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four in one crystal set  
Crystal Set


by John Hassell
VK6JAH

four in one crystal set 2

 

This set is based on a design found in “The Boy’s Book of Wireless” written by Ernest H.Robinson and published in 1923 by Cassell and Co. of London.

 

I built this set using materials similar to those available to constructors in 1923. Brass knobs and fittings along with an ‘Ebonite’ front panel give a nice antique look. Fortunately I had a good sized polished wooden box to build the set into. Although reasonably complex, it was all accomplished using the usual hand tools in my home workshop.



The Circuit

    What makes this set interesting is the ability to change the circuit to any of four configurations by the use of two switches. The S1 allows the selection of either a series or parallel primary (aerial) tuned circuit and a S2 selects the primary or secondary tuned circuit as the signal source for the detector. Continuously variable coupling between the two tuned circuits sets the band pass, and the secondary tuned circuit can also be used as a wave trap to notch out interfering signals.

Click on image below for high res image for printing

 The original circuit showed a 1000pF variable capacitor in the aerial circuit – not easy to find these days – so I incorporated S3 to parallel the second gang of the normal 415pF tuning capacitor when needed.

The other addition I made was to add an audio transformer to provide a better load to the detector and also allow the use of low impedance phones.

 

 

Construction Hints



Front  Panel

    This is made from 3mm thick acrylic sheet measuring about 180mm by 290mm. I found some that was shiny on one side and matte on the other. The matte side looks like the traditional ‘Ebonite’ board used in early radio equipment. Acrylic sheet usually comes protected with peel-off paper on both sides. To prevent scratches leave this on until all the cutting and drilling is done.

 

Hardware

    The brass tuning knobs were from surplus military equipment and originally painted khaki. A rub with some acetone removed the original paint to reveal the brass scales.

The mounting posts for the diode are made from 25mm threaded spacers drilled through the sides to take the diode wires. Thumb screws are from the junk box, as are the antenna and earth terminals

Toggle switches are DPDT from Tandy. The original design used miniature knife switches. Nice if you can find any.



Coils

    The coils are basketweave type wound on a 50mm former. I used a piece of plastic rod but a turned piece of wood would do fine. Draw two parallel lines spaced 10mm apart around the circumference of the former. Drill and push fit 9 pairs of small nails evenly spaced around the two lines.

Smear the former and nails with a little oil to help when removing the finished coil.

I wound the coils with a 1 up 1 down pattern. ( see basketweave coil winding elsewhere on this website)  The exact number of turns is hard to count due to the cross-overs, but any one panel counts 12 turns high. Coil inductance measures about 200uH. Best to wind too many turns then remove some later. Apply clear nail polish to the cross-overs and let dry. Ease out the nails and gently slide the coil from the former. Apply a generous coat or two of varnish to hold the coil together. Glue the coils to acrylic squares to mount on the coil holder. 

 



Coil Holder

    The photograph shows the construction pretty well. Brass stock is from a hobby shop. The base is drilled then bent up in a vyce. Make the front holes in the base a little oversize to allow the bends in the arms to slide through. The springs come from ball point pens. Knobs are wooden beads.



Audio transformer

    The audio transformer is a common type used to match 8 ohm loudspeakers to a 100 volt line in public address systems. It has primary taps of 2k, 4k, 8k, 16k and 32k. I put the detector diode on the 32k tap and the high impedance phones on the 4k tap. Experiment and see what works best with your detector and phones. Low impedance phones go to the 8 ohm secondary.



Performance

    Make no mistake; though this set was designed in 1923 it is a real performer. With an adequate aerial and earth and using a good germanium diode detector it will drag in DX stations from around the country.

John Hassell

VK6JAH





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