Feature: RF
Figure 3: Circuit PCB
the lower end of the dial. Te dial and frequency may be at a mis-step, requiring some adjustment.
• With a screwdriver, turn the oscillator coil very slowly clockwise or anti- clockwise (not more than an eight of a turn at a time), until you hear the correct station clearly.
• Adjustment of the oscillator trimmer will affect the oscillator coil again.
To mount the tuning knob, turn the gang
capacitor’s spindle fully clockwise, and check it moves smoothly. Next, mount the volume control and fit the completely-mounted PCB into the cabinet. Fit the battery and the set is ready, except for alignment.
Alignment By rotating the volume control, you should hear a sound. Slide the antenna coil to bring it on the tip of the rod. Keep the antenna and oscillator trimmers (on gang) in half- open position. Adjust the tuning knob to tune into a station. If you do not get a sound from a station,
touch the gang lugs (except centre lug) with the blade of a screwdriver (from the component side). In case no ‘click’ sound or a station appear, you’ll have to fault-find. Tunning for 455kHz IF section is not
necessary since no IFT coils are used in the IF section. Te oscillator coil is tuned
to cover the lower frequencies, and the oscillator trimmer the higher frequencies of a particular band, whereas the antenna coil and trimmer give tuning sensitivity. For example, the oscillator coil is tuned to 525kHz (lower range), the oscillator trimmer to 1625kHz (higher range), the antenna coil to 600kHz (mid lower range), and antenna trimmer to 400kHz (mid higher range). Alignment allows for maximum sound
and number of stations. A little time spent carefully performing the following steps will be rewarded with great performance. No specialist tool is needed to make the adjustments. • Te dial is marked from 5^3 to 16, corresponding to the 530-1600kHz range of a medium-wave band. Two trimmers are mounted on the gang: one is the antenna trimmer and the other, the oscillator trimmer. Tere are two half circles in each. When one half circle is opposite to the other, it is said that the trimmer is completely ‘open’. When turning the trimmer, and one plate comes above the other, making a half circle, this is known as a ‘closed’ position. In the open position, capacitance of the trimmer is minimum and in the closed position maximum. Keep both trimmers in half-open position and the antenna coil on the tip of the rod.
Figure 4: Breadboard of the simple radio receiver
• Slide the antenna coil in and out of the rod to increase the sound.
• Tune the knob to a known station at 38 June 2021
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• Tune into a weak station between 550-700kHz, and carefully slide the antenna coil back and forth on the ferrite rod to obtain maximum sound output. Lower the volume and adjust the antenna again for better sound. Melt the wax provided on the coil (with a hot soldering iron) at the end of antenna coil so that the coil does not slip off from its position. If the antenna coil is too loose, use sleeving or a piece of card to tighten it. To check the best position of the antenna coil on the ferrite rod, bring another rod of about 5-8cm slowly near the coil. This will result in weakening of the signal (output). A 100mA range of any multimeter in series with the battery will help check the maximum output. This is a very good test for coils, and generally applied by radio manufacturers for fine tunning.
• Tune in a station between 1100- 1600kHz, preferably a weak one. Adjust the antenna trimmer (on the gang capacitor) for maximum sound output. If you have a 455kHz RF signal
generator, use it to align the radio. Connect the signal generator to a standard radiation loop or put a 0.01uF capacitor across its leads. If you hear an AF tone from the IF (455kHz ceramic filter) then your 455kHz IF section is working. It’s worth noting that the signal input
must be kept as low as possible to avoid overload and clipping; the volume control must be kept in its maximum position, except for antenna coil and trimmer alignment; and the trimmer should be kept in those positions where the output signal is the highest.
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