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Old 10th Nov 2015, 9:12 pm   #1
Pitagora
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Location: Leverkusen, Germany
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Default Configurable Crystal Receiver

The Restoration (in fact the Reconstruction) of a Secondary Crystal Receiver

It starts with an old mounting panel and its accompanying parts bought through a well known auction site. Not being able to retrieve the original diagram, I created another one, capable of emulating the most “state of the art” diagrams employed in receiver sets of that period. Reassembled in a new look, the set was finally tested. (And) It works!

The acquired parts, (see below) seem to belong to one (or more) crystal set(s) build by a French amateur in the twenties. Nowadays, even the starting auction prices for (more or less) “original” sets of that period are rather prohibitive for poor collectors (like me) so I had to content myself with less. The pitiful state in which the purchased items are, does not scare me too much because I am more a repairer than a collector, and (for me) a set requiring an intense restoration is much more interesting. An amateur construction does not scare me either, because in that period many “amateur” sets were more intelligently conceived and executed than the “commercial” ones, and to me, the real value of an old wireless set resides mostly in the quantity of intelligence invested in its creation.

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The parts as advertised for selling.

Receiving the parcel, I started the renovation of the (already) detached parts, and I tried to “guess” the schematic diagram out of the existing wiring on the board.
Without any luck! Only three connections linking the coils to the input binding posts were made with old silver-plated squared copper wire and could be considered “originals”. The rest made with low quality electric wire whose isolation fell in crumbs at any (slight) touch, was, obviously, of a recent period and the subsequent circuitry did not make any sense. (At least not to me.)
Continuing the renovation of the detached parts I started looking for a schematic diagram belonging to the same period and who would match (more or less) the parts I got.
It is how I found the article written by John Hassell (VK6JAH) describing the construction of a crystal set inspired by “The Boy’s Book of Wireless” written by Ernest H.Robinson and published by Cassell and Co. of London in 1923:


The set is called ”Four In One” and is capable of emulating four different configurations of crystal sets, just by the use of some switches:

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Four Configurations

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In One

The idea of a configurable crystal set capable of emulating the most used schematic diagrams in France in the twenties suites me fain because allows me to elegantly avoid the embarrassment of choosing a particular configuration and ever regretting not having chosen the other. Subsequently I did try to implement something similar. My problem was the total lack of switches among the parts acquired. In „Four In One” the switch S1 is used to connect series/parallel the variable capacitor to the antenna/ground circuit and S2 to connect the detector either to the primary or the secondary circuit. With the detector connected to the primary, the secondary works as a rejecter. S3 switches the second section of the variable antenna/ground capacitor in parallel with the first one to increase the maximum capacity towards 1000 pF. I can neglect this switch due to the fact that one of the variable capacitors acquired has already a maximum capacity of (about) 1000 pF (and I will put it in the primary).
I happened to found in a German book intituled, „Der Kristall - Empfänger” (“The Crystal Receiver”) written by Hanns Günther (W. De Haas) in 1924 the switching system (presented below) which requires only an extra binding post (A’1) and a wire (S) to short-cut other two binding posts. (No switch!)

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Variable capacitor switched series/parallel in the input circuit.

That’s how I intend to replace S1 from the „Four In One”. (Of course the same method could be applied if there is a variable capacitor in the secondary.)
Next I remembered the switching system encountered in “The Crystal Receiver with Interchangeable Coils” another French crystal receiver in my collection:

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Antenna/Ground switched to primary/secondary.

No switches either! Instead of switching the detector to primary or secondary as in „Four in One”, one switches antenna/ground either to primary or secondary by connecting them to the other pair of binding posts. (Most ingenious!) Also from this set I got the idea of unplugging the unused or unwanted coil. (In „Four in One” the coils could not be unplugged being affixed with screws.)


To be followed
Kind Regards,
Pitagora Schorsch
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Old 10th Nov 2015, 10:14 pm   #2
Ed_Dinning
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Default Re: Configurable Crystal Receiver

Hi Pitagora, a well researched article on crystal sets, I'm sure some of our other crystal set contributors will be interested in it.

I live in the N E of England where there were a great many old leadmines, a useful source of Gelena which seems to work quite well. I am told however the stuff used for crysals was actually manufactured for better results. There would be many trace elements in the natural Galena which may have enhanced or reduced the crystal's properties in a manner akin to semiconductor "doping"

Ed
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Old 10th Nov 2015, 11:36 pm   #3
Pitagora
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Default Re: Configurable Crystal Receiver

Good Evening Mr. Dinning,

Thank you for your kind words. As for the Galena crystal, I haven’t had yet the opportunity to practically experiment too much. I heard also that the natural crystal is supposed to let pass only the negative current with respect to the cat-whisker and the artificial (improved) one, only the positive (or the reverse). I am looking ahead, after making sure that I (finally) have a dependable crystal set to find a way to test these “rumours”
Kind Regards,
Pitagora Schorsch

§ 2
“The Crystal Receiver with Interchangeable Coils”, where I took the switching system,

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The Crystal Receiver with Interchangeable Coils

could not be dated properly but only looking inside at the type of the variable capacitor (presented below) one can tell it belongs to the twenties:

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The variable capacitor from “The Crystal Receiver with Interchangeable Coils”

The set has a piece of paper glued to its bottom explaining in (French) words how to use this switching system and what kind of coils are recommended for each transmitter to be heard in France in those days:

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The original text explaining the switching system and recommending coils.

In order to correlate the text with the set’s schematic diagrams I presented earlier, one has to know that the binding posts “on the fixed coil side” are marked on the diagrams with A2 and T2, and the ones “on the mobile coil side” with A1 and T1.
Combining the two above described switching systems (without switches) and applying the result to a crystal receiver with two variable capacitors we obtain the following schematic diagram:

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Schematic Diagram of the Set

With (only) two extra binding posts (A’1 and A’2), two pieces of wire for making the short-cuts, connecting antenna and ground to various binding posts and unplugging (when necessary) the unused or unwanted coil, one can have nine (9) different configurations:

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Nine in One.

Those who are acquainted with the schematic diagrams currently used on the continental crystal sets of the twenties would surely recognize (if not all, at least) most of them.

To be continued
Kind Regards,
Pitagora Schorsch
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Old 11th Nov 2015, 11:20 am   #4
60 oldjohn
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Default Re: Configurable Crystal Receiver

Hi Pitagora, You have made a nice job of your crystal set. I would guess the tuning capacitor may be older than you think, possibly around 1918 but I have no proof of that. I have never seen anything like it.

John.
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Old 13th Nov 2015, 2:03 pm   #5
Pitagora
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Join Date: Nov 2015
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Default Re: Configurable Crystal Receiver

Good Afternoon Mr. 60 Oldjohn,

Thank you for your nice words. (And I’m glad you like it.) Regarding the peculiar tuning capacitor (until I opened the set I haven’t seen such a thing either): The fixed semicircular plate insulated with (what I believe to be) a sheet of celluloid which (for maximum capacity) is to be covered with the flexible copper fabric, is made of aluminium. I am not a historian but I am not so sure aluminium was in common use at such an early date.
Kind Regards,
Pitagora Schorsch

§ 3

Freed from the embarrassment of implementing only one configuration, (but having to add on board two extra binding posts) I had to rearrange almost all the parts on board. (I am afraid, this restoration looks more and more like a reconstruction!). After drawing some sketches I took a sheet of (black) plastic and I tried to make a prototype:

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The Prototype

The parts on board (in my opinion) are definitely looking better in a symmetrical arrangement . The location of the binding posts for the headphones has not yet been decided upon but the design of the set starts showing.
The box underneath finished by itself as I completed the missing walls with pieces of plywood. Then I painted it “Mahogany Light”:

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The Box.

After putting all parts on the prototype board, I connected them temporarily with flying leads and I spent (almost) the whole night testing the various configurations and comparing results. (It works splendidly!)
Then I started to deal with the ebonite board: I glued it together (it was broken), I filled some old holes, I made some new ones, and to finish, I polished it (three times!) on both sides.
I put the parts on it and I took some pictures:

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The Set is Almost Ready.

Then I semi-permanently wired it (not very accurately at first) with new silver-plated round-copper wire. (I’m sorry, but I put aside the old silver-plated square wires for real restoration.)

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Semi-Permanent Wiring.

After correcting the wiring a bit, (I promise to do it right as soon as I gather some more patience) I enjoyed again playing with it till very late in the night (“just” to test the set):

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The Set in Full Action.

To conclude
As a restoration, the operation became a failure as soon as I realized I haven’t the slightest chance to retrieve the original schematic diagram (the set being originally made by an amateur). Instead of storing it in a corner, I reassembled it anew putting together what information I could gather from several sources of that period (and using my limited knowledge and skill). To me it became a tool: An experimental set, meant to emulate (almost) any configuration of future crystal sets to be restored, in order to know what performance to expect, once the restoration is finished. And this is the reason I dared to post the story into the “Homebrew Equipment” section of this forum.

Kind Regards,
Pitagora Schorsch Leverkusen 13 11.2015
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