|
2. THE "A" SET
(a) General circuit considerations
The "A" set is designed so that when it is tuned to any incoming signal within its frequency range, exactly the same frequency will be radiated without the adjustment of a separate Master Oscillator. Netting is thus automatically achieved, provided that the receiver is tuned accurately.
The principles employed are shown in the block diagram, Figure 25.
The complete set employs 9 valves, 3 of which are used on both "Send" and "Receive".
For reception a superheterodyne circuit with automatic volume control is used, and the sender signal frequency is controlled by the tuning of the receiver 1st Local Oscillator and Beat Frequency Oscillator.
(b) The flick frequency
This set is equipped with a mechanical flick frequency arrangement which permits the pre-selection of any two frequencies in the band, change-over from one to the other being accomplished by the simple movement of each of the two tuning dials.
The flick mechanism consists essentially of two circular plates each of which is located to the main condenser shaft by means of two clamping screws. When these latter are slackened by one half-turn the condenser shaft can rotate independently.
Each plate has a V aperture cut in the periphery, into which a spring loaded locating arm can engage. The latter also operates a flag, thus providing visual identification for each frequency. A three-position flick lever in the "tune" position disengages the flick mechanism and brings the slow motion drive into operation. In the "set" position both flick and slow motion mechanisms are in operation and the tuning dial may be set for "flick". In the "flick" position the slow motion drive is disconnected and the flick mechanism is operating.
(c) Receiver – Technical Description
The receiver employs a 6 valve superheterodyne circuit comprising one stage of R.F. amplification, frequency changer, two I.F. amplifier stages (465 kc/s) followed by a combined signal detector, A.V.C. and output stage. A separate Beat Frequency Oscillator used for C.W. reception may also be operated at exactly the intermediate frequency by means of a toggle switch. Tuning to zero beat with the aid of this will ensure correct adjustment and accurate netting.
(1) R.F. Amplifier
The signal from the aerial, which is tuned to resonance by the variometer L1A and condenser C24A (0.001mF) in series, is applied to the aerial tuning circuit comprising L3A tuned by C3A (0.00054 mF) through a tapping on L3A. The latter is connected to the control grid of V1A, an R.F. variable-mu pentode, type 6K7G, via the condenser C2A (0.0001 mF).
C33B (0.1 mF) and C36A (0.01 mF) isolate the P/A H.T. which is applied to L3A on send. V1A is self-biased by means of R2A (220W) decoupled by CV4B (0.1 mF). Extra cathiode bias for R.F. gain control is provided by R46A (10kW) also in the cathode circuit.
A.V.C bias is applied via L10A, the purpose of which is to prevent loss of signal input to chassis via the bias circuit.
The anode load impedance is the primary of the R.F. transformer L22A and B (H.F.) or L23A and B (L.F.). H.T. to anode is applied via the resistor R5A (2.2k
W) decoupled by C45B (0.05mF or 0.1mF). The screen grid, in common with the screen grid of V1B, (the first I.F. amplifier) receives its H.T. from a potential divider consisting of R33B (27kW), R44A (82kW) and R46A. C45A is the screen decoupling condenser.C9A, one section of a four-gang assembly, tunes the transformer secondary to the signal frequency and C10A is the trimmer for this circuit. C10D, a further trimmer, is necessary on the L.F. band.
(2) Frequency-Changer.
The R.F. amplifier output is taken to the hexode control grid of V2A, a triode-hexode, type 6K8G. The triode section of this valve acts as the local oscillator in a tuned anode feed-back circuit, and it is parallel fed. It works on a frequency, 465kc/s above the signal frequency, which is determined by the variably tuned circuit L24A (H.F.) or L24A and L24C in series (L.F.) and C9B.
Feedback is provided by the coupling coils L24B (H.F.) or L24B and L24D (L.F.) These coils are shunted by R17B and R5F respectively to stabilise oscillator output by preventing resonance in the grid circuit.
C8A (0.005
mF) and C12A (0.002mF) in parallel with C11A (750pF) are padding condensers for the H.F. and L.F. bands respectively. C10G, C35A, C43A and C6A are the trimmers, C6A is a special type with a negative temperature coefficient to correct frequency drift. Bias is provided by the grid leak R6B and condenser C7B.The triode grid is connected internally to the injector grid of the hexode section which functions as the mixer. Control grid bias is provided by R3A (270
W) in the cathode lead, decoupled by C4E. A.V.C. is not applied to this stage.The I.F. component of the output is selected by the primary of the transformer L8A and passed on via the secondary of L8A to the grid of V1B. L8A is permeability tuned, R7A (100k
W) shunts the primary and so assists in providing the desired response curve.
(3) The I.F. Amplifier.
There are two stages of I.F. amplification. The input to each of the I.F. valves V1B and V1C (Type 6K7G) is taken from the secondary of the previous transformer. These form band-pass filters which are permeability tuned to the intermediate frequency (465kc/s). V1B is biased by R3B in the cathode lead. Each valve also receives full A.V.C. bias except on C.W. when V1C is disconnected from the A.V.C. line.
When the send-receive relay is in the "send" position it disconnects the screen grids of V1A, B and C, but does not disconnect the anodes.
(4) Signal Detector, A.V.C. and Output Stage
V3A is a double diode pentode (type 6B8G). One diode acts as the signal detector, the other diode as the A.V.C. rectifier, and the pentode as the A.F. amplifier.
The signal detector section of V3A operates in a series diode circuit, the load resistance being R1B. R7C in conjunction with C14A (100pF) and C15A (500pF) forms an I.F. filter. A.F. is fed through the blocking condenser C17A (0.002
mF) to the A.F. volume control R13A (1MW) which is connected by a screened lead through the system switch and contacts on the send/receive relay to the control grid of the pentode section.The A.V.C. diode receives I.F. via the condenser C18A (20pF). This diode operates in a parallel diode circuit, R8A (1MW) being the diode load resistor. Rectified current through R8A develops a voltage which is applied as negative bias through the filter circuit R8B and C38A (0.1mF) to the controlled valves. As R8A is returned directly to earth, a negative delay bias is applied to the A.V.C. diode equal to the voltage (30V) across the cathode resistors R9E and R11A (3.3kW), plus R5G on C.W. only.
The pentode section of V3A is biased by the cathode resistor R9E which is decoupled by C16A (12
mF). Its anode load impedance consists of the primary of the output transformer T2A, the secondary of which goes to the phones. Both anode and screen receive H.T. through the S/R relay contact S5A/3 when on receive.Selectivity on C.W. is assisted by the inclusion of an A.F. flter consisting of a network of resistors and condensers R7E, R47A and B, R48A, C5A and C40A and B. The filter provides voltage negative feedback excpet over a band of frequencies around 1000 c/s and so a note of this frequency will receive more amplification than other frequencies.
The performance on W/T is further improved by restricting the frequency response of the output stage in order to reduce noise and interference. The reduction in the value of the screen decoupling condenser C29E provides bass attenuation, and the higher frequencies are attenuated by means of the grid filter consisting of R8C and the shunt capacity of the screened leads.
(5) THE BEAT FREQUENCY OSCILLATOR
The triode section of V2B operates as the B.F.O. in a parallel-fed Meissner circuit. The oscillatory circuit consists of L5A tuned by C41A (200 pF) to the intermediate frequency (465kc/s). Feedback is provided by L5B, and the valves is biased by means of the voltage drop across R37B and R27A, C4Q is the bias decoupling condenser.
Oscillator frequency is varies within audible limits of the I.F. by means of the coupling coil L5C which is shunted by the rejector circuit L6A, C42A and R14A, R14A is centre-tapped and variable. The rejector circuit is tuned to the I.F. when R14A ("Het. Tone" control) is in the centre position. Variation of R14A either way changes the reactance of the rejector circuit, which in turn varies the load on the B.F.O. oscillatory circuit and, therefore, its frequency.
The output voltage developed across part (R1D) of the anode load, R6A and R1D in series, is fed to the primary of L8B via C39A.
Foe best C.W. reception the "Het. Tone" control is adjusted so that pitch of the wanted signal coincides with the peak frequency of the filter in the output circuit.
When the netting switch S9A is pressed, the B.F.O. operates at the intermediate frequency (465kc/s) and the set is accurately tuned by adjusting the tuning control for zero beat.
(d) THE SENDER
(1) General
When the "A" set is operating on 'send' the output of the
first local oscialltor V2A (signal frequency plus 465 kc/s) and the B.F.O output (465
kc/s) are combined in the hexode section of V2B. The difference frequency, i.e. signal
frequency, is selected by a tuned circuit and, after amplification in the buffer stage,
V5A, it is applied to a power amplifier stage. On R/T and M.C.W. grid modulation of the
P.A. is employed. On C.W. the screen of the P.A. valve and the anode and screen of Buffer
Amplifier are keyed.
(2) Master Oscillator.
This is the triode section of the receiver frequency
changer valve V2A, which oscillates at a frequency 465 Kc/s above that of the carrier. No
circuit changes take place in this stage when switching from receive to send. Coupling to
the hexode control grid of V2B is obtained by means of the potentiometer comprising C21A
(5 pF), R42C and R7L.
(3) Sender frequency changer.
The triode section of the triode hexode valve V2B oscillates at exactly
465 Kc/s. Oscillator frequency is independent of the "Het. Tone" control by
making the sliding contact o/c on send. A slight variation in frequency on applying H.T.
to the hexode screen on send is compensated by means of the adjustable coil L25A in series
with C4J and shunting L5C. On receive this shunting effect is reduced to a negligible
quantity by C17B. The output is mixed with that from the master oscillator in the hexode
section of V2B. The anode load impedance for this hexode section is the primary of a
transformer L7A and B (H.F.) or L21A and B (L.F.), the secondary being tuned to the
difference frequency (signal frequency) by C9D.
The screen grid voltage is applied by means of a potential divider R45B and R4D. Control grid bias is obtained from the cathode resistors R37B and R27A in series, with R10A in parallel. The hexode grid receives bias developed across R37B only.
(5) Double Diode Bias Control
The control grid of V5A receives further negative bias in the form of
automatic gain control voltage from V6A, a double diode valve (ARDD5). This control
voltage which is delayed is used to maintain a constant pre-determined drive to the grid
of the P.A. One diode of V6A operating in a parallel diode circuit received R.F. input
from C2D, and the voltage developed across the load resistor R15A is applied as control
boas to the grid of V5A through the filter consisting of R1C and C15D and then through the
grid leak R1E. The control voltage is delayed by the other diode which shunts the A.G.C.
line to an extent determined by the positive bias provided by the potential divider R5E,
R43A, R1C and R15A\across the H.T. supply.
(6) Power
Amplifier
The P.A. valve V4A is a beam tetrode (ATS25) operating with an anode voltage
of 450V. Screen voltage is obtained from the normal H.T.line. Grid bias on R.T. amd M.C.W.
is obtained from the voltage drop across R16A, which is in H.T.2 negative line. R16A is
short-circuited on C.W. and bias is obtained by the grid current method.
When switched to receive a large bias voltage is applied via R18Band R19A which renders the valve inoperative, on Send R19A is short-circuited, (see fig 33). The output is tuned by L3A and C3A and then passes on to the variometer L1A which tunes the aerial.
(7) Modulation
On R/T working the pentode section of V3A is used as a modulation
amplifier. Input from the microphone comes through the secondary of transformer T3A to the
control grid via a screened lead. The A.F. output is choke-capacity coupled to grid of the
P.A. valve, the choke being the primary of the telephone transformer T2A. Side-tone is
taken from the secondary of this transformer.
On M.C.W. V3A acts as an A.F. oscillator, the primary and secondary of T2A acting as reaction and grid coils respectively. Keying takes place in the anode and screen circuits.
(8) C.W. working
Keying of the carrier for C.W. is effected by connecting the key in the
following H.T. circuits:-
(a) Screen grid of
V4A.
(b) Anode and screen
grid of V5A.
When operating on C.W. V3A is switched off and no sidetone is provided.
(e) INCIDENTAL EQUIPMENT
(1) Test meter
The meter on the front panel is 0-500 micro Amps full scale deflection, and by means of a 6 position rotary switch the following indications are available:-
(a) Aerial current
R.F. from the transformer L1A is applied to the
bridge rectifier W1A. D.C. output is taken to the meter via L2A, R29A and the screened
aerial lead L2B.
(b) A.V.C.
In this position the meter, in series with
R21C, shunts the cathode resistor R9A in the 1st I.F. stage. The deflection obtained will
depend on the cathode current of V1B, which, in turn, will be affected by the changes in
bias due to the operation of the A.V.C. circuit. The meter reading should show a decrease
when the set is tuned through a strong signal.
(c) L.T.
The meter is switched in series with R26A and
acts as a voltmeter across the L.T. input.
(d) H.T.1.
The meter is switched in series with R24A and
acts as a voltmeter across the input of H.T.1.
(e) H.T.2.
The meter is switched in series with R25A and
measures the H.T.2 voltage.
(f) Drive
In the drive position the meter is connected
across R42B in the cathode of the A.G.C. diode. As the current through the diode will be
in proportion to the drive applied to the grid of the P.A., any variation in Drive will be
clearly indicated.
(2) Variometer
The variometer which tunes the aerial comprises
a rotor and a stator which can be connected in series or parallel by means of switches
operated by the rotor spindles. Between O and 100 on the scale the coils are in series,
and between 100 and 200 they are in parallel. Switch change-over positions are indicated
by red bands on the scale, the set must not be operated when in this condition. To
increase inductance from minimum to maximum the dial must be rotated first from 100 to 200
then from 100 to 0.
WS No.19 contents Low power wirelesses contents Radio Pages Home Page
Rod Beavon 17 Dean's Yard London SW1P 3PB
e-mail: rod.beavon@clara.net