51S-1 Detailed Physical & Operational Description



The 51S-1/1A/1F/1AF/1B Receiver receives USB, LSB, AM, and CW signals in the range of 0.2 to 30.0 MHz. Coverage is continuous in thirty 1-megahertz bands. The model 51S-1 is mounted in a perforated wrap-around cabinet and equipped with an ac power supply capable of 115- or 230-volt, single-phase, 50-to 400-Hz operation. The 51S-1A is similar, except that it is fitted with a 28-volt dc transistorized power supply. The rack-mounted ac version is model 51S-1F, while the rack-mounted dc version is model 51S-1AF. The 51S-1B is similar to the 51S-1, but it has a rear-mounted junction box that provides military-type connectors for power, control, audio, and antenna lines.


The 51S-1 and 51S-1F Receivers require 115-or 230-volt, single-phase, 50- to 400-Hz power at approximately 125 watts. The 51S-1B requires 115-volt, single-phase, 50- to 400-Hz power at approximately 125 watts. The 51S-1A/ 1AF Receiver requires 28 volts dc at 4.5 amperes. The 51S-1/1A Receiver may be mounted on table or bench for fixed station operation, or may be mounted with a mounting plate similar to the 351E-4 on shelf, bench, or table in moving aircraft, ground vehicle, or boat.

51S-1/ 1F/1A/1AF Receivers require a 4- or 600-ohm speaker or headphones for local audio monitoring, but monitoring devices of any impedance may be matched with 600-ohm line-to-monitor transformers at remote locations) to several miles. Alternately, the 600-ohm line termination may be connected to telephone lines, or the 600-ohm local output may be used with a phone patch. The 51S-1B has the same local audio provisions as those described above, but the remote audio line has a 150-ohm impedance. 51S-1 series receivers require a good antenna with 50-ohm unbalanced feed.


Frequency range…………    0.2 to 30.0 megahertz in thirty 1-megahertz bands continuous coverage.

Modes………………….    Upper sideband, lower sideband, AM or CW.

Power consumption……….    125 watts.

Type of service…………    Fixed station attended with provision for remote control of rf gain.

Rf input impedance ……..    50 ohms, unbalanced.

500-kHz if. output at J9…    50 mv minimum into 50-ohm load with 5-uv input signal.

Matching speaker impedance.    4 or 600 ohms.

Balanced line out impedance.   600 ohms balanced, center-tap ground reference or floating.
(For 51S-1B, 150 ohms floating.)

Matching phone patch impedance (local)………..   500 to 600 ohms.

Frequency stability………   During temperature change from 0 to +50 øC,
after 20 minutes warmup, audio output frequency will not vary more
than +/-885 Hz for carrier frequencies from 2 to 7 MHz. From
7 MHz to 30 MHz, stability varies from 36 PPM +/-400 Hz at 7.00 MHz
(652 Hz) to 27 PPM +/-400 Hz at 30 MHz (1210 Hz). For +/-10% line voltage
variation, frequency varies not more than + 100 Hz.

Calibration accuracy……..   When zeroed to nearest 100 kHz calibration
point, the frequency will be within +400 Hz.

Dial backlash……………   Not more than 150 Hz.

Audio-frequency response AM.   100 to 2500 Hz +/-6 db (line channel).
200 to 2500 Hz +/-6 db (local channel).

  SSB (high-frequency limit determined by filter bandwidth)…………….   350 to 3050 Hz +/-3.5 db
(line channel). 350 to 3050 Hz +/-3.5 db (local channel).

Audio output distortion (SSB test signal 100-microvolt input, 1.0-watt local output, 1-mv (0 dbm) line output)

  Local…………………   Not more than 10 percent.

  Line………………….   Not more than 1.2 percent.

Q-multiplier rejection notch depth……………..   Not less than 40 db.

Receiver sensitivity (nominal)

  AM……………………   3.0 microvolts for not less than 10-db signal + noise/noise (2 to 30 MHz).

                               15.0 microvolts for not less than 10-db signal + noise/noise (0.5 to 2 MHz).

                               20.0 microvolts for not less than 10-db signal + noise/noise (0.2 to 0.5 MHz).

                               With 55G-1 Preselector, 5.0 microvolts for not less than 10-db signal +
noise/noise (0.2 to 2.0 MHz).

  SSB and CW…………….   0.6 microvolt for not less than 10-db carrier on carrier off (2 to 30 MHz).

                               3.0 microvolts for not less than 10-db carrier on carrier off (0.5 to 2.0 MHz).

                               4.0 microvolts for not less than 10-db carrier on carrier off (0.2 to 0,5 MHz),

                               With 55G-1 Preselector, 1,0 microvolt for not less than 10-db carrier on
carrier off (0.2 to 2,0 MHz).


  CW (at 6 db points)…….   800 hertz bandwidth, nominal. (650 Hz
minimum, 950 Hz maximum, 300-Hz maximum
bandwidth optional),

  SSB (at 3.5db points)…..   2.75 kilohertz bandwidth (2.4 kHz  bandwidth

  AM (at 6 db points)…….   5.0 kilohertz bandwidth minimum,

    (at 60 db points)…….   22,0 kilohertz per second bandwidth maximum.

Spurious responses (above 2 MHz)

Internal spurious signals…   Less than one microvolt equivalent signal.

Other spurious signals……   Not less than 70 db down, except from 4.8
to 5.2 MHz, not less than 40 db down.

Image response…………..   Not less than 50 db down from 2 to 25 MHz;
not less than 40 db down from 25 to 30 MHz; referenced to midband.

Size……………………   Cabinet version: 7-3/4 in. high by 14-3/4 in. wide by 14 in. deep,
Rack-mounted version: 8-3/4 in. high by 19 in. wide by 15 in. deep.

Weight………………….   28 pounds

Under the hood…



Figure 3-2 is a block diagram of the 51S-1, and figure 7-1 is a schematic diagram of the 51S-1. Figure 7-2 is a schematic diagram of the 51S-1A. Figure 7-3 is a partial schematic of the receiver, showing the complete front-end switching arrangement. The 51S-1 is a dual- or triple- conversion communications receiver which operates in the range of 0.2 to 30 megahertz. The 0.2- to 2.0-MHz portion of the coverage is intended for laboratory applications and broadcast monitoring. In this range, internally generated spurious whistles occur at 333 kHz, 666 kHz, 1000 kHz, 1500 kHz, and 2000 kHz. Triple conversion is used for the 0.2- to 7.0-MHz bands, and double conversion is used for the 7.0- to 30.0-MHz bands. For 7.0- to 30.0-MHz operation, the 14.5- to 15.5-MHz bandpass network and second mixer are bypassed.

The 51S-1 is basically a 2.0- to 30.0-MHz receiver with a built-in low-frequency converter. The tuning mechanism, counter dials, and turret are arranged so the two lowest bands, 0.2 to 1.0 MHz and 1.0 to 2.0 MHz, use the 28.0- to 29.0- and the 29.0- to 30.0-MHz bands of the receiver as a variable if. (conversion) frequency. As the megahertz counter is reduced in setting below 2.0 MHz (lowest band on the turret), a segment switch, S6, connects the low-frequency converter and its bandpass filter between the antenna and the turret input, which is now the 29.0- to 30.0-MHz band. When the megahertz counter setting is reduced below 1.0 MHz, the segment switch, S6, maintains the low- frequency converter connection, but the turret is changed to the 28.0 to 29.0 MHz band. In this manner, the 28 positions of the turret plus two positions of over travel provide 30 bands, each 1 megahertz wide. The 0.2-MHz limitation of the lowest band is a function of the frequency roll-off in the bandpass filter and mixer considerations.


RF Amplifier

Signals from the antenna are fed from J1 through S6 contacts to an impedance-matching transformer, L30. The output of L30 is coupled to the first section of the double-tuned input network. Refer to figure 3-1. The double-tuned input circuits are composed of C40, L33, L32, L31, C71, L69, L68, L67, and the components mounted upon turret wafers A1 through A5. All rf section components and turret wafers are shown in figure 7-3. The first section of this network is tuned by C40, Cp, Lp – Lm and L33-L32-L31. For any position of the turret, L33, L32, L31, and C40 are in the circuit, and the band changing is accomplished by connecting the turret-mounted components in shunt. The tuning slug of L32 is coupled mechanically to the tuning control of the receiver, and is varied to accomplish tuning throughout the 1-MHz band. The second section of the network is tuned by C71, Cn, Ln, Lm and L69-L68-L67. The tuning slug of L68 is ganged to the tuning control of the receiver to accomplish tuning in the same manner as that of L32 in the first section of the network. The turret-mounted components are selected by the MEGACYCLES control. This control positions the turret wafers so that the proper set of components is connected into the circuit according to the megahertz information on the counter dial. Coupling between the two sections of the input network is provided by mutual inductance Lm The output network consists of a single-tuned system using a band-switching and tuning scheme similar to that of the input network.

First Mixer

The first mixer, V2A, is a triode. The rf signal is fed to the grid, and the hf crystal oscillator signal is injected at the cathode. The output network consists of a 14.5- to 15.5-MHz bandpass filter for 2- to 7-MHz operation and a 3- to 2-MHz variable, triple-tuned network for 7- to 30-MHz operation. The slugs of the 3- to 2-MHz variable if. network inductors are coupled mechanically to the tuning control of the receiver and tracked with the slug-tuned inductors in the rf circuits to produce the 1-MHz coverage for each band.

Second Mixer

During 2- to 7-MHz operation, the second mixer, triode V3A, uses a 3- to 2-MHz variable if. for its output network. This is the same output network that is used by the first mixer during 7- to 30-MHz operation. The signal from the first mixer plate is fed through the 14.5-to 15.5-MHz bandpass filter network, T12 and T13, to the grid of the second mixer. The 17.5-MHz oscillator signal is injected into the cathode circuit of this mixer. The second mixer is inoperative during 7- to 30-MHz operation.

Third Mixer

The third mixer, pentode V4A, receives its input signal from the 3- to 2-MHz variable if. network. The input signal from the first or second mixer is fed to the grid of the third mixer and the vfo signal is injected into its cathode. An external vfo signal may be injected through J6 if external frequency control is desired. Such an external injection signal might also be a selected crystal oscillator frequency if precise fixed channel tuning is desired. In such a case, the tuning dial would have to be set to the channel frequency in order to properly resonate all the rf and if. gang tuned circuits.

The output network of the third mixer is selected with the EMISSION switch on the front panel. In USB and LSB positions, mechanical filters FL2 and FL3, are used. These mechanical filters provide a 2.75-kHz bandwidth for single-sideband reception on upper or lower sideband, respectively. The CW position of the EMISSION switch selects a crystal filter, FL4. The crystal filter provides a 800-Hz bandwidth for reception of CW signals. The AM position of the EMISSION switch selects a network composed of two lightly coupled 500-kHz if. transformers, T14 and T15, which provides a bandwidth of 5-kHz for reception of amplitude-modulated signals. The first if. amplifier, pentode V5, receives its input signal from the third mixer through one of the four selective networks described in paragraph 3.2.4. The output signal is coupled to the Q-multiplier through if. transformer T1.


The Q-multiplier, V6, is a twin triode. The first triode section is a cathode follower, the output of which is coupled to the cathode of the second triode section. When REJECTION TUNING is being used, the signal from the plate of the second triode is coupled through a parallel-tuned circuit to the grid of the second if. amplifier. The parallel-tuned circuit consists of L108, C145, and C146 and a small voltage sensitive capacitor. These components, plus R33 and R34, form a bridged-T rejection notch filter. The end of the parallel-tuned circuit, away from the plate of the second triode section, is coupled to the grid of the second triode. This feedback arrangement forms a Q- multiplier. The Q of L108 is 250. The feedback loop has a gain of 10, resulting in an overall Q of 2500 and a rejection notch depth of not less then 40 db. Turning the REJECTION TUNING control fully counter clock-wise deactivates the rejection network by forward biasing capacitance diode C315 into conduction.

Second IF Amplifier

The second if amplifier, pentode V7, receives its input signal from the Q-multiplier network. The output network of the second if. amplifier is if. transformer T2. The secondary of T2 is coupled to the third if. amplifier, V8, and cathode follower V11A.

Third IF Amplifier

The third if amplifier, V8, receives its input signal from the second if. amplifier through transformer T2. The third if.amplifier output is coupled to the product demodulator through if. transformer T3 and to the AM detector through C158.

The product demodulator is composed of CR1, CR2, CR3, and CR4 in a diode-ring configuration. Signal from the beat- frequency oscillator, V17, is injected into the product demodulator at the junction of R135 and R136. The audio output is fed to the SSB/CW preamplifier, Q1. The bfo supplies a reinserted carrier to replace the suppressed carrier of the SSB signal. The demodulator functions as a mixer, and its output is a full-wave rectified signal consisting of the if. and bfo signals plus their mixing products. C161, L123, and C310 form a low-pass filter that passes the if. and bfo mixing difference frequency and blocks the rest of the demodulator output. The mixing difference frequency is the desired audio signal.

SSB/CW Preamplifier

The output impedance of the diode demodulator is approximately 600 ohms. Transistor Q1 provides impedance match and gain between the product demodulator and the following audio amplifier grid. The SSB/CW preamplifier is an NPN transistor, connected in a common emitter configuration. Audio signals from the product demodulator and sidetone signals from the cathode follower, V11B, are coupled to the base of Q1. The SSB/CW preamplifier output signal is coupled from the collector of Q1 through C165 to switch S2C. During SSB and CW operation, the contacts of S2C connect the audio output signal to first local af amplifier, V14B, and the first line amplifier, V14A.

Audio Amplifiers

The 51S-1 includes two, two-stage, audio-frequency amplifiers. The local amplifier, consisting of V14B and V12, provides audio power to local headphones, speaker, or phone patch. The line amplifier, consisting of V14A and V13, provides power for a 600-ohm remote line.

The line output impedance of 51S-1B is 150 ohms. Figure 7-5 is a partial schematic, diagram of the 51S-1B output circuit. The first local and the first line af amplifiers obtain input signal from either the SSB/CW preamplifier, Q1, or from AM detector CR15. The signal source, Q1 or CR15, is selected by contacts of the EMISSION switch, S2. The first local and first line af amplifiers drive their respective second local and line amplifiers V12 and V13. The line amplifier distortion is reduced by use of negative feedback from output transformer T4 to the cathode of V14A.

Low-Frequency Mixer

For receiving signals in the 0.2- to 2.0-MHz range, the 51S- 1 uses a low-frequency mixer, V10A-V16A, and converts the signal to the 28-and 29-MHz bands. The low-frequency input to the mixer is passed through a bandpass filter, and the output of the mixer is tuned by the turret and slug-tuned circuits. External tuners for low-frequency operation may be used. Jacks J14 and J13 on the rear apron are provided for this use. When an external low-frequency tuner is used, the jumper between J14 and J13 must be removed.


The calibration oscillator, V16B, is a crystal-controlled oscillator operating at 100 kilohertz. Variable capacitor C227 trims the frequency of the oscillator. The output of the calibration oscillator is coupled to the antenna jack, J1.

The low-frequency crystal oscillator, V10B, uses a 14-MHz crystal. The plate circuit of this oscillator is tuned to the second harmonic of the crystal. The low-frequency crystal oscillator operates only when the 51S-1 is receiving signals in the 0.2- to 2.0-MHz bands. The output of this oscillator is coupled to the low-frequency balanced mixer, V10A and V16A. Capacitor C2 trims the crystal oscillator to frequency.

The high-frequency crystal oscillator, V2B, operates on all bands. Frequency of oscillator operation is determined by one of sixteen crystals mounted on a wafer in the turret (see table 3-1). The proper crystal is selected by positioning the band-switch MEGACYCLES control. Individual turret-mounted piston trimmer capacitors trim each crystal to frequency.

The 17.5-MHz oscillator, V3B, is crystal controlled. This oscillator operates only when the 51S-1 is operating in the 2- to 7-MHz range. The crystal may be trimmed to frequency by variable capacitor C233.

The variable-frequency oscillator is a Collins 70K-7 permeability-tuned oscillator. The frequency of this unit is varied by changing the inductance of L501. This change of inductance is accomplished by turning the 51S-1 tuning knob which is coupled mechanically to the slug of L501. The output of the oscillator tube, V15, is coupled to the cathode of the third mixer through T501.

The beat-frequency oscillator, V17, is a 500-kHz crystal- controlled oscillator which operates only when the EMISSION switch of the 51S-1 is in USB, LSB, or CW position. No beat- frequency oscillator is needed for AM operation. The output of the bfo is coupled to the product demodulator. There is no provision for trimming the bfo frequency.