A Self-Contained Short-Wave Two

[Variometer]

My interest in vintage radio “re-ignited” last week and I decided to build a “Self-Contained Short-Wave Two.” I began by making the box. Its external dimensions are 6 inches by 6 inches and 3.8 inches high. It is veneered with sapele veneer on the outside and is now in the process of being French Polished.
I then made the battery compartment in the bottom of it. Six 9 Volt batteries clipped back to back and with end clips fitted, form the HT. These are contained in a wooden frame covered with a piece of clear acrylic that slots under a wood strip at one end, and is secured with a brass bolt at the other. The LT battery is a single cell, that is a push fit in its own section. The outputs from HT and LT go to a four-pin DIN socket, so that the receiver can be completely disconnected from the box during circuit adjustments or servicing.
Next, the top panel was cut from a piece of black 3mm acrylic sheet. The controls are: left to right - aerial trimmer, tuning and reaction. A jack socket for a 3.5mm stereo plug is fitted in the lower centre. At the top, the aerial connection, the on/off switch and then the earth connection.
The valves, a 1T4 and a DL96 are slightly recessed in the old style. The two holders are on a small aluminium chassis mounted beneath the front panel on steel spacers.
The coil was pure guess-work. A one-inch diameter plastic tube with an aerial coil of 7 turns of solid insulated hook-up wire and an 8-turn reaction winding of the same hook-up wire wound in the same direction, about 1/8th inch apart.
Quite a standard circuit, of detector, followed by audio output, initially resistance capacity coupled.
On testing, it didn’t work! Found I had made a mistake in the wiring. On correcting it, it was not loud enough. I therefore changed the coupling to choke capacity, using half of an LT44 output transformer as the LF choke. It was then too loud and very unstable. To reduce the volume, I put a screen dropper in G2 of the output valve that was initially connected direct to the HT.
It now receives a lot of stations at comfortable headphone strength.
Normal low resistance stereo headphones are used and a small mains transformer is used for matching. The jack socket is connected up for mono.
At the moment, I have no idea what waveband it receives, but there are quite a few stations that sound like Far East and several USA. The first station I brought up was Bucharest!
A pleasant little project, that progressed without too much stress. Several component values had to be juggled to improve the performance.
At the moment, it suffers from slight hand-capacity and the reaction control needs very careful adjustment.
I hope to improve on it presently.
Bob

[Ed_Dinning]

Hi Bob, a superb effort as always.
Drop me a PM if you want some silk or cotton covered copper wire for a "visible" coil, I also have some Litz that might be useful.

Best regards, Ed

[Variometer]

Thanks Ed,
I do have some silk & cotton-covered wire, but did not think it would look as good as the insulated wire that I used. Here is the coil before I began the wiring.
Don't think Litz would be of any benefit on short waves. Thanks for offer though.
Bob

[Alex728]

Originally Posted by Variometer

At the moment, I have no idea what waveband it receives, but there are quite a few stations that sound like Far East and several USA. The first station I brought up was Bucharest!

Impressive - being relatively young I've only read about these sets in old books and seen black and white photos of them...

As for the receiver frequency I'd take a guess at either 41m or 31m, as the Romania international service, Radio China International and some US produced religious broadcasters are very commonly heard there (I can receive them on an Éton G3 with internal antenna even in an urban area plagued by QRM).

What sort of antenna is used with this set? A random wire and earth?

[paulsherwin]

Don't be mislead by hearing RCI or US evangelical stations - they are relayed by transmitters in Europe. It's perfectly possible to get intercontinental reception on radios like these of course.

Super job! Whish I had thought of it for a project. To reduce hand capacitance effects try a sheet of tin (any conductor) on the inside of the front.

[Variometer]

Thanks for replies.
I am in a temporary workroom at the moment due to major house decorations. The aerial is just a simple piece of stranded, insulated bell wire about 20 feet long tied to a tree branch. The other end is just trapped in the top of the plastic window frame. The earth I am using is the radiator. I have clipped it to the brass tap.
As it is already built, I don't really feel like going to the trouble of taking it to pieces and putting a metal plate behind the front panel, although I did know that is the accepted solution. I did not put one there to begin with because I wrongly assumed the chassis would perform the same task, as the coil and variable capacitor are on the other side of it. It is not all that bad, but still noticeable. When I get things sorted out, I will find out what frequency it tunes across, but it must be pretty high with a 7-turn aerial coil. The capacitor is about 200pF. The aerial trimmer is 35 pf!
There are lots of variations on these old designs, so plenty of scope for those wishing to do something more fancy. I just chose the simplest form of a box. Some had sloped fronts with the valves either along the top or sticking out of the front.
Incidentally, I got the black acrylic on Ebay. There is always a good supply to be had of varying thicknesses and colours, often in A4 paper size. Makes great front panels!
Bob

[Anthony Thomas]

Nice work

Any chance of a schematic at all?

As an LF choke you could perhaps use a 48 volt relay coil, this I have seen suggested elsewhere.

Tony

[Variometer]

Thanks,
Yes, a relay coil would no doubt work very well and I have a few reserved for that purpose. In this one, space was getting to be a problem and the LT44 is very small and I had a number of them lying around, so used one.
I will sort out a circuit diagram presently. It has changed a lot from my original plan and at the moment is just an untidy mess of notes. I always draw up the final diagram after I have got them to work, then I am on safe ground
Bob

[Ed_Dinning]

Hi Bob, yes the close up of the coils looks good.
How about a rotatable frame aerial to go with the set, types with tuning will give good Dx results even in the MW band and are useful at nulling out QRM.

Ed

[Variometer]

I wouldn't know how to connect one The tuning circuit is as basic as one can get. The tuning capacitor connected is just connected across the 7-turn aerial coil. There is no coupling coil or centre tap or anything fancy. The reaction coil is the red one above it! I am surprised at how selective it is!
I suppose a frame aerial would take the place of the tuning coil, but what physical size, I haven't a clue!
Bob

[Ed_Dinning]

Hi Bob, the frame aerial plugs into the A & E sockets and can be both rotated and tuned.
If you look on the Dave Schmarder site there are some designs. It involves both woodwork and electronics, 2 of your specialities.

Ed

[Variometer]

Ed,
I just looked at the Dave Schmarder site - very interesting - I had not seen it before. Looking quickly over the loops, I can't really see them being suitable for short waves. May be wrong, as I am not all that well up on theory these days.
Bob

[Variometer]

Completed the circuit diagram this afternoon.
Component list below:
Bob

Parts list:
V1 1T4 - V2 DL96.
VC1 35pF - VC2 200pF.
C1 100pF - C2 .047uF – C3 .0047uF – C4 .01uF – C5 .01uF.
R1 1.5K – R2 2.7M - R3 3.3M – R4 1.5M – R5 560.
VR1 47K.
L1 = 7 turns solid bell wire on 1 inch plastic tube. L2 = 8 turns solid bell wire on 1 inch plastic tube spaced 1/8th inch above L1.
L3 = 1mH choke. L4 = 1mH choke.
L5 = high impedance half of LT44 miniature output transformer (Maplin).
T1 = small mains transformer 120-0-120 / 12-0-12
Bob

[terrybull]

What did you use to draw your schematic Bob.

[Variometer]

I made a computer folder with all the individual symbols stored in it as small jpegs.
To make a diagram, I open a new Word document and put the drawing grid on it. (this is in Microsoft Word and although it shows on the screen, it doesn't print). I insert the symbols one-by-one and align them using the grid. When they are all in, I print it out and then join all the components up using a drawing pen. I then scan the result, turning it into a jpeg. Using Adobe Photoshop, I can then add all the necessary writing. The black dots on the connections are obtained by using the "n" key in "Marlett" font."
Bob

[Herald1360]

You can also do something similar in Paint- just draw up an initial drawing template with all the symbols you want including loops and blobs etc. Each new drawing just open this template , save it as whatever then carry on from there using copy paste etc. Connections are easy simply use the paint standard line tool. Any new symbols you might come up with in a new drawing can just be copied back to the template version for future use.

If you're really lazy, scan a suitable sized circuit diagram with a good selection of components on it. Open it up in paint and simply copy individual components into the template!

To do circuit diagrams consider a free PCB CAD package, lots about... http://en.wikipedia.org/wiki/Comparison_of_EDA_software