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====== E-629: Shortwave Receiver Collins 51J-4 ======
**E-629**, **Shortwave Receiver Collins 51J-4**; manufactured by [[Collins|Collins Radio Company, Cedar Rapids, USA]].

After WWII, [[Collins]] presented their first all wave receiver covering all frequencies from mediumwaves to 30,5 kHz shortwaves, the 51J - after a short time in 1949, the 51J-1 appeared and was built in small production numbers. After major technical improvements, the 51J-3 was built in larger quantities, it's military tropicalised version carried the R-388 designation. Only the later model 51J-4 was equipped with Collins' famous mechanical IF filters - the U.S. military designation of these receivers with the mechanical filters installed was R-388A.

The Swiss Army acquired 84 of these sets to be used on high power stations, in embassy communications and for monitoring, the Swiss sets seem to originate from a late production batch as all sets are equipped with S-line knobs.

{{:images:e-629-collins-51j4.jpg?direct&800|Collins 51J-4}}

===== Technical Data =====
  * [[Principle]]: [[Double conversion|Triple conversion]], [[IF]] 1st IF 1.5-2.5 or 2.5-3.5 MHz, 2nd IF 500 kHz
  * [[Modes]]: [[A1]] with BFO, [[A3J|SSB (A3J)]] with BFO, [[A3|AM (A3)]]
  * [[Frequency range]]: 500 kHz - 30.5 MHz in thirty ranges of 1 MHz
  * **Analog dial**, linear analog dial, accuracy better then 1 kHz.
  * [[Sensitivity]]: [[A3]] 2 - 3 μV / [[Selectivity ]] (-6 / -60 dB): 1.5 / 4 kHz, 3 / 8.5 kHz, 6 / 14.4 kHz

==== Power Supply ====
  * [[Mains]]: 115 / 230 V; 85 W

==== Dimensions ====
  * 536 x 317 x 333 mm, 24.9 kg

==== Accessories ====
  * 75 ohms asymmetrical antenna connector for a **L-Antenna**.
  * headphones

----

{{:images:e-629-collins-51j4-front.jpg?direct&600|}}
===== Operation =====
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 analog linear drum dial displaying the selected 1 MHz segment with 100 kHz marks. Underneath, just next 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, you have 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 (audio frequency) gain control. A second big rotary control will act as bandswitch, it moves the dial drum and 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 set 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 activates the [[BFO]] used for [[CW]] and [[SSB]] reception and another knob controls 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|

===== Technical Principle =====
{{:images:e-629-collins-51j4-block.jpg?direct&400|}}\\ 
The HF circuit of the 51J-4 is nearly as complicated as the mechanical synchronisation of the preselection using very impressive gears. 

The RF signal from the antenna input first has to pass a tuned circuit and a RF amplifier stage (V101: [[de:6AK5]]).

In the  **0.5 - 1.5 MHz** frequency range, which in fact can only be used from 540 kHz up due to the IF of 500 kHz, the 51 J-4 acts as a triple conversion receiver: the RF signal is converted to a 11.5 - 10.5 MHz intermediate frequency in the first mixer (V102: [[de:6BE6]]) with a 12 MHz quartz oscillator (V105: [[de:6AK5]]) signal. In a second mixer stage (Band I mixer, V103: [[de:6BE6]]) it is mixed with an oscillator frequency of 8 MHz to generate the standard IF of 3.5-2.5 MHz. The third conversion is done in V106 ([[de:6BE6]]) when the signal is mixed with the VFO signal of 2 - 3 MHz (V001: [[de:6BA6]]; V002: [[de:6BA6]]) to generate the third IF of 500 kHz.

In the **1.5 - 3.5 MHz** frequency range, the receiver acts as single conversion receiver; the signal is fed directly to V106 ([[de:6BE6]]), thus generating the IF of 500 kHz.

In the other shortwave bands from **3.5 up to 30.5 MHz** , the receiver operates as a double conversion superhet: after the first HF amplifier stage (V101: [[de:6AK5]]), in the even frequency bands, the signal is converted to a 2.5 - 1.5 MHz intermediate frequency in the first mixer (V102: [[de:6BE6]]). In the odd frequency bands the intermediate frequency is 2.5 - 3.5 MHz. In the second mixer (V106: [[de:6BE6]]), the product is also converted to the second IF of 500 kHz by mixing it with the 2 - 3 MHz signal of the highly linear VFO (V001: [[de:6BA6]]; V002: [[de:6BA6]]).

After that, the 500 kHz IF signal first has pass through the switchable crystal filter and then the bank of mechanical filters (with two 500 kHz IF amplifiers V301: [[de:6BA6]] and V302: [[de:6BA6]]), after two additional IF amplifier stages (V108: [[de:6BA6]] and V109: [[de:6BA6]]) it is fed to the detector (one system of the V110: [[de:12AX7]]) and after the noise limiter (one system of the V112: [[de:12AX7]]) to the audio frequency preamplifier stage (second system V112: [[de:12AX7]]). After the AF final stage (V113: [[de:6AQ5]]), the audio signal is fed to the connectors for the 600 ohms and 4 ohm output.

The second system of the V110 ([[de:12AX7]]) acts as the first and the first system of the V111 ([[de:12AU7]]) as the second AGC amplifier, the other system of the V111 ([[de:12AU7]]) acts as amplifier for the 500 kHz IF output. Another tube [[de:6BA6]] (V104) is used in the crystal calibrator,  a further [[de:6BA6]] (V114) in the BFO, whose signal is fed to the detector for [[CW]] reception.

The mains rectifier is a [[de:5V4G]] (V115), a [[de:0A2]] (V116) acts as voltage stabilizer. 

==== Valve setup ====
V001 and V002 ([[de:6BA6]], VFO 2-3 MHz );
V101 ([[de:6AK5]], RF preamplifier); V102 ([[de:6BE6]], 1st mixer to 2.5 → 1.5 MHz ); V103 ([[de:6BE6]], 2nd mixer, Band I); V105 ([[de:6AK5]], 12 MHz oscillator); V106 ([[de:6BE6]], 3rd mixer to 500 kHz)
V301 ([[de:6BA6]], 1st IF amplifier 500 kHz); V302 ([[de:6BA6]], 2nd IF amplifier 500 kHz); V108 ([[de:6BA6]], 3rd IF amplifier); V109 ([[de:6BA6]], 4th IF amplifier); V110a ([[de:12AX7]], detector); V112b ([[de:12AX7]], Noise Limiter); V112a ([[de:12AX7]], AF preamplifier); V113 ([[de:6AQ5]], audio final amplifier)
V110a ([[de:12AX7]], 1st AGC amplifier); V111a ([[de:12AU7]], 2nd AGC amplifier; V111b ([[de:12AU7]], amplifier 500 kHz IF output; V104 ([[de:6BA6]], calibrator); V114 ([[de:6BA6]], BFO)\\ 
V115 ([[de:5V4G]], mains rectifier), V116 ([[de:0A2]], voltage stabilizer).

==== Development ====


==== Field use ====
Most sets used by the Swiss Army have been modified with a S-meter zero knob next to the signal strength meter, so you can recognize them easily.
{{tablelayout?colwidth="300px,300px"}}
|{{:images:e-629-collins-51j4-old-knobs.jpg?direct&400|}}|{{:images:e-629-collins-51j4-new-knobs.jpg?direct&400|}}|
|Usually, You find Dekaware knobs on the Collins 51J-4 / R-388A.|Only the sets from the latest series made by Collins have been equipped with the newer S-line knobs, like in this example of a set having been used by the Swiss Army.|

==== Variants ====

{{gallery>:images?e-629*.jpg&0&80x80&lightbox&showtitle&titlesort}}
===== Manuals =====
  * {{ :manuals:e-629-collins-51j4-manual.pdf | 51J-4 Communications Receiver, Instruction Book, Collins Radio Comp.}}

==== Additional information ====
  * [[http://armyradio.ch/radio-d/e-629.htm|E-629 / Collins 51J-4 auf www.armyradio.ch]]
  * [[https://www.hamfu.ch/de/geraete/geraet.php?id=189|E-629 / Collins 51J-4 auf der Website der Hamfu]]
  * [[http://www.radiomuseum.org/r/collins_51j_4.html|E-629 / Collins 51J-4 auf der Website von www.radiomuseum.org]]