| Superhet Radio Alignment |
Superhet Radio RF & Oscillator Ganging or Tracking Ganging or tracking, as it's often called, is where the aerial section of the tuning capacitor follows, or tracks, the oscillator section. In other words, whichever frequency the oscillator is tuned to, the aerial section of the tuning capacitor is tuned to optimum signal level for that particular receive frequency. Ideally, the aerial tuned circuit will be at optimum no matter what frequency the radio is tuned to but, in reality, problems arise. This reason is that, although the tracking may be perfect at one end of the band, as the tuning capacitor travels towards the other end of the band, the two variable capacitors don't track correctly. Although the two variable capacitors are on the same shaft, there are bound to be slight variations in capacity at any given point during its travel. For example, when one side is set to 169pF, it may be found that the other side is actually 161pF. There is a way round the problem. Usually, a padding capacitor (or trimmer) is included across the variable capacitor which may be adjusted to allow the aerial circuit to be tuned for maximum signal at the high frequency end of the band (vanes fully open). Inductors are sometimes fitted to the more expensive radios which which may be adjusted to bring the low frequency end of the band (vanes fully meshed) into alignment with the oscillator section. So, what about the middle of the band? You may have noticed that the outer rotary vanes of tuning capacitors have cuts or splits in them. This is to allow very small variations in capacitance to be made by bending each split vane in or out - i.e. closer to or further away from its neighbouring stationary vane. Setting this up is straight forward, although it can be fiddly and time consuming. THE INITIAL ALIGNMENT Tune the radio dial to the high frequency end of the medium wave band, say, 200 metres. Set the signal generator to this frequency (1.5mHz) and adjust the oscillator padder or trimmer so that the pointer reads exactly 200 metres on the dial. Using an audio output meter (or by watching the AGC line voltage) tune the aerial padder or trimmer for maximum signal. WARNING. The following procedure should only be carried out by a qualified radio engineer. Bending tuning capacitor vanes too far may cause shorts which, at the very least, will cause crackling during tuning or, at the very worst, there will be or no reception at all. THE OSCILLATOR SECTION Start with the tuning capacitor fully open, vanes unmeshed. Now turn the shaft until the first split vane is half meshed. Using a signal generator, check that the dial reading corresponds with the signal generator frequency. If adjustment is required, bend the half meshed vane in or out slightly until the dial reads correctly. Now turn the shaft until the second split vane is half meshed. Note the dial reading (say, 250 metres) and set the signal generator to this frequency (1.2mHz) and make sure that the dial reads correctly. Bend the half meshed vane in or out to obtain the correct dial reading. Repeat this operation with each split vane in turn until until the capacitor is fully meshed. THE AERIAL SECTION Again, start with the capacitor fully open, vanes unmeshed. Now turn the shaft until the first split vane is half meshed. Set the signal generator to this frequency and, using an audio output meter (or the AGC line voltage) bend the half meshed vane in or out for maximum output. Half mesh the second vane and repeat the operation. Do this until all vanes have been adjusted and the capacitor is fully meshed.
The above photo clearly shows the cuts in the outer vanes. It is the sections between the cuts that must be bent in or out for aerial and oscillator alignment. By the way, the osc and aerial sections may be the other way round on some radios.
Wavelength - kHz conversion chart here
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