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Introduction
In Part 1 we constructed the audio amplifier module for the system and tested it in a very simple way. If you did what was suggested and built the simple Crystal Set to use as a signal source, you will know just how well the amplifier works.
The case
From the photo below, you can see the way the components are mounted. The audio amplifier is seen at the top right of the base, to the right of the tuning capacitor VC1. The next in this series will deal with the variable-frequency oscillator (VFO) and VC1. The current part deals with preparing the case to receive the components.
Figure 1 Fixing holes for each module are best measured from each printed
circuit board or matrix board.
Figure 2 Position the slow motion drive to allow viewing of the tuning dial.
Figures 1 and 2 show the markings for preparing the front panel and base.
All the circuit boards and the tuning capacitor are mounted on the base using 10mm stand-off pillars with 6BA bolts. The board locations are shown in Figure 1. The front panel control positions are shown in Figure 2, together with the small rectangular hole for viewing the tuning dial.
The best way to mark out the holes for the boards is to lie the boards flat on the base (before you've started soldering the components in) and marking the base through the holes in the boards. This minimizes the scope for errors!
A reduction drive is used between the tuning knob and the capacitor shaft.
This is simply a gear mechanism that slows down the capacitor shaft by a factor of six compared with the tuning knob, and makes tuning very much easier. The recommended variable capacitor also has a pulley wheel mounted on the shaft. Glued to this wheel will be a scale marked with frequency and is visible through the rectangular hole in the front panel.
The next part ...
The variable-frequency oscillator and mixer will be added to the project.