Choccy Block Transistor Radio
Modifications
Other More Advanced
Projects and Modifications
Long Wave
There is nothing to stop you making a LW coil to make a long wave
radio, but you'll need about 180 turns of wire wound up in three
or more layers. This can be quite bulky when made with PVC
covered wire, so you might choose to use thinner enameled copper
wire or a ready-made LW coil. It's relatively easy to
connect one end of both coils together and arrange a switch to
connect either one of the other ends to the tuning capacitor.
Short Wave
The pictures shown above are from the alternative breadboard
version but the effect is the same.
If you remove the ferrite rod, make the main coil about 25 turns
and wind the coupling coils as shown in the picture, you will be
able to tune the shortwave bands from about 7 to 30MHz. At
this stage you will need to link in a few meters of aerial wire
by connecting it to one end of the short coil on the left and
connecting the other end of the short coil to a heating radiator
or earth connection. Don't connect to a mains power earth
pin. The tuning will be quite broad but you will be able
to hear several strong international broadcasters, especially
after dark. Sliding the ferrite rod back in will lower the
frequency range of the tuning.
More Signal
One way of sharpening up the tuning and getting more signal on the
station that you want is to use an external tuned loop
antenna. This requires no physical connection to the radio
and can be very effective. You can see how to make one
here: Tuned
MW Loop Antenna. You should use a separate external
capacitor to tune the other loop.
General Magnetic Pickup
I've not given this a
proper try yet, but because
our radio only has one tuned circuit, it is quite easy to change
it to pick up other magnetic signals that are around in the
environment, and we can choose to have a high-Q tuned circuit or
not. We can also choose whether to have an AM demodulator or
not. It's easier when playing around with magnetic pickup if
you use headphones instead of the loudspeaker, as the strong
signal from the coil in the speaker has an even greater tendency
to get back into the input. A good magnetic pickup for audio
frequencies is the primary of a small mains transformer with the
core removed, connected instead of the normal input coupling
coil. The demodulator will be fairly ineffective at audio
frequencies and if you want to remove any demodulation effect, you
could remove C6. If you wanted the AM demodulator to work
down to lower frequencies you may need to increase the value of
C6.
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© Henry J. Walmsley 2014