4. In practice the MHz dial should be set roughly to indicate the MHz portion of the frequency required. Subsequently
it may be adjusted slightly in order to strengthen the reception. It may also be adjusted to eliminate certain types of
interference due to image channel effects. This displacement shifts the images until they cannot be heard, without
affecting the main tuning. This is an advantage of a multiple heterodyne design over a conventional heterodyne design
where such an image effect cannot be shifted. An image effect is usually heard as a whistle on the required station
which varies in pitch as the receiver is tuned. A whistle which does not vary in this way is usually due to other causes
such as an adjacent station. Ensure that any interference encountered cannot be eliminated by slight adjustment of this
dial.
5. Having tuned the station on the MHz and kHz dial the antenna control should be peaked up to give maximum
signal on the signal strength meter which is provided for this purpose. Alternatively this may be done by ear,
particularly on a weak station where the optimum antenna tuning is easily heard. A strong station may not change
much audibly with antenna tuning, but should nevertheless be peaked up accurately on the meter, otherwise optimum
reception will be lost should it weaken or fade deeply. The meter is provided for this purpose although also useful for
comparing alternative transmissions etc.
When searching generally over a limited band e.g. an amateur band or particular broadcast band, the aerial tuning
may be peaked up on any station in the band, or simply on the noise, and ignored until a station is located for
protracted listening, when a final touch up may be made. This procedure is very effective over most of the coverage of
the receiver, but at lower frequencies and the medium wave frequencies particularly accurate antenna tuning is usually
called for.
Reception of Time Signals and Calibration or adjustment of Dial Scales
Radio time signals from observatories and similar institutes are continuously receivable in most parts of the world.
The majority of signals of interest are transmitted on exact multiples of 1 MHz (most often 5 MHz, 10 MHz, 15 MHz
and 20 MHz). This receivers internal crystal produces continuous unmodulated signals at each exact multiple of 1
MHz as can be observed by setting the kHz dial to 000 or 1000 on each MHz band) which will beat with the time
transmissions.
This is strictly a shortcoming of this type of crystal controlled receiver which cannot be completely eliminated without
making it unduly expensive for its intended use. However, the time signals, unless they are very weak, can usually be
satisfactorily received on these whole number MHz, as the precision of the internal crystal is sufficient to produce only
low level inaudible beats with the time transmissions.
This low frequency beat which can often be observed on the signal strength meter serves to check the internal crystal,
and internal adjustment in the receiver is provided for setting this crystal accurately. These 1 MHz marks are also
useful to check the accuracy of the kHz dial and are perhaps best observed with the aerial down and U.S.B. mode in
use to centre the tuning exactly to zero beat.
Adjustment of the extremities of the kHz scale can then be made (by anyone sufficiently skilled) by means of the
internal adjustment of the trimmer and inductance of this kHz oscillator, which is the section closest to the scale drum,
the 000 being set on the trimmer and the 1000 being adjusted by the inductance, alternatively until the extremities are
both in exact agreement. Although it is not intended that such adjustment should be necessary nor undertaken except
by a skilled person, it is useful that this can be achieved
in circumstances where no instruments other than the receiver itself are available.
3. Adjustment of B.F.O.
The change in frequency of the B.F.O. from L.S.B. to U.S.B. is set by a trimmer adjustment to about 3 kHz change.
The use of accurate instruments to set this is necessary and there is little reason for this to change with time provided
it is not disturbed.
The absolute frequency of the B.F.O. may however suffer change with time or abnormal climatic conditions, and this
can be accurately set without the use of instruments by adjusting the inductance (lower corner can) until the noise with
aerial down, sounds the same in the two modes, indicating the 3 kHz side-step is symmetrically placed on the band
edges. Only a small fraction of a turn would normally be required to reset.
