• REJECTION TUNING by a single control provides 50 db nominal rejection of unwanted heterodynes and carriers.
• VARIABLE BFO results in additional convenience to CW and RTTY operators. The variable BFO can be utilized in either the CW or SSB mode.
• OPTIONAL 500, 800 or 1500 cps Mechanical Filter offers sharp selectivity for CW. An optional 200 cps crystal filter is also available.
• 2.1 KC MECHANICAL FILTER assures sharp skirt selectivity and optimum passband for SSB. Background noise is effectively reduced by restricting bandwidth to only that required for communication. It can also be used for CW and RTTY.
• ZENER REGULATED OSCILLATORS provide increased stability and less variance due to line voltage changes.
• AGC CONTROL KNOB disables AGC or selects two AGC decay time constants. This permits choice of AGC characteristics to suit operating conditions.
• CONCENTRIC RF AND AUDIO GAIN CONTROL arrangement on the 75S- 3B receiver offers increased operating convenience. AUDIO OUTPUT of 3.0 watts assures adequate speaker levels.

Coverage outside the amateur bands, or additional 10 meter- band frequency coverage, can be obtained by plugging in the appropriate crystals.

The pitch for CW reception can be varied by turning the BFO control on and adjusting the knob for the most pleasing beat note, with the desired signal centered in the filter passband. The calibration marks can be used to shift from high mark to low mark when you are copying RTTY.

When the BFO knob pointer is at the 0 reference mark, the BFO frequency is approximately the same as crystal BFO in USB position. With the knob in the off position, the crystal BFO is in operation.

RTTY operation requires a conventional converter and printer. Fine tuning in this mode is easily accomplished with the variable BFO.

Also available is the 75S-3C, providing extended range coverage. It is identical to the 75S-3B except that an additional HF crystal board and front panel selector switch are included. The board for additional crystals is located on the top of the chassis and the standard group of amateur band crystals is mounted in the board on the underside of the chassis.

75S-3B/3C SPECIFICATIONS

FREQUENCY RANGE: 3.4-5.0 mc and 6.5-30 mc, for the following bands:

80 meters – 3.4-3.6 mc, 3.6-3.8 mc and 3.8-4.0 mc.
40 meters – 7.0-7.2 mc and 7.2-7.4 mc.
20 meters – 14.0-14.2 mc and 14.2-14.4 mc.
WWV       – 14.8-15.O mc.
15 meters – 21.0-21.2 mc, 21.2-21.4 mc and 21.4-21.6 mc.
10 meters – 28.5-28.7 mc.

MODE: Selectable USB, LSB, CW or AM.

TYPE OF SERVICE: Selectable single sideband, CW, RTTY and AM continuous.

POWER REQUIREMENTs: 115 v, 50-60 cps. Power consumption is
approximately 85 watts. Power can be provided by an external
supply which delivers 185 v dc at 125 ma and–62 v dc at 5 ma.
Filament power can be ac or dc as follows: 6-7 v at 5.5 amps,
12 4 v at 2.75 amps or 24-28 v at 1.4 amps.

HARMONIC AND OTHER SPURIOUS RESPONSE: Image rejection better than 50 db.
Internal spurious signals below 1 uv equivalent antenna input.

AUDIO NOISE LEVEL: Not less than 40 db below 1 watt.

AMBIENT TEMPERATURE: 0 -50 C.

AMBIENT HUMIDITY RANGE: 0%-90%.

ALTITUDE: 0-10,000 ft.

PROTECTIVE DEVICES: The ac line has a 1 amp SB fuse.

SIZE: Receiver with feet – 14-3/4″ W, 7-3/4″ H, 12-1/2″ D.

WEIGHT: 20 Lbs.

CALIBRATOR: 100 kc crystal.

FREQUENCY STABILITY: Within 100 cps after warm-up.

CALIBRATION ACCURACY: 1 kc after midband calibration.

BACKLASH: Not more than 50 cps.

VISUAL DIAL ACCURACY: 200 cps on all bands.

SENSITIVITY: 0.5 uv for 10 db signal-plus-noise-to-noise ratio
in SSB mode.

SELECTIVITY: SSB – 2.1 kc, 2:1 shape factor. CW – two switch
positions and sockets; no filters supplied. AM–5 kc.
Optional filters for 0.2, 0.5, 0.8, 1.5, 3.1, 4.0 or 6.0 kc
are available.

VARIABLE BFO: Tunes 452.35-458.35 kc.

AUTOMATIC GAIN CONTROL: AGC threshold – 1.5-3.0 uv, 1.5 nominal.
Selectable AGC time constant, Fast, Slow and Off. Attack time
is 1 millisecond in both Fast and Slow. Fast release time is
190 milliseconds. Slow release time is 600 milliseconds.

AUDIO OUTPUT LEVEL: 1 watt at AGC threshold, 3 watts maximum.

ANTENNA INPUT: 50 ohms nominal +/-50%.

AUDIO OUTPUT REQUIREMENTS: Speaker – 3-4 ohms.
Headphones – 500 ohms or higher.

AUDIO DISTORTION: Not more than 10% at 1 watt.

MUTING: Receiver silenced during transmit.

Under the hood…

75S-3B/3C CIRCUITRY

Double conversion is used with injection voltage for the first conversion provided by a crystal-controlled oscillator. A bandpass if 200 kilocycles wide is used to couple the first and second mixers. Injection voltage for the second mixer is furnished by a vfo with a tuning range of 200 kc. The 455-kc output frequency of the second mixer is coupled through the if. system to separate AM and SSB detectors . Injection voltage for the product detector is provided by either a crystal-controlled bfo or a tunable bfo. The 75S-3C is electrically identical to the 75S-3B, except that it is equipped with an extra hf crystal mounting board on the chassis, a crystal board selector switch on the front panel, and associated components.

RF and Mixer Circuits

The rf amplifier grid, high-frequency mixer grid, and crystal oscillator plate circuits are resonated by slug- tuned coils. The slugs are mechanically ganged and linked to the PRESELECTOR tuning knob. The required tuning ranges of these circuits are obtained by switching appropriate values of fixed capacitance in parallel with the coils. The total 3.4- to 30-mc tuning range of the receiver is divided into five segments for band switching purposes, as noted in table 2-1. The tuned-circuit LC ratio is thereby varied within appropriate limits for each of the five segments.

Signals within the particular 200-kc band selected are amplified by V2, the rf amplifier, and coupled to the control grid of V3A, the first mixer. Injection voltage is coupled to the cathode of V3A. Products of mixing are selected in the plate circuit of V3A tuned from 3.155 to 2.955 mc which is the bandpass intermediate frequency. Signals are coupled to the control grid of second mixer V4A with vfo injection voltage applied to the cathode of this tube.

Oscillator Circuits

CRYSTAL OSCILLATORS

High-frequency crystal oscillator V3B provides injection voltage for the first mixer. The crystal oscillator output frequency is always 3.155 mc higher than the lower edge of the selected band. On bands below 12.0 mc, the oscillator plate circuit is tuned to the crystal frequency. At 12.0 mc and higher, the plate circuit is tuned to the second harmonic. The secondary winding of T2 couples injection voltage to the first mixer cathode circuit and furnishes a dc return to ground for mixer tube V3A. Dummy load R41 simulates the load presented by a transmitter when connected for transceiver operation.

Crystal-controlled bfo V8B and associated circuitry furnishes injection voltage for the product detector. Crystals Y15 and Y16 provide the proper bfo frequency relationships to the mechanical filter passband to yield optimum audio response from the product detector. Crystal Y15 (453.650 kc) is used for lower sideband reception, and Y16 (456.35 kc) is used for upper sideband. This is due to sideband inversion in the first mixer. Capacitor C95 and coil L12 form a broadly resonant circuit at 455 kc. Oscillator voltage is developed across R49 and coupled by C100 to the cathode of V8A, the product detector tube.

The crystal calibrator circuit provides marker signals at multiples of 100 kc. Variable capacitor C61 provides for adjustment to zero beat with WWV. The output of this oscillator is coupled to the receiver antenna circuits. Diode CR8 assists in the generation of the higher frequency harmonics.

VARIABLE OSCILLATORS

The vfo uses fixed capacitance and variable inductance to produce the required tuning range of 2.50135 to 2.70135 mc for LSB reception and 2.49865 to 2.69865 mc for USB, AM, and CW reception. Capacitor C303, in the frequency-determining network, is paralleled by variable capacitor C308 in series with diode CR301. This diode switches C308 in or out of the circuit depending on the polarity of the bias voltage impressed across its junction. With the MODE switch in the LSB position, diode CR301 is reverse biased and switches capacitor C308 out of the frequency-determining network. This condition will result in the tunable 2.50135 to 2.70135 mc signal desired. With the MODE switch in the USB, AM, or CW position, diode CR301 is forward biased and switches C308 into the frequency-determining network lowering the output frequency to the tunable 2.49865 to 2.69865 mc signal desired. Note that when C308 is properly adjusted, it shifts the vfo frequency by an amount equal to the frequency separation of crystals Y15 and Y16. This allows either sideband to be selected without retuning or recalibrating the dial. The vfo output voltage is coupled to the cathode of second mixer tube V4A and to the control grid of cathode follower V4B. The cathode follower prevents loading of the vfo circuits by cable capacity when operated in transceiver service.

Tube V11 and associated circuitry comprise a 452.35 to 458.35 kc tunable bfo. The bfo tuning control is potentiometer R81. This control varies a positive dc voltage applied to the junction of voltage-variable capacitor CR4. The junction capacity of this device is proportional to applied voltage. Adjustment of R81 therefore varies the output frequency of the bfo. Voltage for the tuning circuit is stabilized by a regulator consisting of zener diode CR5 and resistor R82. Switch S13 completes the cathode circuit of either V8B or V11 thus turning on the desired bfo and turning off the other. The output circuits of both oscillators are coupled to the product detector.

IF and Detector Circuits

Output from the second mixer is connected to T4 and then to one of three mechanical filters FL1, FL2, or FL3 (FL2 and FL3 are not supplied) or to the tuned circuit of transformer T5. Mechanical filter FL1 (centered on 455 kc with a nominal bandpass of 2.1 kc) is selected for SSB reception, while FL2 and FL3 are optional filters to be used for CW operation. For AM operation, 455-kc transformer T5 is used to provide an increased bandwidth of approximately 5 kc. Output from these circuits is amplified by the if. preamplifier, V5A. Transformer T9 matches the preamplifier to the Q-multiplier, V5B. Control R57, the IF GAIN ADJUST, sets the receiver gain for the proper agc threshold sensitivity. The S-meter circuit is connected from the screen circuits of V6 and V7, the two if. amplifiers, to the cathode of V7. Under no-signal conditions, the voltage developed across R13 is equal to that developed across R21, and the meter reads zero. Application of agc causes the cathode current of V7 and the combined screen current of V6 and V7 to decrease. The voltage across R13 increases, the voltage across R21 decreases, and the meter reads up-scale by an amount which is proportional to signal strength. Output voltage from the second if. amplifier is coupled to the product detector, V8A. It is also coupled to separate AM and agc diode detectors. Bfo injection voltage is applied to the cathode of the product detector.

Q-Multiplier and Notch Filter

The notch filter is composed of coil L8 and associated capacitors and resistors. The rejection notch occurs at the resonant frequency of this circuit and is centered at 455 kc. Capacitor C132 is mechanically coupled to the REJECTION TUNING control which allows the notch frequency to be moved across the receiver if. passband. Potentiometer R77 is adjusted to provide optimum Q and depth of notch. Switch S10 shorts the filter circuit in the OFF position. The Q- multiplier is a feedback circuit which includes L8. This circuit multiplies the Q of L8 approximately ten times, thereby obtaining a much deeper and narrower rejection notch than would be provided by the filter alone.

AGC and Control Circuits

Signal voltage is coupled from the secondary of transformer T6 to one of the diode plates in V9 and rectified. This rectified signal voltage then passes through filter network R50 and C49 to the agc network consisting of resistors R24 and R88, and capacitors C50, C137, and C153. The agc network develops the desired agc signal and then applies it to the rf and if. amplifier stages. The parallel combination of R88 and C153 present the fast charge-discharge rate desired for elimination of small time duration interference; the parallel combination of R24 and C50 present a longer RC time constant allowing for a smoothly developed agc signal. Generation of agc voltage is delayed until the signal voltage at the diode plate exceeds the cathode bias on V9. Potentiometer R57 in the secondary of T9 is normally adjusted so that agc action is initiated with a receiver input signal of approximately 2 microvolts. This point is referred to as agc threshold.

Manual control of rf gain is also accomplished through the agc line. A voltage divider circuit consisting of resistors R33, R55, and RF GAIN control R56 is connected across the negative 65-volt bias line. At the maximum gain setting, this circuit places a one-volt static bias on the agc line to furnish proper operating bias for rf amplifier V2. At lower control settings, increased bias is provided which reduces the gain. The dc grid return for the first mixer stage and MUTE jack J11 are connected to the junction of resistors R33 and R58. When the receiver function switch is placed in the STBY position, aground at J11 causes the receiver to operate in a normal manner. Removal of this ground causes cutoff bias to be applied to the mixer grid and increases bias on the agc line, thus muting the receiver.

Audio Circuits

Audio voltage from the appropriate detector is selected by S8A on the MODE switch and is coupled to the AFGAIN control. The CW SIDETONE jack, J10, is also connected to this point. A sidetone audio voltage of approximately 0.2 volt will produce a comfortable listening level at average gain settings. Audio is amplified in a 2-stage amplifier consisting of tubes V8 and V10. Capacitor C106 limits the audio response to 3 kc for AM and SSB reception, and capacitor C164 reduces it to 1.5 kc for CW reception. Three audio outputs are provided. Jack J8 is a4-ohm outlet for a speaker. The headphone jack is connected to a resistive divider across the 500-ohm tap on the output transformer. The divider provides a load for V10 when the impedance of headphones used is relatively high. The ANTI-VOX jack, J12, is also connected to the 500-ohm tap. At normal audio gain settings, 5 to 15 volts of audio are available at J12 for use with the anti-vox circuits in an associated transmitter.

Power Supply Circuits

The internal power supply furnishes filament, plate, and bias voltages for the receiver. Three high voltage values are developed consisting of a 190-volt dc unregulated voltage at the positive side of C59B, a 140-volt dc regulated voltage at the cathode of zener diode CR6, and a 135 volt dc unregulated voltage at the positive side of C59A. The high voltage winding of transformer T8; diode CR1, CR2, and CR6; resistor R86; and the filter network consisting of capacitors C59A, C59B, and C59C, resistor R51, and choke L6 make up the full-wave rectifier system which generates the three high voltage values mentioned above. Bias voltage is obtained by rectifying ac voltage from a voltage divider connected between one leg of the high voltage secondary and ground. The tube heaters and pilot lamps are connected to allow operation from a 6, 12, or 24-volt source. Heater, plate, and bias voltages may be furnished by an external source such as a mobile power supply.