DETAILED CIRCUIT DESCRIPTION
1. Reference should be made to the circuit diagram at the end
2. A 75-ohms unbalanced aerial source is connected to the tuned
amplifier trough a three-section 30 Mc/s low-pass filter and a
position attenuator covering a range of 0 to 40 dB. Switch S2
band 75-ohms or wideband (high impedance) or any one the five
aerial coils L4-L8 for tuned operation. These aerial coils are
means of dust iron cores. The aerial is tuned by a capacitor C18A/B
is switched out of circuit in both wideband positions.
3. The incoming signal is fed via C28 and grid stopper R25 to
of V3B; the r.f. stage (V3) employs a variable-mu, low-noise double-
triode; the two halves of the valve are connected in cascode so
as to utilize
the low-noise high-gain properties of the valve. A delayed a.v.c.
derived from a shunt diode network, is applied to the grid of
V3B when the
signal level is approximately 10µV. The capacitors C40 and
C41 ensure that
the cathode is adequately decoupled over the wide frequency range.
beads have been fitted to the heater lead, connected to pin 4,
the anode of
V3A and the cathode of V3B adjacent to C41, to prevent parasitic
30 MC/S LOW-PASS FILTER
4. The amplified signal is passed to a 30 Mc/s low-pass filter
a substantially flat responseover the frequency range. L27, C47
R28 constitute the first 'L half Section' of the filter. The signal
fed at low impedance (680-ohms) trough the coupling capacitor
C74 and the grid
stopper R45 to the control grid of V7, the first mixer stage.
capacitance of V7 forms the capacitance to chassis betweeenL15
and L17 required
to the filter network.
NOTE: This capacitance is not critical, therefore no adjustment
necessary should V7 be changed.
FIRST VARIABLE FREQUENCY OSCILLATOR (VFO-1)
5. This circuit comprises a cathode-coupled Hartley oscillator
which may be continously tuned over the frequency range of 40.5
69.5 Mc/s. The frequency determining components are an inductance
L36 and a
variable capacitanceC76. Alignment is accomplished by adjusting
of L36 and the trimming capacitor C77. The variable capacitor
C76 is coupled
to the Mc/s dial which is calibrated from 0 to 29 Mc/s. The anode
of L20, a compensating inductance which is wound on a 470-ohm
The oscillator is coupled via C85 to the signal grid of the first
V7 and also via C42 to the control grid of the harmonic mixer
NOTE: The Mc/s dial calibration may be affected if V5 if changed.
necessary correction may be made by adjusting C77 with the Mc/s
set to 29 Mc/s.
FIRST MIXER (M1)
6. the outputs from the 30 Mc/s low-pass filter and the variable
oscillator VFO-1 are fed to the signal grid of the mixer stage
which produces a signal at 40 Mc/s. The signal is then passed
to a 40 Mc/s
band-pass filter which forms the anode load of this stage.
40 MC/S BAND-PASS FILTER
7. The 40 Mc/s band-pass filter consists of eight over-coupled
circuits connected in cascade and is tuned by the trimming capacitors
C21, C33, C43, C53, C61, C70, C79 and C88. This filter, which
has a passband
of 40 Mc/s ±650 kc/s, ensures that only the required 1
Mc/s spectrum of
signals is passed to the second stage. This filter is deliberately
set to a
slightly wider passband than is theoretically required, to allow
drift in VFO-1.
1 MC/S CRYSTAL OSCILLATOR/AMPLIFIER
8. The frequency of the crystal oscillator V1 may be set precisely
1 Mc/s by adjusting the trimming capacitor C2A. The crystal XL1
is connected between the control grid and the screen grid is electron
to the anode. The anode coil L2 is adjusted to resonate at 1 Mc/s
by means of
a dust iron core. The fixed capacitors C9, C10 and C11 complete
circuit. When an external signal is applied to socket SK3, the
as an amplifier.
9. The output from V1 is capacitance-coupled to the harmonic generator
V2 and via SK2 to a "T" adptor for feeding a 1 Mc/s
input into the
l.f. converter and also the control grid of the mixer valve V13.
10. The 1 Mc/s signal is fed via coupling capacitor C8 to the
of the harmonic generator V2. The h.t. is fed to the screen grid
R12 and is decoupled by C8A. Harmonics produced at this stage
are passed to a
32 Mc/s low-pass filter.
32 MC/S LOW-PASS FILTER
11. The megacycle harmonics are fed trough a 32 Mc/s low-pass
circuit to prevent harmonics other than those required from passing
to the harmonic mixer (V4). Limited control over the cut-off frequency
provided by C7 which is adjusted to equalize the output from yhe
the frequencies corresponding to 28 and 29 Mc/s on the MEGACYCLE
12. The outputs from the 32 Mc/s low-pass filter and VFO-1 are
the harmonic mixer by applying the filtered megacycle harmonics
the suppressor grid and the output from the VFO-1 to the control
37.5 Mc/s output is selected by the tuned anode load, consisting
of a fixed
capacitor C50 and an inductance L28 which may be adjusted by means
of a dust
iron core, and coupled by C51 to V6. R36 is grid stopper.
2-STAGE 37.5 MC/S AMPLIFIER (1)
13. The anode load of V6 is a tuned circuit consisting of a fixed
C67 and an inductor L33 Which is tuned to 37.5 Mc/s. Frequency
adjustment is by the dust iron core L33. This stage feeds the
via C68 to the following stage V8. The 37.5 Mc/s signal is then
passed to the
37.5 Mc/s band-pass filter. The anode load of this stage is provided
37.5 MC/S BAND-PASS FILTER
14. The 37.5 Mc/s band-pass filter consists of eight under-coupled
circuits arranged in cascade. These filter sections may be tuned
C24, C35, C45, C55, C63, C72, C81 and C91 respectively. This filter,
has a passband of 300 kc/s, allows for possible drift in VFO-1.
passband and high rejection to frequencies outside the passband
spurious signals from reaching the second mixer stage (V9).
37.5 MC/S AMPLIFIER (2)
15. The filtered 37.5 Mc/s signal is further amplified by V10
passed to the second mixer stage (V9). To prevent interaction
the 40 Mc/s band-pass filter and the 37.5 Mc/s tuned circuit (L50
and to enable either circuit to be adjusted without affecting
the other, a
balancing circuit is included which is shown in simplified form
in fig.4. The
40 Mc/s signal is introduced into the 37.5 Mc/s tuned circuit
at a point of
zero r.f. potential since L50 is centre tapped and C108 is adjusted
equal to the total of the capacitance of V10 anode to chassis.
C107 and the
input capacitor of V9.
NOTE: The anode load of V10 is adjusted to 37.5 Mc/s by adjusting
iron core in L50. The balancing circuit will be affected if V9
SECOND MIXER (M2)
16. This mixer (V9) produces the second intermediate frequency
of 2-3 Mc/s
by mixing the 40 Mc/s i.f. and the 37.5 Mc/s signal. The tuned
formed by L300, C300 remove the 37.5 Mc/s frequency whilst the
circuit formed by L301, C301 remove the 6 Mc/s frequency so that
second i.f. is passed to the 2-3 Mc/s band-pass filter preceding
2-3 MC/S PRE-TUNED BAND-PASS FILTER
17. This filter consists of two pre-tuned band-pass filter sections.
characteristic impedance of the filteris 1000-ohms.
18. The output from the 2-3 Mc/s band-pass filter is resistance-capacitance
coupled to the signal grid of V25 together with the output (3.6-4.6
Mc/s) from the second v.f.o. amplifier V11 when the V.F.O. switch
(S300) is set
to the INT. position. With the V.F.O. switch set to the EXT. position,
operates as a buffer amplifier. This mixer (V25) produces the
frequency of 1.6 Mc/s. The signal is then fed to a 1.6 Mc/s band-pass
which forms the anode load of this stage.
19. The 1.6 Mc/s band-pass filter consists of two double-tuned
formers, the first section of the filter is formed by C320, L306,
and C325 and the second section by C332, L313, L314, C334. This
filter has a
bandwidth of 13 kc/s.
SECOND VARIABLE FREQUENCY OSCILLATOR (VFO-2)
20. The second variable frequency oscillator, covering a frequency
3.6 to 4.6 Mc/s, is an electron coupled Hartley circuit embloying
half of double-triode V12. The oscillator frequency is determined
inductance L55, two fixed capacitors C303, C305, a trimming capacitor
a variable capcitor C301. The KILOCYCLES scale which is calibrated
and 1000 kc/s is coupled to this variable capacitor.
21. The output from VFO-2 is resistance-capacitance coupled to
the grid of
V12A, a cathode-follower stage. With the V.F.O. switch set to
position the output from V12A is fed via PL305 and PL300A to the
of the second v.f.o. amplifier V11. In the EXT. position the external
4.6 Mc/s signal is fed ti V11.
22. the output from the 1.6 Mc/s band-pass filter is directly
the signal grid of a pentagrid valve V26; it is mixed with a 1.7
signal from V27 fed via the coupling capacitor C339 to the oscillator
V26. The resistor R68 completes the d.c. path from this grid to
100 kc/s output from this mixer stage is then fed via SK6, PL6
to the crystal
1.7 MC/S CRYSTAL OSCILLATOR/AMPLIFIER
23. The frequency from the crystal oscillator C27 may be set precisely
1.7 Mc/s by adjusting the trimming capacitor C337. The crystal
which is connected between the control grid and the screen grid
coupled to the anode. When an external signal is applied to socket
valve operates as an amplifier. The output from this circuit is
fed via C339
to the oscillator grid of the fourth mixer V26.
24. Six alternative switched i.f. bandwidths are available as
100 c/s ) Crystal 1.2 kc/s )
300 c/s ) 3.0 kc/s )
6.5 kc/s ) L - C
13.0 kc/s )
25. In the crystal positions the fourth mixer anode is connected
in the crystal filter. L47 and L49 provide a balanced output which
tuned by capacitors C109 and C110. In the 100 c/s position, the
output is connected via crystals XL2 and XL5 to the first tuned
section of the
100 c/s L-C filter. The differential trimmer C118 is the phasing
this bandwidth. XL3, XL6 the capacitor C119 form a similar circuit
300 c/s position. Damping resistors R64 and R65 are connected
across the tuned
circuits to obtain the required bandwidth.
100 KC/S L-C FILTER
26. This filter consists of four tuned circuits arranged in cascade.
L-C bandwidth positions, the signal is fed to the tuned circuit
by L61 and the combination of the capacitors C145, C146, C146A
and C147. The
second section consists of L62 and L63 in series with C152, C152A
and C153. The
final section consisting of L68 and L71 in series with C161 and
C162, is damped
by the series resistors R86, R87A and R88 according to the bandwidth.
L-C positions the output is taken from a capacitive divider formed
by C161 and
C161A with C170, to equalize the gains in the L-C and crystal
27. The L-C banwidths are obtained by varying the degree of coupling
between each section of the filter in addition to the damping
in the final stage. The capacitor C175 is included to compensate
effective reduction of the input capacitance of V14, appearing
across the tuned
circuit, when switching from crystal to L-C positions.
28. To maintain the input capacitance of the L-C filter, in the
positions, a trimming capacitor C148 is switched into circuits.
trimmer is adjusted to be equal to the output capacitance of V26
screened cable. In the crystal bandwidth positions, the L-C filter
in its narrow bandwidth positions, i.e. 1.2 kc/s.
NOTE: The 470-kilohm damping resistors R77 and R80 are disconnected
except during filter alignment.
FIRST 100 KC/S I.F. AMPLIFIER
29. The output from the L-C filter is passed trough a coupling
C164 to the control grid of the pentode amplifier valve V14. This
is returned via R96 to the a.v.c. line which is filtered at this
point by R102
and C173. The screen potential is derived from a potential divider
R93, R97 and RV4. This stage is coupled to the second i.f. amplifier
i.f. output stage by a double tuned transformer having an over-coupled
SECOND 100 KC/S I.F. AMPLIFIER
30. The signal from the first i.f. tranformer is fed trough the
stopper R114 to the control grid of the second i.f. amplifier.
supplied to the screen via the dropping resistor R113 and is decoupled
The anode load is tuned circuit consisting of L77, C192 and C191.
is heavily damped by R112. The secondary winding L78 and L79 is
tuned by C195
and C195B with R120A as a damping resistor. The output is fed
to the diode
31. The low potential end of L79 is connected through the r.f.
R128, C210, C219 and C211) to the diode load R130. With the meter
switched to R.F. LEVEL, the meter indicates the detector diode
resistor R131 is incluced to complete the diode detector circuit
when the meter
is switched out of circuit.
32. The noise limiter diode (pins 2 and 5 of V21) is connected
in a series
circuit to operate at approximately 30% modulation. its operation
explained with reference to Fig.5.
33. The d.c. path from point A is trough R134, R135, the diode
The a.f. signal path from detector diode load is through C216,
diode and C218 when S8 is open. In the presence of a signal, a
potential varying with the depth of modulation, will be developed
at point A
thus causing the diode to conduct. The negative potential at B,
will be lower
than at A and will be maintained at a constant level due to the
constant of R134 and C217. R135 allows the cathode potential to
sympathy with the modulation provided the modulation depth does
not exceed 30%.
The potential appearing at the cathode of the noise limiter diode
consists of a steady negative potential with the modulation superimposed.
noise impulses corresponding to high modulation peaks appear at
point A and via
C216 at point C, the voltage across the diode changes sign thereby
diode to stop conducting and open-circuit the a.f. signal path.
With S8 in the
OFF position the limiter is inoperative.
A.V.C. AND T.C. DIODE
34. The signal appearing at the anode of V16 is passed through
C139 to the anode of the a.v.c. diode. The diode load is formed
A positive potential derived from R120, R121 and R122, supplies
a.v.c. delay voltage to the cathode of this diode.When A.V.C.
switch is in the
SHORT position and the SYSTEM switch set to a position in which
the a.v.c. is
operative, i.e. A.V.C., CAL. or CHECK B.F.O., the anode of the
a.v.c. diode is
connected to the a.v.c. line via L81 and R127. The choke L81 is
tuned by C203
to a frequency slightly below 100 kc/s so that is presents a small
at 100 kc/s, thus R127 is prevented from shunting the diode load.
signal level falls, the capacitors C182 and C173 discharge through
and L81 into the diode load resistor R116. The a.v.c. potential
is brought out
via R123 to the tag strip at the rear of the receiver for external
required. With the SYSTEM switch set to the MANUAL position, the
a.v.c. line is
connected to the R.F./I.F. GAIN control RV1, thus the gain of
the 100 kc/s
amplifiers may be varied by adjusting the negative potential applied
35. Audio frequencies are applied to the control grid of V23B
via RV2 the
A.F. GAIN control. The output transformer (T2) provides four separate
outputs as follows: 1W into 3-ohms, and three windings supplying
3mW into 600-
36. The internal loudspeaker (which may be switched out of circuit
operating S11) is connected across the 3-ohm winding. The headphone
jacks JK1 and JK2 are connected across one of the 600-ohms windings.
A.F. LINE OUTPUT
37. The audio frequencies are also applied to the grid of V23A
via RV3, the
A.F. GAIN LEVEL control; this control presets the level from output
transformer T3. The transformer provides a 10mW output at 600-ohms
suitable for direct connection to landlines. A bridge rectifier
MR1 is connected
across the output via R142 and R143. Th meter may be switched
rectifier circuit so that the operator can monitor the a.f. output.
BEAT FREQUENCY OSCILLATOR
38. The beat frequency oscillator (V19) employs an electron-coupled
circuit. The oscillation frequency is determined by a fixed inductor
L82 and a variable capacitor C200 in parallel with C202 and C201.
capacitor C201 is adjusted to produce an output frequency of preisely
when the beat frequency oscillator frequency control is set to
zero. Bias is
applied to this valve by C199 and R125.
39. The b.f.o. output is coupled to the diode detector anode via
b.f.o. is supplied with h.t. via S7 except when SYSTEM switch
is in the
CAL. or STANDBY positions.
100 KC/S I.F. OUTPUT
40. The control grid of V17 is connected to the secondary of the
100 kc/s i.f. transformer which feeds the stage with the 100 kc/s
signal. The screen resistor R108 and the cathode bias resistor
R115 are of the
same values as used in the scond 100 kc/s i.f. amplifier, hence
characteristic of this stage is identical to that of the main
anode load resistor R109 feeds the auto transformer L76 via blocking
C189. This transformer provides a 70-ohms output at PL8 and PL9
NOTE: PL8 and PL9 are connected in parallel, therefore only one
output is available at 75-ohms, and to avoid a mismatch the other
connection should be made at high impedance.
41. The crystal calibrator, controlled by the 1 Mc/s crystal or
1 Mc/s standard input to V1, feeds signals at 100 kc/s intervals
the signal grid of the third mixer stage to provide calibration
The calibration can only be carried out when the V.F.O. switch
S300 is set
to the INT. position.
42. The 1 Mc/s signal, fed through SK2, is connected through PL2
grid stopper R83 to the first grid of the mixer valve V13. The
load consists of a 100 kc/s tuned circuit (L70, C167) and is coupled
control grid of V15 through the capacitor C168. The anode load
of V15 (L75,
C117) is tuned to 900 kc/s and is coupledvia C178 to the third
grid of V13.
V15 is heavily biased so that it functions as a frequency multiplier.
43. An output of 900 kc/s, appearing across the tuned circuit
is coupled to grid 3 of V13 thereby producing a difference frequency
of 100 kc/s relative to the 1 Mc/s input. The 100 kc/s output
the anode tuned circuit (L70, C167) and is fed to the control
grid of V15. The
ninth harmonic is selected in turn by the anode tuned circuit
(L75, C177) of
V15 and fed back to the third grid of V13 to provide the beat
100 kc/s with the 1 Mc/s input. This crystal controlled regenerative
is thus self-maintaining. The 100 kc/s output is obtained from
the coil L69
which is mutually coupled to L70 and fed via the octal plug (PL7)
44. The primary of the mains transformer is tapped to provide
of 100-125 and 200-250V. To remove mains-borne interference the
capacitors C224 and C225 are incluced. The secondary winding of
T1 feeds a
bridge-connected full-wave rectifier MR4, MR5, MR6 and MR7 whose
filtered by C206, L80 and C198 and fed via the receiver muting
to the SYSTEM switch S5. A 120-ohm resistor R124 is connected
negative line and earths thus providing a negative 25V d.c. supply
45. The following conditions exist for each setting of the SYSTEM
The link on h.t. adaptor terminal is assumed to be in position.
(1) STANDBY S5A disconnects the h.t. from all stages
and connects R119A across the h.t. as a
(2) MANUAL (a) The h.t. passed through S5A, S5B
and S5C to all stages except the
(b) S5F connects h.t. to the b.f.o. when
S7 is switched on.
(c) The a.v.c. line is disconnected from
the a.v.c. diode by S5D and connected
to the R.F./I.F. GAIN control (RV1)
(3) (a) (2) (a) and (2) (b) are applicable.
(b) S5D connects the a.v.c. line to the
(4) (a) H.T. is applied via S5A, S5B and S5F
to all stages except:-
The r.f. amplifier (V3)
The first v.f.o. (V5)
The first mixer (V7)
The second mixer (V9)
The final 37.5 Mc/s amplifier (V10)
(5) CHECK B.F.O. (a) (4)(a) applicable except that h.t. is
also applied to the b.f.o. via S7.
(b) (3)(b) applicable.
46. The "S" meter is connected between the cathode of
V14 and a point of
preset (RV4) positive potential. It is calibrated to provide an
indication of signal strengh; a 1µV signal provides a typical
between "S1" and "S3" and ascendinc "S"
points in approximately 4 dB steps.
The variation in treshold is dependentupon the gain of the r.f.
should be remembered that only with the R.F./I.F. GAIN control
at maximum is
the correct calibration maintained.