Solid State Shortwave Receivers For Beginners - The Listeners Guide

Components List for Fig.36
R1 390 ohms
R2 390 ohms
R3 5.6k.
R4 39k.
R5 150k.
C1 100mfd. 10v.w.
C2 500pf compression trimmer.
C3 1.2nf polystyrene (5%) or mica.
C4 500pf compression trimmer.
C5 100nf plastic foil.
C6 8.2pf ceramic.
C7 15nf plastic foil.
Tr1 2N3819.
Tr2 BC109.
I.C. NE555V.
S1 S.P.D.T. toggle.
S2 D.P.S.T. toggle.
S3 S.P.S.T. toggle.
T1 Denco transistor useage Yellow Range 2T coil.
Hardware.
The R.F. oscillator uses Tr1, and this is basically the same
oscillator circuit that was used in the 80 Metre D.C. receiver. It uses a
different coil though, and is adjusted to operate at a frequency of
1MHZ using C2. When S1 is in the opposite position to that shown
in Fig.36, C3 and C4 are shunted across the tuned winding of T1.
These have a much higher combined value than that of C2, and they
produce a much lower frequency of oscillation. In fact, C4 is adjusted
to produce an output at 200kHZ.
It is helpful to be able to modulate the output of the oscillator
with an audio tone so that the calibration signals can be easily identified,
and are not confused with other signals that may be received during
the calibration process. In this circuit an NE555 timer I.C, used in the
astable mode provides the audio modulation tone. Tr2 is connected in
the source circuit of Tr1, but it will not normally have any effect on
the circuit as it will pass only minute leakage currents.
When S2 is closed, the astable circuit comes into operation, and
its output is connected to Tr2 base via R5. R5 is a current limiting
resistor which is included for the protection of Tr2.
When the output of the I.C. is in the high state, Tr2 is turned
hard on and it increases the current through Tr1, and hence also the
amplitude of its oscillation. When the output of the I.C. is in the low
state, Tr2 is turned off and has no effect on Tr1. Thus the R.F.
oscillator is amplitude modulated with an audio tone when S1 is
closed.
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