TYPE 686 P


TYPE 686 TRANSCEIVER


DATE OF DESIGN 1944
FREQUENCY RANGE Not stated as such. A very strange set up for this one-off transceiver.  See 'method of producing oscillations' below.
POWER SUPPLY 12 Volts [50 AH] Battery [AP W3433A] This allows for 11 hours of continuous operation if used in periods of 10 minutes transmission 10 minutes reception.
TYPE OF SET VALVE
WAVEFORM CW or MCW
METHOD OF PRODUCING OSCILLATIONS SELF.{VFO}. The transmitter  has  two-bands over which it can work. On band 1 there are three choices of SPOT FREQUENCIES which are colour coded A =Blue, B = White and C = Yellow: on Band two, three more choices, C =Yellow {common to bands 1 and 2} D = Black and E = Brown.  A transmitter can have either but not both.  The transmitter shown below in the photograph section is tuned to operate on C,D and E spot frequencies only {click also on this thumbnail}Click to enlarge

{The apparent marking before 'A' and after 'C' are screwheads}
whereas another set will be prepared with a tuning dial showing the spot frequencies A,B and C. The frequencies are NEVER mentioned either to the operators of the set or to the planners of the communication circuits - all that is spoken/written of is the COLOUR/SPOT FREQUENCY LETTER. The set is tuned by a special test set ashore or in the HQ ship before issue for use.  The receiver tuning dial allows the operator to tune across the whole frequency range of the receiver which covers A,B,C,D,E frequencies. Theses latter frequencies are shown by a continuous heavy black line marked on the RX tuning dial.  {See the photograph below and also click on this thumbnail}

Click to enlarge

{the two apparent markings above the tuning scale are screwheads}. The best way to describe the alignment of the 686 transmitters is by using this block diagram of the system.

 Here you can see the Test Set and plugged into that,  are the earphones and an output from the 686's variable frequency oscillator [VFO]. The test set generates harmonics of the 3Mc/s crystal, which, with the exception of the fundamental frequency, are fed to a mixer stage. The output of the mixer contains frequencies which are the sums and difference of the two inputs, plus the input frequencies themselves.  The low pass filter feeds the difference to the AF amplifier and the zero beats can be heard when the crystal oscillator coincides with the free running LC oscillator.
                                             

Transmitter Tuning Dial Readings Beat No Frequencies
14 1 Not used
26 2 'A' = Blue
37 3 'B' = White
49 4 'C' = Yellow
61 5 'D' = Black
74 6 'E' = Brown
If all was well with the transmitter [and assuming that the test set was never in doubt to its accuracy of functionality] by setting the transmitter tuning dial to say 26 [Blue dot on scale] a beat note would be heard in the test set meaning that the VFO's frequency agreed with that of the CO's frequency which would be 6Mc/s.  When considering the frequency of 12Mc/s the 686 dial set should read 49 etc.  If it didn't, then the technician would altered [trim] the Capacitor or Inductor of the tuned circuit until the beat was heard in the earphones. The 6th beat would represent 18 Mc/s.  With the type of aerial supplied, one would assume that the transmitting/receiving frequencies would be very high, certainly in the VHF range, and this is further supported by the power output and ranges achieved. Although there is no mention of frequency doubling, trebling, quadrupling etc, it is possible that the VFO {once checked correct} was ⅛ of the output frequency.  If this were the case [and this practice at VHF frequencies was not unheard of] the frequency range was in the order of 48 - 144Mc/s.

WHERE USED/FITTED Type 686.  The set is not fully tropicalised.  This set is used for communications between Combined Operations craft over distances of 2-10 miles according to whether R/T or MCW is employed, and also depending upon the maximum height at which aerials can be fixed in the craft concerned.  The set, which is transportable, has been designed for operation by unskilled personnel.  To ensure efficient operation it is most essential that adjustments of the set before issue for service should be correctly carried out. It has two types of aerial supplied [a] a dipole for omnidirectional working, and [b] a 4-element array for unidirectional working.  The dimensions are 11" x 10" x 15".  The battery accommodation is a further 12" x 6⅞" x 9".  The weights are: Set = 44lbs :  Dipole Box = 24lbs : Battery = 61lbs : Array Box = 20lbs.
POWER OUTPUT  3 Watts - R/T Range 2-5 miles/MCW Range 3-10 miles depending upon height of aerial.
PHOTOGRAPHS