Solid State Shortwave Receivers For Beginners - The Listeners Guide

Components List for Fig.25
As Fig.24, except C4 is reduced to 100nf plastic foil.
Add the following:-
R3 330k
R4 390 ohms
C6 100mfd. 10v.w.
Tr2 BC109.
high impedance speaker.
The audio output from R3 is coupled to the volume control
(VR1) via D.C. blocking capacitor, C6. The output from the slider of
VR1 is taken to the I.C. output stage. This will supply ample output
to any speaker, regardless of impedance. The available output power
increases as the speaker impedance is reduced. A 25 ohm unit gives
what is probably the best compromise between battery consumption
and available volume. By the addition of a suitable socket, any type
of headphone or earpiece can be used with the set.
S1 is the usual on/off switch. Very effective supply decoupling
is needed, and this is the purpose of Cl, C2, and R1.
This set is used in much the same way as the previous design,
except that it will be much easier to use on reception of amateur C.W.
and S.S.B. signals.
One problem that frequently arises when this type of set is used
for amateur band reception is that of hand capacity effects on the
tuning. This is where moving ones hand near the tuning capacitor
slightly alters the tuning. This is unlikely to be noticed on the broadcast
bands, but can be a severe problem on the amateur bands where
the use of C.W., and more particularly S.S.B., makes the tuning far
more critical.
It is advisable to construct the receiver in a non-metalic case, or
if a metalic case is used, this should be completely insulated from the
circuit. It is also a good idea to use a non-metalic extension shaft between
the tuning control knob and VC1. The same applies to any form of
bandspread control that may be fitted and also to the volume control.
Shaft couplers are readily available from the larger component retailers.
The additional length of spindle can be a piece cut off a potentiometer
(these normally have rather long spindles) on a piece of ¼ in. dia. dowel
can be used. As just mentioned, potentiometers usually have fairly long
spindles, and an extension may well not be necessary in the case of VR1.
The components are mounted on a bracket behind the front
panel, and their spindles protrude through 9/32 in. diameter holes in
the front panel. This is usually fairly easy to arrange, and the only
real disadvantage of this is that it slightly increases the size or the case
which is needed to house the project.
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