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 Post subject: SOE - resistance TRF receiver Type 31/1.
PostPosted: Sep Mon 15, 2008 3:43 pm 
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Joined: Jan Wed 23, 2008 2:55 pm
Posts: 132
Location: Littlehampton England
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During WWII resistance groups needed a small inconspicuous receiver to listen to the BBC news transmitters, presumably to hear the various coded messages sent by the BBC within their news programmes. 5000 of them were parachuted to the Norwegian resistance.
Its measurement are:- 5'' long, 4-1/4" wide and 1-1/4" thick.

This little receiver shown here is the Type 31/1, a 3 valve battery TRF set designed by Willy Simonsen a Norwegian graduate electronics who escaped to England in 1942. From his own work in resistance groups he knew what sort of receivers were needed and was employed by the Inter Services Research group where he designed this set. Charmingly he named it the "Sweetheart" because he was keen on a girl who worked with him on the project. Lucky man! I've never met a girl who was 'persuaded' by a radio I've built.

A customer/friend of mine has had this one for about 60 years from when he was a boy. It was modified to work on Medium Waves which it did very successfully all those years ago. He found it recently after many years of disuse and felt the need to have it working again but on its proper frequency this time.

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The receiver is a very simple detector and two audio TRF with R/C coupling between the stages. Designed to operate on 6 - 12 MHz into crystal type earpieces. The filaments were in series and the HT supply (B+) was 2 hearing aid batteries giving about 30 volts. Filament current consumption was 50 m/A at 4.5 volts and HT consumption was only 0.5 m/A giving a long battery life.

This is the circuit diagram.

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There was no on/off switch. The battery box plug was simply pulled out to disconnect the batteries.

After many years lying about the in the garage the set was in a poor state. The electrolytics were open circuit and all the paper capacitors had turned into low value resistors. Several of the resistors were in a poor state from having lost their paint covering even if the value was within reasonable limits.
An interesting part of the design was the use of the triangular shape formed between two adjacent valve envelopes and the chassis to stand the screen decoupling caps there. Their position is seen quite easily in the following pic.

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It was a relatively easy repair. Modern components being so much smaller than the old ones were much easier to fit. I have a lot of lowish voltage capacitors 160 volts or so but couldn't find them and had to use 400 volt ones which seemed a waste on 36 volts.

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I assumed it was originally built on some sort of jig to get the component lead lengths correct and then slid into place and secured into the chassis.

I did a few sensible mods. I made a battery box with on/off switch and a low current LED to show it was switched on. Proper headphone jack and battery lead connectors to make it reliable and simple to use.


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The most difficult part was winding the coil. Fortunately the circuit shows the winding details and after several false starts with the Norwegian words for winding the individual coils sections I was able to get on with it.

The coupling coil is wave wound. Not easy without a machine. I slipped two large grommets on the coil former in the right position for the winding with two 'washers' of silicon finish baking paper between them to form the sides of the coil. A few turns of wire to start and then I dropped a blob of hot wax off the soldering iron between the washers and on to the turns to hold them in position. Whilst the wax was hot I then rotated the former, oscillating the wire from side to side in time with the turns, dropping more wax as the process went on. Finally when I had a few turns too many I measured the inductance and removed some turns until it was correct applying the iron gently to evenly melt the wax. A little later I removed the grommets and there stood the coil ready to be connected to its tags. The other windings were quite straightforward being only 18 and 25 turns.

All of my radio equipment is in storage at the moment after a house move and all I had to check its frequency was an R 1155. With the tuning cap fully closed and the reaction increased enough to make it oscillate so that I could find it on the '55 it was only 50 k/cs off frequency which was easily adjusted with the core.

The dial is calibrated in metres and is surprisingly accurate. I have no equipment handy with which to measure its sensitivity but it performs surprisingly well and seems able to hear anything the R1155 can. Tuning and reaction settings are critical, safecrackers fingers almost, when using it for Dxing which was of course not its original purpose. The lid is missing which creates a slight hand capacity effect so the owner is going to make a new one as a replica.
I'm running it on 36 volts. HT (B+) at which its current consumption is 0.75 m/A. Less surely than transistors would take!

Jim G4XWD


Last edited by g4xwd on Sep Mon 15, 2008 4:28 pm, edited 1 time in total.

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 Post subject:
PostPosted: Sep Mon 15, 2008 4:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: seabeck,wa. usa
Thanks for sharing. Good report.


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 Post subject:
PostPosted: Sep Mon 15, 2008 4:36 pm 
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Location: Muscletown, USA
Beautiful


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 Post subject:
PostPosted: Sep Mon 15, 2008 5:27 pm 
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Location: Mission Viejo, southern California
Very nice work! Thanks for sharing it.

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 Post subject:
PostPosted: Sep Mon 15, 2008 5:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: S. Dartmouth MA 02748-1225 USA
Jim,

That's the sweetest little radio I have seen for the job it does in a long time. FWIW This guy is even EMP proof!

Looks like it covers 80M does it get down to 40?

How's reaction on SSB?

How did you manage the crystal earphone?

Are you considering posting more through documentation?

How about here on ARF?

I have everything I need in my junque box for this set. With exception of the dial which could be mapped to a flat surface and rendered to an image. What you have presented seems to cover how well the set is constructed.

Other than a PARA set, are you looking for a similar type of transmitter?

Nice restoration job!.

Very Best,

Chas

Charlie WA1JFD

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 Post subject:
PostPosted: Sep Mon 15, 2008 7:55 pm 
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Joined: Jan Wed 23, 2008 2:55 pm
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Location: Littlehampton England
Thanks everyone for your comments,

Chas, I'll get some more info together including an opened out tracing of the chassis with measurements and post tomorrow.

Jim


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 Post subject:
PostPosted: Sep Tue 16, 2008 2:49 am 
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Location: Brunswick, ME 04011 (USA)
...looks pretty good from here...

...coil details (if I read them correctly):
L1: (85µH) 80 turns, 36 ga., wave-wound, 1/2 in. inside dia.
L2: (7.1µH) 18 1/3 turns, 30 ga., single layer, 1/2 in. inside dia.
L3: (14.5µH) 25 1/3 turns, 38 ga., single layer, 1/2 in. inside dia.
(but no dimensions given for spacing between the windings or for the width of L1)...

...I imagine you had to do some substituting for R1 and R10; I see a 470k in there (that must be R10) and R1 is partially obscured, but it looks like you used a 4.7M...

...but that's a fine job by any standards!...


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 Post subject:
PostPosted: Sep Tue 16, 2008 8:32 pm 
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Location: S. Dartmouth MA 02748-1225 USA
I may have some suitable blank Amphenol Bakelite coil forms. I would need to know the overall length of the form. I see the O.D. should be 1/2" There are no terminals or mounting method for the form. It is just a blank cylinder with a few holes pre-drilled. When I see the dimensions I will check against these forms if it is very near correct, I do have a few.

Start, finish and direction of the wind need to be known or there will be some wire swapping to get the radio to work properly. Is it all enameled wire, litz or textile covered wire? Potted in bee's wax?

These forms will mount into a Morris coil winder to make a universal wind.

Best,

Chas
WA1JFD

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 Post subject:
PostPosted: Sep Tue 16, 2008 9:10 pm 
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Topic continued here: http://antiqueradios.com/forums/viewtopic.php?t=96057


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