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Collins Radio Company, Cedar Rapids, USA

51 J - 4

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überarbeitet am 14.12.2007

After WWII, Collins presented their first amateur radio receiver 75A-1 with an analog linear dial and covering the ham radio bands; the 75A-2 had a similar aspect like the 51J-4, but still covered only the amateur radio bands. The first all wave receiver covering all frequencies from mediumwaves to 30,5 kHz shortwaves was the 51J and a bit later the 51J-1 presented in 1949 and built in small production numbers only. After major technical improvements, the 51J-3 has been built in larger quantities, it's military tropicalised version carried the R-388 designation. Only the later 51J-4 was equipped with Collins' famous mechanical IF filters - the military designation of these sets with the mechanical filters installed was R-388A.

double / triple conversion, ZF 1,5-2,5 or 2,5-3,5 MHz, 500 kHz

linear Analog dial 1 kHz

AM, CW with BFO

Sensitivity 5 MHz
AM <5 uV

Selectivity -6/-60 dB
1,5/4 kHz, 3 / 8,5 kHz, 6 / 14,4 kHz

RF-Gain, AGC, Noise Limiter, Crystal Filter

The Collins 51 J-4 is a typical "boatanchor" equipped with a total of 19 valves, it's dimensions with the cabinet included are 53,6 x 31,7 x 33,3 cm, quite frequently it has been sold as 19" rack version. It's weight is 21,3 kg, the set can be powered by 115 or 230V AC and has a power consumption of 85 Watts. The 51J-4 has no internal speaker, at 4 Ohm speaker or a 600 Ohms high impedance spaker can be connected to the speaker connectors at the rear of the set.

In the centre of the front panel, You find the linear analog drum dial displaying the selected 1 MHz segment with 100 kHz marks. Underneath bext to the main tuning knob You find a window displaying a segment of a circular dial calibrated in 10 kHz marks, between the 1 kHz-lines You can read a tuned frequency with an accuracy of around 500 Hz.
Due to the circuitry, in some 1 MHz segments marked with red lettering on the MHz drum, Yo uhave to read the frequency off the red numbers of the circular kHz dial.

In the lower row of controls You find from the left to the right the mains switch, in the "Standby" - position all the valves heaters are on and the receiver will work immediately when switched ON, then at the left of the main tuning knob the RF (radio frequency) gain and at the right side the volume / AF (aufio frequency) gain control. A second big rotary control will act as bandswitch, moves the dial drum controlles the preselection over several mechanical gears. Two small controls in the middle underneath the main tuning knob will operate the moving calibration line in the kHz-dial window and the antenna tuner.
The round signal strength meter in the right upper corner indicates the level of an incoming signal in dB and can be switched to indicate the AF level. Three smaller switches below the signal strength meter will activate the internal 100 kHz - calibrator, the AGC (automatic gain control) and the Noise Limiter.
In the left upper corner of the front panel, You find the controls for the crystal filter, the rotary switch activating the BFO used for CW and SSB reception and another control for the BFO pitch note. A small metal lever is used to switch the three IF bandwidths of 1,5 / 3 and 6 kHz.
Only the 51J-4 version is equipped with the famous mechanical Collins IF filters, I consider the 3 and 6 kHz filters as very useful for AM reception, in some situations they are even better suited then the filters found in the R-390A. Another special feature of the Collins 51J-4 is the "Crystal Filter" which will on one hand reduce the IF bandwith and on the other hand will eliminate unwanted interfering signals like a HF notch filter when the phasing function is activated.

Position Selectivity Filter 1,5 kHz (-6/-60 dB) Filter 3 kHz (-6/-60 dB) Filter 6 kHz (-6/-60 dB)
0 1.2-1.6 / 4 2.8-3.4 / 8.5 5.7-6.3/14.4
1 1.3-1.55 / 3.5 2.27-2.75 / 6.4 3.0-4.0/11.5
2 0.8-1.2 / 3.0 1.2-1.5 / 5.5 1.25-1.6/11.0
3 0.3-0.5 / 2.65 0.3-0.5/5.25 0.5-0.7/10.0
4 0.1-0.3 / 2.5 0.3-0.3/5.0 0.3-0.5/9.5

The HF circuit of the 51J-4 is nearly as complicated as the mechanical synchronisation of the preselection using very impressive gears. In the 0,5-1,5 segment, the 51J-4 uses triple conversion, the signal will be converted to 11,5-10,5 MHz in the first mixer and to the usual 3,5-2,5 MHz in the second mixer. In the 1,5-3,5 segments, the receiver is switched as single conversion and the signal directly proceeds to the second mixer. In the other shortwave bands from 3,5-30,5 MHz, the typical double conversion circuit takes action: In the shortwave bands with even MHz counts, the signal is mixed to 1,5-2,5 MHz after having passed the first RF amplification stage, in the bands with uneven MHz counts to 2,5-3,5 MHz. In the second mixer, the signal is converted to the second intermediate frequency of 500 kHz. The signals first has to get passed the switchable crystal filter and the through the bank of mechanical IF filters. After another RF amplification stage, the signal is passed over to the detection stage and into the AF stage.

In practical use, the accuracy of the frequency dial is better then 1 kHz thanks to the linear frequency response in the 1 MHz segments. When the dial is calibrated using the internal crystal calibrator, You can rely on what's displayed on the dial.
Many other "boatanchor" receivers of these years use only a non-linear coarse analog dial with insufficient accuracy in the high frequency bands and a accurate fine tuning dial which lacks calibration - so the frequency readout scheme of this Collins set is absolutely superior to that of many other sets from the fifties. The sensitivity is not superior to contemporary sets, using a really long long wire antenna, You won't have any problems pulling in very faint signals - and the 51J-4 is one of the receivers very unlikely to be impressed by really long long wires: the mechanically tracked preselection does an excellent job. As I mentionned before, the filter selection is very adequate for reception in tropical bands as well as in the crowded broadcast bands in Europe. Using the crystal filter, You can substantially reduce IF bandwidth for CW reception.

The 51J-4 is a bulky "real boatanchor" delivering very pleasant AM reception in the broadcast and tropical bands with good useable selectivity and very low noise floor. SSB reception is substandard as the set comes only with a BFO, but using the narrow filters, I did most of my tropical bands DXing in AM mode.

Usually, You find Dekaware knobs on the Collins 51J-4 / R-388A. Only the sets from the latest series have been equipped with the newer S-line knobs, like in this example of a set used by the Swiss Army.

further reading:
d: Betagt, aber kein altes Eisen - Collins R-388/URR, Oldie KW-Empfänger, Nils Schiffhauer
e: the Collins 51 J-4 Receiver, Dallas Lankford, fine tuning proceedings 1989
d/e: Collins 51J-4 at www.radiomuseum.org

© Martin Boesch