in the chassis. The TV tuner is placed in this cut-out, held in place by two 3" angle brackets cut from aluminum stock. Height of the tuner above the chassis is determined by the shaft of the tuner which must pass through the bushing drilled in the lower left corner of the dial escutcheon. Likewise, the placement of the three gang tuning capacitor G is determined by the shaft height of the main dial bushing. G is therefore mounted above the chassis by means of long 6-32 bolts and steel spacers.
Placement of the major components may be seen in figures 19 and 20. The two mechanical filters are mounted to the left of the tuner, directly behind the 6BA7 mixer tube and the 1695 kc. i-f transformer, Ti. The filter selection switch Si is mounted below chassis so that the switch segments fall over the filter terminals. This switch is made up from a switch kit, and is mounted in place by means of two brackets cut from soft aluminum. One bracket holds the front panel bushing of the switch, and the other is placed just forward of section SiB and holds the two fixed shafts of the switch. A hole is cut in the center of the bracket to clear the rotary shaft of the switch. This bracket also serves as a shield isolating the output circuits of the mechanical filter from the input circuits. The particular switch used is highly recommended, and it is suggested that no substitution be made. It is imperative that the drive shaft of the switch be metal, and that it is securely grounded. Some switches have fibre drive shafts, or metal shafts held in position with a cotter pin. Fibre drive shafts permit signal leakage along the shaft, deteriorating filter performance, and the cotter pin-type switch does not insure the shaft is securely grounded which is mandatory, when the metal shaft-type switch is used.
The three intermediate frequency stages are spaced along the back of the chassis. At the right rear are the 6AK6 audio stage, the volt-
Figure 19 TOP VIEW OF AMATEUR COMMUNICATION RECEIVER
Mechanical filters are at left with three stage l-f amplifier along rear of chassis. The TV tuner It mounted in chassis cutout to the left of main tuning capacitor. Power supply and audio section occupy right-hand section of steel chassis. Six channels of tuner are used to cover the amateur bands between 80-and 6-meters. Six spare channels may be used to cover shortwave broadcast bands, or portions of 100 Mc. region.
age regulator tube, and the 12AU7 S-meter tube. In front of the last i-f transformer can are the two 6AL5 tubes and potentiometers Re and Ra-
At the right end of the chassis is the power transformer, and between it and the panel is the b-f-o transformer and the 12AT7 au-dio/b-f-o tube. Between the power transformer and the three gang tuning capacitor are the power supply filter capacitor and the auxiliary filament transformer. The i-f gain control potentiometer Ri is placed between the filament transformer and the main tuning capacitor.
Placement of the smaller components below the chassis may be seen in figure 20. The TV turret and mechanical filter switch are at the right of the chassis, with the audio output transformer mounted to the back wall of the chassis near the turret. The filter choke for the power supply is mounted to the left side wall of the chassis, with the two selenium rectifiers in front of it. The 10 watt dropping resistor for the regulator tube is mounted to the chassis beween the filter choke and the rear tube sockets.
Construction in two parts to simplify construction. The power supply, audio and 455 kc. i-f sections are built first, and the receiver is placed in operating condition up to the 1695 kc. i-f channel. The TV tuner is then modified, using the rest of the receiver for alignment and adjustment tests.
All major components should be marked in position on the paper wrapper of the chassis which is then drilled and punched. All major components with the exception of the TV turret are mounted in place. The ground connections at each tube socket are made first, then the filament leads are wired in position. Use #14 insulated wire for the filament leads, as the current drain is quite high. The power supply should be wired first and tested. It should deliver approximately 140 volts across a 2000 ohm, 10 watt resistor used as a temporary dummy load. When the power supply section is completed, the audio stages, b-f-o, S-meter, second detector, and noise limiter stages can be wired. Operation of the audio system may be checked by injecting a small audio signal on pin #2 of the 12AT7 stage and checking the volume of the tone in the loud speaker.
The next step is to wire the three i-f amplifiers and the a-v-c circuit. Five terminal phenolic tie-point strips are mounted in front of each i-f tube socket. The a-v-c, cathode bias, and screen dropping resistors are mounted between the socket pins and the terminals of the strip. The ceramic bypass capacitors are soldered directly between socket pins with the shortest possible leads. All interconnecting wiring is done between the terminal strips-After the i-f strip has been wired up to the arm of switch S>, segment B, the amplifier may be tested. Pin # 1 of the first 6BJ6 i-f tube is temporarily returned to the a-v-c bus by placing a 100 K resistor across the output pins of the mechanical filter socket. Switch Si is adjusted to place the resistor in the circuit.
Figure 20 UNDER-CHASSIS VIEW OF AMATEUR BAND RECEIVER
To the left of the chassis are the selenium rectifiers and the filter choke. At the extreme right is the mechanical filter selector switch mounted on two aluminum brackets. The controls along the panel are (left to right): BFO, AUDIO GAIN, MAN—AM—CW, tuning control, TV turret switch, R-F GAIN, and filter selector switch. Output transformer is mounted to back of chassis.
A signal generator having a tone modulated 455 kc. signal is attached to the input grid of the 6BJ6 tube, and an a-c voltmeter is connected across the speaker terminals. To insure that the i-f system is aligned at the center of the filter passband it is imperative that the alignment frequency be very close to 455.0 kc. The use of a BC-221 or equivalent signal generator is recommended. As the transformers are brought into alignment the input signal to the i-f strip should be decreased. A vacuum tube voltmeter may be employed to measure the negative voltage developed on the a-v-c bus. A maximum voltage of about — 8 volts is developed with a strong input signal to the i-f strip. Overall gain is controlled by the setting of Ri when switch S3 is placed in the "A-M" position.
After i-f alignment has been completed, the 100 K input resistor should be removed from the filter socket and the 6BA7 mixer stage wired. The capacitors connected between pins 1 and 2 and ground of the 6BA7 socket determine the level of excitation to the 2150 kc. crystal. If difficulty is encountered in obtaining consistent oscillation, the value of one or both of these capacitors may be altered until reliable oscillation is obtained. Operation of the crystal oscillator may be monitored with a nearby receiver tuned to the crystal frequency. When the oscillator is working properly, a 1695 kc. tone modulated signal should be coupled through a 0.01 /xfd. blocking capacitor to the "hot" end of the primary winding of transformer T. This transformer is adjusted for maximum output signal. The receiver is now ready for tuner installation.
Tuner The tuner used in this re-
replacement type unit using a 6BQ7A cascode r-f amplifier and a 6j6 mixer stage. Several different variations of this tuner can be purchased at nominal prices through large wholesale outlet stores. The tuner has snap-in coil strips for 12 TV channels, and may have either a 21 Mc. or 42 Mc. i-f output range. Several simple changes must be made to the tuner for use in this receiver. The i-f output coil and the cascode neutralizing circuit must be removed. The best thing to do is to remove all the clip-in coils and trace out the tuner circuit, and then compare it with the circuit shown in figure 18A. Cathode bias must be added to the first section of the cascode and several of the resistors in the tuner must be changed. Rewire the tuner according to the circuit of figure 18A. Note that regulated voltage is applied to the oscillator section of the 6J6 and that a-v-c voltage is applied to the first section of the 6BQ7A. The main tuning capacitor C1A-B-C is connected to the appropriate points in the tuner by three short leads. A M-inch coil is drilled in the rear of the
Was this article helpful?