Experiences constructing the "Diotran" Radio Receiver Kit

Here is an account of my experiences constructing the "Diotran" radio kit. John Davies very kindly sent me one after a telephone conversation at Easter during which I expressed an interest and volunteered to undertake a re-write of the instructions. The receiver is a 'no-frills' regenerative circuit which was novel to me because of its extreme simplicity (but probably very familiar to some old hands out there). It is basically a germanium diode crystal set with a BC109 feedback circuit, with the transistor collector load being one of the old telephone 'balanced armature' earpieces - and you don't see many of those around nowadays! These medium-impedance earpieces are pretty sensitive, and I was encouraged to see one included. There is also a nice slow-motion drive air-spaced variable capacitor for tuning.

My son Alexander (nearly six at the time) and I sat down to build it one Sunday after lunch. The instructions were familiar to me, having transcribed them earlier, but are very readable and detailed. The author claims that no knowledge of electronics is required and I think that the instructions are clear enough to support that. Alexander's attention span is not very long, but he got quite enthusiastic about the nice little "Diotran Radio Club" badge that was included, and developed his skills with screwdriver, wire cutters and strippers (in fact, I had to discouage him from helpfully stripping the insulation off every cable in sight). The entire circuit is constructed on an eight-way terminal strip with some components carefully prepared to fit. The purpose of some of the 'additional' components is not always clear, but I guessed that the '4 oz. coffee jar lid' and the 'cotton reel (sawn in half)' together form a makeshift tuning knob in conjunction with the 'contact adhesive', even though they are never mentioned again!

Construction was not entirely without its frustrations; at first we couldn't work out which way round the tuning capacitor went (it sits up on its side); then we suffered the 'MFI syndrome' of the mounting hole in the (wooden) base being slightly off-mark so that the screw wouldn't mate with the tuner; and finally, we both laughed upon discovering the inevitable wood screw with no slot cut in the head for a screwdriver! This final device reminded me of the comedian Jack Dee's definition of a screw as "crinkle-cut nail"! A little bodgery with bradawl junior hacksaw soon put these little problems to rights and thereafter, all was plain sailing. The trickiest bit was winding the coil, but again, the instructions are very clear and with the aid of a young lad solemnly counting my windings up to 60, all was completed without diffiuclty. The whole job, problems and all, took a surprisingly brisk one-and-a-half hours to complete.

We carefully took the completed device, now looking like a real wireless set, up from the kitchen table to the workshop in the roof and there connected it to a 4.5V battery, a 6m long aerial wire and an earth. We held our breath, put the earpiece up to our ears and switched on. Success first time! We picked up eight stations loud and clear straight off (mind you, we do live within 20 miles of London). Alexander, clearly impressed, listened to some music and the football commentary and then went off to play 'Galaxians' on my old BBC microcomputer. Kids these days, eh...?

The balanced armature earpiece gives excellent results, often almost at loudspeaker volume (it also has a restricted frequency range that cuts out a lot of irritating noise and provides a pleasant tone). The regeneration, via a trimmer capacitor, operates much more smoothly than some potentiometer circuits I have tried, which largely stops the set from bursting into oscillation without warning. As the instructions emphasize, a good earth connection is clearly essential, but this should not be too difficult to arrange (John and I have changed the bit in the instructions that recommends using a main plug to provide 'a convenient earth'!). At half-past ten the following evening, the radio provided reception of 25 different stations, loud and clear, of which at least ten were overseas, with France, Germany, Holland and Sweden clearly audible (sorry DXers, I didn't identify them all). At the far end of the dial, I also picked up a slightly noisy and distorted full-duplex 'amateur' transmission which I eventually identified as my wife talking to her mother on the cordless 'phone!

I am amazed at the performance of this little set, and highly recommend it. Clearly, a lot of thought has gone into both the circuit and the kit and the designer obviously knew what he was doing. My thanks to John for his kind provision of the kit which has brought Alexander and myself so much enjoyment.

Philip Miller Tate

How to get a Radio Kit

I have been able to obtain some radio kits for the club which I hope will be of interest to our members. I have 11 of these kits available. They are described as 'Diotran Radio Kit Medium Wave diode-transistor radio for all ages. No soldering required'. I have built one and I sent another to Philip Miller Tate who kindly offered to write the review of this radio kit. He also re-wrote the instructions.

If any of our members are interested in building one of these radio kits I would require that they sent me a £4 cheque or postal order made out to myself.

In the original article that came with this kit it mentioned a Darlington amplifier. I am unable to find out any infomation on this amplifier but if I have enough interest myself and Philip Miller Tate would be willing to produce an odd amplifier kit. We hope this will be based on a Darlington amplifier as they can be built with only a few components.

John Davies

Order of Construction

(Refer to schematic diagrams below)

External wire connections

(All to nearer terminals on block)

Operating Suggestions

IMPORTANT: Check that the battery is connected the correct way, as it will damage the transistor if it is not. Before switching on, make sure the transistor and all other leads are correct, and that no wires are touching unless supposed to. Say a prayer and switch on! Adjust tuning and reaction to give the loudest and clearest signal. A shorter aerial increases selectivity but decreases sensitivity. Turning the tuning control anti-clockwise tunes to the lower frequencies (higher wavelength) of the band. Turning the reaction control anti-clockwise increases volume and selectivity, but if turned too far will cause oscillation (motor-boating) and distortion.

If receiver does not work

Philip Miller Tate

List of Components

The kit is intended to bridge the gap between the simple crystal set and more complicated radio receivers requiring soldering and a knowledge of electronics. It is the result of years of experimentation with circuits and construction techniques among Primary School children. The kit will be of interest to some children of or seven years upwards and with a little guidance, excellent results can be achieved. No soldering or knowledge of electronics is needed and full instructions and diagrams are supplied.

Technical The receiver covers the Medium Wave Band. It uses slow-motion tuning, a ferrite rod coil, one diode and one low noise transistor with reaction to increase volume and selectivity. The output is taken to a medium impedance balanced armature ear piece, which gives a pleasing sound.

Please check components against list

Quantity  Item	                 Quantity   Item
1    	  Wooden 'breadboard'	        1   Condenser 220nF
1	  Panel, plywood	        1   Resistor 1 Meg ohm
2	  No. 6 x ¾" screws             1   Resistor 68k ohm
1	  No. 5 x 3/8" screw	        1   Length of 32 s.w.g wire for coil
2	  4BA x ½" bolts.	        1   Ear piece
4	  4BA washers 1	                1   Ferrite rod
2	  4BA nuts               	1   Plastic strap + fixing screw
1	  2BA nut	                1   Length of foam
1	  2BA washer     	        1   Length white PVC twin flex
2	  6BA x ½" bolts	        1   Short copper distance piece
5	  6BA nuts	                1   Black pointer knob
2	  6BA washers	                1   Piece gummed paper
1	  6BA x ½" brass bolt           1   Piece sandpaper
4 or 2	  Copper plated nails	        1   Screwdriver
1	  8 way terminal block or	1   Diotran Radio Club badge
	  two 4 way blocks	        1   Set of detailed instructions
1	  2 gang tuning condenser	    and diagrams.
1	  Reaction Control (trimmer)
1	  Switch with leads
1	  Diode 0A91
1	  Transistor BC109

Tools required

Other items needed

John Davies
Radio1
Schematic Diagram

Radio2
Component Layout