Introduction
Designed specifically to complement the modified air-band portable (also described in this series), this can be used with some success on many receivers suffering from 'deafness' on VHF.
The circuit
An RF preamplifier is a device which improves the input signal to an existing receiver, enabling it to work more effectively. Because of the noise which is added to the signal by the preamplifier, very weak signals may not be usefully enhanced, but stronger signals will be improved considerably.
Figure 1 The integrated circuit is mounted upside down. Make sure you identify
pin 1 which has a small dot next to it
This little circuit (shown in Figure 1) uses a GEC/Plessey integrated circuit type SL560C. With the addition of four capacitors, it is used between your aerial and the aerial input of the radio's PCB.
Putting it together
1. Use a small piece of prototype (matrix) board about 25mm square. Use an 8-pin DIL socket for the integrated circuit (don't risk soldering the chip - it is seldom a risk worth taking). Figure 1 shows the connections to the socket, looking from underneath.
2. Make special note of the pin numbers, so that you know how to put the chip into the socket when you have finished. The positive and negative connections to the circuit are taken from the main PCB after the ON/ OFF switch - so that the switch operates the preamplifier, too.
3. Unsolder the lead to the radio's telescopic aerial and connect it to the free end of C4, as shown. Then solder a short lead between the telescopic aerial and C2.
4. Sometimes it is possible to cajole your little preamplifier into the radio's plastic case, provided there is room and that you make sure that none of the soldered joints on your little board touch any of the metal inside the case when you replace the back and screw it on again.
5. If there is not enough room inside, then put the preamplifier into its own box, with battery and switch, and its own aerial. Keep the lead from the preamplifier to the aerial connection of the radio as short as possible - perhaps using screened cable.
Because your preamplifier is untuned, you will find not only that it helps with reception on 2 meters, but also that reception on the FM broadcast band is improved!
Parts list
Integrated circuit:
IC1 GEC/Plessey SL560C
Capacitors:
C1 1 nanofarad (nF) ceramic
C2, C4 10 picofarads (pF) ceramic
C3 10 nanofarads (nF) ceramic
Additional item:
Prototype broad approx. 25 × 25mm