Geoff's Crystal Sets – Page 3

Variometer Receiver

Please click on the photographs for a larger picture of this beautiful wireless receiver.
Variometer Crystal Receiver Front View

Hi Dave (my webmaster), I was going to have a quiet restful Whitsun holiday but it turned out to be one of my busiest. I was quite excited though as I was asked last Thursday evening to make a simple 1920's style crystal radio plus kit of parts for the props department of the Television Series 'Mr Selfridge' on ITV.The set was to be used in one of the episodes of the series which would be televised later in the year or in January 2016. The deadline for the set to be completed due to the need filming was days away just a little bit short notice but I like a challenge.

The set had to be in character with the early 20's sets but not too commercial looking so no box just an open layout ,a dual slider was hinted at but that would have been difficult in the time scale so I plumbed for a Variometer , also I had no tuning capacitor which would look convincingly 1920's. Variometers were very popular on the earlier sets of the day in UK so I thought I would have a try at making one.

Into the project I plunged friday lunchtime. It all went very smoothly with non of those constructional hitches and this set turned out to be a little gem one of my simplest but neatest sets. I emerged late Sunday night with the completed set and a bag of parts for the props crew to assemble for the edited prop.

There was no post today due to Bank holiday Monday so we arranged to meet up and I handed over the completed set to Faye one of the props crew team of ITV ,she was very pleased.

The Variometer coils turned out better than expected with no snags at all in the winding .I had only made one Variometer before which was on the Acrylic former but this one had to be in character with the 1920's , so I pulled out a piece of 4 inch diameter paxolin I was saving for such a job and wound the coils.They look great dont you think.

I fired up the set on my 132 ft long wire Aerial and It was pretty close to tuning my local radio station considering it was a bit of guess on the number of turns I added a fixed capacitor between coil and Aerial as this seemed the traditional way .The tuning was quite broad as expected but for a simple set it was fine but the visual effect of the large coil was possibly more in key with the original idea. In the day these sets wouldnt need much selectivity or tuning range as it certainly was limited .So as you might guess there was some station overlap and I could here two stations at once . If I had a little more time I would sort out that problem perhaps using a coupling coil for the Aerial.

The receiver is made of solid Oak from an old 1920's chest of draws that I rescued from the bonfire pile . I sanded down the old wood and re stained it using one of my antique leather dyes which I discovered was very good on wood as well as leather. Then I used a wood oil finish called 'Danish oil' to seal the grain. The baseboard was 9 inches by 6 inches and 7/8th of an inch thick.I still like to use the old imperial measurements because being Old school I understand them ,its just visually easier than metric measurements to me and just seems right on these older looking sets.

The coil was wound on a 4 inch diameter paxolin former for the outer coil and a 2 1/2 inch for the inner coil. I made all the brass parts to my standard designs on the brass parts page of my website. All the wiring is done under the baseboard. I dont like wires going all over the place on the top of the baseboard. I used one vintage component the aerial coupling capacitor this was from the time period and was made of moulded bakelite made by a company called TCC. I wish I had more of these but they are getting rare. This one was a lovely red coloured moulded bakelite.

The Galena Catswhisker is my standard design which I still sell separately. I also put a diode under the board to quick tune the set instead of fiddling with the catswhisker , its useful when you first need to find a station. All the labels are engraved and not printed the dial is solid brass with the background blacked out so that the numbering stands out clearly and can never wear off.This is how it all used to be done and nothing is from the instant age ,except perhaps the maker LOL.

When I have sorted out a few tuning problems I think I would like to sell this receiver as a kit also as a finished set. It is a visual exercise rather than a highly tuned crystal radio but can still satisfy the listening fascination of the early vintage crystal radios.

Altogether from start to finish from the raw materials this set took two long days to make.

Variometer Crystal Receiver Side View
Variometer Crystal Receiver Back View

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Slider and Variometer Radio

Crystal Radio, Variometer and Slider Project
Click image for larger view.

Having built a few Loose couplers earlier, I wanted to see if I could improve reception by adding tuning capacitors to both coil sections ie. the larger inductance of the main aerial coil on the left of the set and the detector coil on the right. The detector coils on originals has traditionally been a tapped coil however a tapped coil is know to have a much lower Q factor than a straight coil.

So why did they not use a Variometer coil instead? This was always a bit of a puzzle to me. I chose to use an unconventional construction for the Variometer coil being offset arrangement for the inner inductance instead of the standard arrangement of splitting the outer coil into two and having a shaft go through the middle gap for the outer and inner coil. The offset coil is much simpler to make and also gives 180 degrees of dial swing instead of the usual 90 degree swing from minimum to maximum inductance. This arrangement is very rarely seen on the internet searches for Variometers and I just wonder why because the offset method has a much higher Q Factor.

The aerial coil or ATU section was a series tuned arrangement with a slider on top to vary inductance from approximately 1000 micro Henries down to about 50uH plus a 500pF variable capacitor in series with this to give a tuning range that covers most of the popular broadcast sections of the Medium Wave band. A shorter Longwire is easily tuned by this arrangement.

The Detector Variometer coil had a swing of from about 50uH to 300uH plus a parallel tuning capacitor of 500pF gave me a good tuning range over the whole of the band. The ATU and Detector were mounted on rails so I could reduce the sensitivity of the set for the very powerful Droitwich transmitter 500kWatt and only 30 miles away, this system works out well and I now us it on all of my more advanced crystal sets.

On tune up of this receiver I first used the diode detector and a signal generator set to 1000kHz this position was marked on the ATU coil and tuning capacitor so that I could return to that spot each time. The detector was calibrated in the same way. I would first find a station on the detector coil in maximum position and tune the capacitor to find the station then peak the ATU capacitor for maximum signal.

I found the set to be very sharp and peaky to tune but not easy to calibrate without the use of a signal generator I would have been lost as there are four variables to take into consideration. The overall sensitivity of the set compared to my reference 'Mystery Crystal set' is about twice as loud and far superior in separating stations than conventional single coil and capacitor arrangements. The set arrangement as a whole would only be bettered by using Litz Wire but I wonder if the difference in price would be really justified as this is a first class receiver in standard copper coils and is much more pleasing to my eye esthetically because of the use of attractive metals of brass and copper.

This dual coil and capacitor arrangement is now my favored construction method even on less 'engineered' projects.

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