====== P-709: VHF direction finding system SCR-634A ======
**SCR-634 A**; manufactured by [[Bendix]] / U.S. Signal Corps.

The US-American VHF direction finder SCR-634 was used by the Swiss Air Force.

{{:images:p-709-scr-634.jpg?direct&300 |P-709: SCR-634A}} {{:images:p-709-bc-639a-bw.jpg?direct&300 |P-709: SCR-634A}}
===== Technical data =====
  * [[Principle]]: Adcock - direction finder
    * **Receiver**: **BC-639**: [[single conversion superhet]], [[IF]] 12 MHz
    * **Transmitter** (target): **BC-655**: VHF beacon transmitter for calibrating the direction finding system
  * [[Operation modes]]: [[A1]], [[A3|AM (A3)]]
  * [[Frequency range]]: 100 - 156 MHz
  * [[Sensitivity]]: [[A3|AM (A3)]] 2 - 3 μV / [[Selectivity]]: 

==== Power supply ====
  * [[Mains]]: 110 V AC (to power rectifier RA-42, receiver is supplied with anode voltage 210 V DC and heater voltage 6.3 V)

==== Dimensions ====
  * receiver **BC-639**: 480 x 270 x 270 mm, 19 kg

==== Accessories ====
In the original American setup, the direction finder consists of
  * the BC-639 receiver
  * the RA-42 rectifier
  * the PE-75 or PE-214 petrol generator
  * RC-153-B antenna system
  * Target transmitter BC-655
  * Shelter HO-34
  * Control Unit RM-38 

{{:images:p-709-bc-639a-front.jpg?direct&600 |P-709: SCR-634A}}
===== Operation =====
The BC-639 receiver in its metal cabinet is a veritable boat anchor with its 480 x 270 x 270 mm and weight of 19 kg, even though the entire VHF direction finder system was intended for air transport by the U.S. Air Force, the few kilograms for the receiver probably didn't make much difference compared to the weight of the shelter, power generators and antenna system.

The tuning knob with a mechanical blocking is located on the left-hand side of the front panel, the frequency dial calibrated in MHz ([[air band|aeronautical VHF range]]) above it.

The tuning instrument is at the top right of the front panel and two rows of controls below. The top row of controls activate a switchable -6 / -12 dB attenuator (which has to be activated in the vicinity of strong signals to prevent overloading), the BFO control for [[A1]] reception and the power switch.\\ 
In the bottom row are the [[RF gain]] control, the switch for automatic gain control ([[AGC]]), the AGC can be turned off for [[CW]] reception and direction finding, and the volume control ([[AF gain]]).

{{ :images:p-709-bc-639a.jpg?direct&300|P-709: SCR-634A}} The operation of the reciver, e.g. for receiving weather reports from the nearest airport, is straightforward: The set is turned on, the [[RF gain]] control is set fully clockwise, the automatic gain control to AVC, the attenuator to 0; the frequency is tuned with the tuning knob and the volume is controlled with the [[AF gain]] control.

The corresponding USAF manual must be studied for information how to use the receiver for direction finding.

===== Technical principle =====
{{:images:p-709-bc-639a-block.gif?direct&600|}}\\ 
The antenna signal coming from the an antenna first is fed through a  RF preamplifier ([[9003]]) and fed to the mixer stage, where it is converted to the intermediate frequency of 12 MHz. The oscillator signal ([[9002]]) is doubled in a doubler stage ([[9003]]).\\ 
The intermediate frequency of 12 MHz in amplified in three IF amplifier stages (each [[6SG7]]) and finally fed to the [[6SQ7]] tube, then the signal is demodulated and the AF signal is amplified. From the AF final stage ([[6K6]]) the signal is fed to the headphone jacks.\\ 
For [[CW]] reception, the signal from the BFO ([[6SG7]]) oscillating from 11.9 - 12.1 MHz is mixed in before demodulation.

==== Tube configuration ====
V1 ([[9003]], RF preamplifier); V2 ([[9003]], mixer stage); V3 ([[9002]], oscillator); V4 ([[9003]], doubler); V5 ([[6SG7]], 1st IF stage); V6 ([[6SG7]], 2nd IF stage); V7 ([[6SG7]], 3rd IF stage); V8 ([[6SQ7]], demodulation, AVC and AF preamplifier); V9 ([[6K6]], AF final stage); V10 ([[6SG7]], [[BFO]]).

==== Development ====
The VHF direction finding system and the BC-639A VHF receiver were probably developed by Bendix with the help of the U.S. Signal Corps and were used in air transport during World War II.

==== Use ====
Apparently, the SCR-634-A VHF direction finder system was used by the air force in Switzerland.

{{gallery>:images?p-709*.jpg&0&80x80&lightbox&showtitle&titlesort}}

===== Technical documentation =====
  * [[http://www.radionerds.com/images/f/f9/TO-31R2-3BC639-22.pdf|TO-31R2-3BC639-22]], manual for the BC-639 A

==== Further information ====
  * [[https://www.radiomuseum.org/r/bendix_scr_644_bc_639bc_639_bc63.html|BC-639 on the website of www.radiomuseum.org]]