Inhaltsverzeichnis
Collins 51 S - 1
Manufactured by Collins Radio Comp., Cedar Rapids.
In the late 1950s, Collins began developing sets for the amateur radio market that fit on the desk of a shortwave listener and which had a civilian appearance. Not everyone is comfortable placing a Collins R-390A on his desk or in their living room.
In 1958, the 75S-1 double conversion receiver was released, which only covered the amateur radio bands. In 1959, the time had come for a general coverage receiver with a continuous frequency coverage of 0.2 - 30 MHz; the 51S-1 was met with enthusiasm by government agencies, amateur radio operators and shortwave listeners who could afford the $2500, and th set still fetches good prices on the used equipment market today.
Variants
- 51S-1: standard version, older sets with „winged“ Collins logo, newer ones with „round“ logo
- 51S-1F: rack version
- 51S-1A: 28 V DC version for aircraft
- 51S-1AF: rack version with 28 V DC power supply
- 51S-1B: with military connectors, 115 V AC
Technical data
- Frequency range: general coverage, 0.2 - 30 MHz
- Frequency display: linear analogue display, 1 kHz
- Frequency memory: none
Power supply
- Mains operation: 115, 230 V
Dimensions
- 343 x 170 x 381 mm, weight 12 kg
Accessories
Operation
With its grey painted metal cabinet and modest dimensions of 37.5 x 16.7 x 33.2 cm and a weight of 11.8 kg for a top hollow state recievr, the Collins 51S-1 is only slightly larger than the popular receivers from the Japan Radio Company. It is designed to operate on 115 or 230 V AC at a power consumption of 125 watts to operate the 17 tubes; the 51S-1A(F) models could be operated on 28 VDC.
The connector options on the rear of the receiver match the origins of the 51S-1 as a piece of professional equipment. In addition to a high- or low-impedance loudspeaker, there are connectors for an external VFO, the IF of 500 kHz and an external LW/MW preselector.
The front panel is very plain; in contrast to the „big“ Collins receivers, the 51S-1 has only nine controls. The frequency dial dominates the front planel, and the large main tuning knob can be mechanically locked with a lever to prevent accidental detuning. The frequency can be read with an accuracy of better than 1 kHz from the analog dial in the sector-shaped window above the tuning knob; there are labels for every multiples of 10 kHz. The 100-kHz digit of the mechanical digital display „jumps“ when cranking over a 99-101 transition; thanks to the metal gears, tuning is silky smooth. The MHz digits are also displayed mechanically in a small window above the tuning scale, the MHz knob is located to the left of the tuning knob. The Zero Set knob can be used to shift the display's calibration line based on the reception of a standard frequency station or the crystal calibrator. At the top left is the power switch, which also has a standby function (in which the tube heaters are already turned on to increase frequency stability) and a position for activating the crystal calibrator.
The RF gain control is located on the left below the tuning knob, the volume control is located at the bottom right of the front panel. Between this and the tuning knob, the modes selector can be found, for single sideband reception, both sidebands can be selected separately. The IF filters are switched simultaneously with the operation mode, for AM reception, a not coil filter with not so steep skirts is used, as a more expensive option, a mechanical 6 kHz filter was available. In SSB mode, a 2.4 or optionally, depending on the receiver variant, a mechanical 2.75 kHz filter is used; in CW mode, an 800 Hz crystal filter is used, optionally a 200 Hz filter could also be ordered. Above the mode switch is the knob for the Q-multiplier with notch function; an interfering signal can be eliminated. At the top right, the large analogue S-meter is located; it is calibrated in dB and can be switched from displaying the signal strength to the audio level.
The much-discussed small difference can also be seen in the pictures: earlier sets buiot until around 1967 came with the „winged logo“, the Collins emblem with side stripes; later sets had the „round logo“. The S-meter was white in earlier sets like the 51J-4; later it was brown.
An RF signal first passes through a bandpass filter for each megahertz range and the mechanically coupled, gear-driven automatic preselector. For reception in the 0.2 - 2 MHz range, the frequency is converted internally via a converter circuit into the 28 - 30 MHz range after passing through a 2 MHz low-pass filter. After an RF amplifier stage, in the first mixer in the 2 - 7 MHz ranges, in which the receiver works as a triple conversion superhet, is converted to the first IF 14.5 - 15.5 MHz. The third mixer gets its signals in the 2 - 7 MHz range from the second mixer and in the other bands directly from the first mixer, and converts them from 2 - 3 MHz to the third IF of 500 kHz. In AM mode, this signal is now fed through the 6 kHz LC / coil filter, in SSB mode through the mechanical 2.4 kHz USB or LSB filter and in CW reception through the 800 Hz quartz filter. The Q-multiplier circuit with the notch filter function and two further IF amplification stages follow. In AM, a diode demodulation is used to generate the audio signal, in SSB mode, a more complex product detector is used. The AF amplification is again done with two tube stages.
In practical use, the 51S-1 is excellent for AM and SSB reception; the sensitivity and thanks to the mechanical tracking preselector, the large signal behaviour on long antennas are outstanding. The budget LC filter used in AM and the fact that the IF filters cannot be selected independently of the operation mode, so that the excellent 2.4 kHz filter cannot be used for AM, are a drawback of the receiver.
The 51S-1 is best used for search reception in a band segment; checking a few parallel frequencies of a station in several different shortwave bands can turn into a tedious process. However, this applies to all tube receivers and also to older transistor radios; the age of microprocessor-controlled shortwave receivers with their memory functions had not begun, when the 51S-1 was designed. Compared to other high-end tube receivers, the Collins 51 S-1 is extremely straightforward and intuitive to use, and despite the considerable mechanical and electronic complexity, the tube receiver takes up very little space. The compactness and the dignified appearance compared to military shortwave „battleships“ have helped the Collins to a very high place on the popularity scale and also to a high price when sold second hand.
Technical description
An RF signal first reaches a bandpass filter for each megahertz range and the mechanically coupled, gear-driven automatic preselector. For reception in the 0.2 - 2 MHz range, the frequency is converted internally via a converter circuit (V16) to the 28 - 30 MHz range after a 2 MHz low-pass filter. After an RF amplifier stage (V1), the conversion takes place in the first mixer (V2) in the 14.5 - 15.5 MHz range for the 2 - 7 MHz range in which the receiver operates as a triple conversion superhet. The third mixer (V4) receives its signals in the 2 - 7 MHz range from the second, and directly from the first mixer in the other MHz segments, and converts them from 2 - 3 MHz to the final IF of 500 kHz. In AM mode, this signal is now sent through the 5 kHz coil filter, in SSB mode through the mechanical 2.4 kHz USB or LSB filter and in CW reception through the 800 Hz quartz filter. The Q-multiplier circuit with the notch filter function and two further RF amplification stages follow. In AM, a diode envelope demodulation is used to generate the AF, in SSB mode, a complex product detector is used. The AF amplification is again done with two tube stages.
Tube configuration
V1 (6DC6, RF preamplifier); V2 (6EA8, 1st mixer, oscillator); V3 (6EA8, 2nd mixer, oscillator 17.5 MHz); V4 (6EA8, 3rd mixer); V5, V7, V8 (6BA6, three IF amplifier stages); V 6 (12AX7, Q-Multiplier); V9 (6BA6, AGC amplifier); V10 (6EA8, LF band converter and oscillator); V11 (5670, AGC decoupling); CR1-4 (four 1N34A, product demodulator); CR15 (1N34, AM demodulator); V12 (6BF5, AF power amp; V13 (6AK6, AF power amp line out); V14 (12AX7, AF preamp AF out & line out); V 15 (7543 or 6AU6, VFO); V16 (6EA8, mixer stage LF band converter, crystal calibrator); V17 (7543, BFO).
Various rectifiers in the voltage regulation.
