Mystery Crystal Set

My version of the Mystery Crystal Set, a design first published in the Brisbane Sunday Mail, in 1932. The mystery is that it works so well, with  the aerial and earth connected to opposite sides of the circuit, with no apparent connection between them. The connection comes from the way the two coils are interleaved, providing a coupling between the two parts of the circuit.

The original Brisbane Mail articles showed a set which looked simple to build, but that would perform well. It could also be fit onto a relatively compact base.

The coil I used was 48 turns for the tuning coil, wound onto a 3.25" cardboard tube. The original specified 50 turns on a slightly smaller diameter tube, but this was the closest I had. The detector coil is 25 turns interleaved with the middle section of the tuning coil. The tuning range just missed the top of the AM band, next time I have it apart I'll remove one turn from the tuning coil.

It performs nicely on local stations, as the original was designed for. With several strong stations at the top end of the dial, the broad connection is not much use here. The sharp connection gives reasonable volume.