Mystery Double Plus Radio
Hi my name is Nick and I have been involved in Ham Radio for nearly 30 years. I have always been more interested in the radio/electronics homebrew side rather than operating.
In my opinion many people these days are buying “black boxes” rather than appreciating the quite respectable performance of simple basic receivers. The idea for this radio started its life by my daughter challenging me to make a radio from household “junk” and although to the purist this is not a “true” crystal radio because of the inclusion of an audio amplifier, it can be used as a simple crystal radio with very acceptable results. A well matched antenna and earth are needed to get reasonable performance on the short wave bands.
My first step was to decide on the specification I wanted:
1 No external power
2 Maximum sensitivity with good selectivity.
3. General AM broadcast band coverage
The mystery plus set appeared to suit my criteria perfectly with different coils for different bands although the selectivity was a bit coarse in its adjustment, either all or none. So I instead of winding the antenna coil directly either at one end or on the top of the main coils, I wound it on a slider. By moving the position of this coil I could optimise the coupling and selectivity/sensitivity.
The next step was to make the coils themselves. The coils are wound on 16mm OD Perspex tube using 28SWG enamelled copper wire. The windings are soldered to a 7 pin din plug keeping the wires as short as possible. The use of internal shorting links enables the use of the variable capacitor in parallel (LW), singly (MW) and series (Short waves)
Coil winding details, number of turns (adjust as required)
The assembly and coil label are then covered with clear heat shrink to hold it together and give a nice professional looking end product.
The major problem was the physical coil lengths for MW and LW so I used a ferrite core joined to a plastic (nylon) nut and bolt which is a light press fit into the Perspex tube allowing adjustment of the ferrite core and hence the inductance.
The number of turns on the coils depend on the frequency coverage required. I have heard stations up to around 14MHz quite clearly, mainly late evening.
73, Nick - G0CWA