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Detailed Circuit Description
19
SECOND VARIABLE FREQUENCY OSCILLATOR (VFO-2)
20. The second variable frequency oscillator, covering a frequency range 3
.
6 to 4
.
6 Mc/s,
is an electron coupled Hartley circuit embloying one half of double-triode V12. The
oscillator frequency is determined by an inductance L55, two fixed capacitors C303,
C305, a trimming capacitor C306 and a variable capcitor C301. The KILOCYCLES
scale which is calibrated between 0 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 the INT. position the output
from V12A is fed via PL305 and PL300A to the control grid of the second v.f.o.
amplifier V11. In the EXT. position the external 3.6 to 4.6 Mc/s signal is fed to V11.
FOURTH MIXER
22. The output from the 1
.
6 Mc/s band-pass filter is directly coupled to the signal grid
of a pentagrid valve V26; it is mixed with a 1
.
7 Mc/s signal from V27 fed via the
coupling capacitor C339 to the oscillator grid of V26. The resistor R68 completes the
d.c. path from this grid to earth. The 100 kc/s output from this mixer stage is then
fed via SK6, PL6 to the crystal filter unit.
1.7 MC/S CRYSTAL OSCILLATOR/AMPLIFIER
23. The frequency from the crystal oscillator C27 may be set precisely to 1
.
7 Mc/s by
adjusting the trimming capacitor C337. The crystal XL300 which is connected between
the control grid and the screen grid is electron coupled to the anode. When an external
signal is applied to socket SK303A the valve operates as an amplifier. The output from
this circuit is fed via C339 to the oscillator grid of the fourth mixer V26.
CRYSTAL FILTER
24. Six alternative switched I.F. bandwidths are available as follows:-
100 c
/
s
300 c
/
s
Crystal
1
.
2 kc
/
s
3
.
0 kc
/
s
6
.
5 kc
/
s
13
.
0 kc
/
s
L
−
C
25. In the crystal positions the fourth mixer anode is connected to L48 in the crystal filter.
L47 and L49 provide a balanced output which is tuned by capacitors C109 and C110.
In the 100 c/s position, the balanced 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 control for this bandwidth. XL3, XL6 the capacitor C119 form a similar
circuit for the 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. In the L-C bandwidth
positions, the signal is fed to the tuned circuit formed by L61 and the combination
of the capacitors C145, C146, C146A and C147. The second section consists of L62
Содержание RA-117
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