GM0NEG,S crystal set

I was in Robins shack a while back and happened to notice what looked like a real piece of vintage gear, it turned out to be a crystal set but some of the wiring inside was broken, Having experimented with and made crystal sets when I was a kid I asked him If I could have a go at restoring it.

The first thing I checked was the detector and surprisingly it was ok with a good forward reading on the ohm meter and nothing in reverse, although in the photo below it is not clearly seen it is an enclosed point contact type and looks very old, I would guess pre or about second world war ( any info appreciated )

I have drawn a diagram of the crystal below

The diagram of the set is shown below and its a bit unusual in that the coil is tapped with the aerial going to the centre tap but this admittedly is a guestimate as a couple of connections were broken on the coil and I had to guess what was what mainly by wiring layout,

Anyway on re-making the wiring and connecting an aerial and headphones 2 Medium wave stations could be tuned in and were clearly heard

So how does the set work ?

Well the aerial which is a long wire picks up the signal and it appears as a very small alternating current at its end, similar to domestic electricity coming out of the sockets on your wall but much smaller in voltage probably in the region of one thousandth to one ten thousandth of a volt compared to to the domestic 250 volt mains, and also very much higher in frequency, for the medium wave band around 500000 hertz to about 1500000 hertz as opposed to the 50 hertz mains

One of the most important functions of any radio is its ability to tune only to the station you want and reject others, and how good it is at this is called its selectivity, in the crystal set this is performed by the variable capacitor and the coil (inductor ) they both have a certain reactance at any specific frequency but at one frequency their reactances are equal and at this frequency the circuit resonates and the small alternating voltage is magnified considerably.

However an high frequency alternating voltage is no good for driving headphones it has to be converted to DC ( albeit a varying DC voltage ) and this is the function of the detector

The detector has the property of only conducting electricity in one direction and so it removes one half of the alternating cycle leaving a pulsing high frequency DC voltage, in some diagrams you will see a capacitor connected across the headphone socket to average out these pulses but this is not necessary as the headphones being electro-mechanical devices cannot respond to this anyway

Below are the voltage waveforms in the cct as they are bitmap they will take a while to load