The G0CWA Mystery Super Crystal Radio
Hi I’m here again (Nick G0CWA) here is one of my other versions of the mystery crystal set, I claim no originality for the basic idea of this radio and its efficiency. The mystery radio is a well known basic design which bears a close similarity to a Telefunken design that Dave has made, his set number #37 Crystal Set with the only difference being the bi-filar winding technique used in the mystery set.
The Mystery and Mystery Plus sets are covered in the excellent easy to follow article by Ray Creighton in the link below.
I couldn’t help but feel this set was capable of much better performance than either the basic or plus versions could provide and decide that although very sensitive was not very selective, and made my mind up to try to improve this already excellent design. I first experimented with tapping the main tuning coil for the antenna feed, this although showing promise made winding the bi-filar section difficult so how about tapping the tuning capacitor? This was achieved by using a dual 500pF capacitor with both gangs in series and feeding the antenna to the centre, it worked perfectly increasing not only the sensitivity but selectivity as well.
Why Is Still A Mystery!
The only feasible explanation I can give at this time is two-fold in that it provides a balanced input to the detector coil, and significantly reduces any loading of the tuning coil.
The tuning coil inductance had to be increased to approx 300 to 350 micro-H to still cover the full MW band as only a 250pF capacitance swing was available but the excel spread sheet I have included makes the calculations a lot easier.
The included schematic diagram shows the tapping of the capacitor and the two “standard taps” for the mystery set. In practice you will probably find that only the tap between the caps is used. In my set I have also include a standard galena type of detector, which subjectively performs at least as well if not better than the diode detector, as also noticed by several other people.
The use of an audio impedance matching transformer on the output also yielded an improvement in performance. If a crystal or ceramic earphone is used C3 needs to be changed to a resistor of around 10K ohms.
73, Nick - G0CWA
I would invite you to download my mystery coil spreadsheet nick-mystery.xls and the explanations file Simplified-method-for-calculating-mystery-crystal-radio-coils.doc.