Page 137 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

+ 1.5
.01 ut
+ 1.5 "
421Fl01
VCl RF VCl RF Pad Re
11[ lOOK
140 pI 20 pf C13
C7 + 1.5 MPSH10 0022 ut
T2 D3
nl · ll Primary .01 ut
11 421Fl0l R7 CI2
680 U: Mp~~~~==lIIC
3 12K 1" ~ '"" R9
Cl< UM
R6
m . l~u~eCOndary R5 1----, R13 12K l~
R"
+ 1.5
C6 lOOK lOOK
12K T
.15Ul
m
R12
VCl OscVCl Osc Pad Rll 56K
+ 1.5 + 1.5
60 pf 20 pf C<
33~ RlD f-
C2 R' '" C15
'''' R3 56K lOOK 06 33~
ClD 2N5089
Cl Ol 02 56K
MPSH 1 0 MPSH 1 0 ,,~
C3 Dl
C5 lN914
Note # 1: Capacitor Removed D2 Note # 2: Use Low Side Tap forT2,T3 & T4
From Tl, 421Fl03; vel_Ost RI .01 of R2 33~1
lN914
to High Side Tap ofTl. 12K 12K

FIGURE 9-2 Schematic diagram of a six-transistor low-power
superheterodyne radio.
For the oscillator circuit, a different approach is needed to achieve oscillations at
low transistor collector currents. Also, because a conventional or off-the-shelf
oscillator coil has too high a turns ratio, some other type of oscillator coil must be
used for a low-power oscillator circuit.

Since the third IF transformer (the 42IF103) has a turns ratio of 6: 1 instead of
20:1, it is possible to use it as an oscillator coil for a low-power circuit. The first
step is to remove the internal capacitor by using a small-blade screwdriver to break
out the ceramic material on the bottom of the 42IF103 IF transformer. The leads of
the capacitor can stay and do not need to be removed. Alternatively, Xicon, which

manufactures the IF transformer and oscillator coils, also makes a version of the
42IF103 without the capacitor, which is the 42IF106. The required inductance is
about 330 ~H, which is achieved across the high-side tap. Thus the variable
capacitor Vcl is connected to the high-side tap connection of the primary of Tl.
This time the low-side tap connection of the 42IF103 or 42IF106 primary winding is
left alone as an open circuit.

At resonance via VC1_0sc and the primary high-side tap winding of Tl, the
secondary winding has a sufficiently high load resistance for Q1 to set up a
sustained oscillation via positive feedback via Q2. The oscillator's output signal is
about 170 mV peak to peak at C2. This oscillation signal is added to the