Crystal Radios a Growing Fascination.
Anyone whose visited my Beltane designs site Beltane Products
will see I like to make things from raw materials like leather and wood and brass. Mostly functional things
with an artistic theme running throughout the design but there is also the scientific side of me that also
is bursting for expression.
Since my early youth I had a training in electronics and radio. My workshop
has grown with some basic engineering equipment like lathes engraving machine, drill presses and
fly presses many hundreds of hand tools all crying out to be used. It seemed a logical step for me to
blend both the scientific and the artistic and come up with a different form of expression.
The crystal radio strange as it might seem has a fascination and a satisfaction within my creative ability.
Trouble is I've never really made anything for myself in all these years things go out of my workshop
to other people. I guess I really enjoy giving and making for others more than keeping things for myself.
Crystal Radio Earth Station Receiver Mk1
This was the first radio I made using a home made oak box and engraved Gravoply. I wanted to create a simple but classic look.
I used a Calrad Type Vernier dial for slow motion tuning but although it worked well I was not exactly happy with the visual
appearance being too modern.
The coil on this set was wound with an AVO Douglas coil winding machine that I renovated.
The old AVO's machine was completely worn out when i got it and I had to get a new traverse thread made before it would wind
anything resembling a coil. I now wind all my coils from .15mm up to .5 mm wire on this machine for
heavier coils I use a modified Lathe.
(See AVO Photo).
The Mk1 set whilst looking ok was prone to the old problem with simple single coil sets of overloading and damped
tuning range, it was an interesting experiment though which has lead the way to other projects of a more advanced design.
Crystal Radio Earth Station Receiver Mk2
I wanted to extend the limited tuning range of the Mk 1 radio and I added a tapped coil to this radio.
I could switch the main inductance from 100- 400 micro henries. I also put on an aerial coupling coil
which helped reduce the overloading and damping.
I have wound all the coils I have made so far on 'Paxolin' which is a trade name for a form of Bakelite.
It is still one of best insulating material for coils because it is 'Dry' electrically it looks and
smells great remember that old radio from the 20's and 30's this was the material that gave it that
characteristic smell. It is very hard to find this material today as its not made anymore in large
quantities. I was very lucky to find quite a few feet of it from a de- commissioned transmitting
station it is Genuine Vintage Tube.
Mystery Crystal Radio Mk3
This set was my first to use all brass fittings. I prefer to use solid brass rather than nickel plate it seems to
enhance the look of quality and I can make many of the parts on the lathe to match up all the fittings.
I made a plug in coil for medium wave band and I can simply plug in a Long wave or Short wave coils on this set.
I did not want to restrict what type of coil design I used ultimately so I can swap over to spider or basket weave
coils later to make comparisons with performance.
I made the box from solid oak and finished it with 'Rustins Teak Oil'. I liked the look of the natural oak its grain
showed up much better than staining the usual dark brown colour of vintage oak boxes.
I always enjoy listening on these projects when they're finished it gives me a sense of satisfaction hearing as
well as seeing the end results.
This set has a Bogen type T725 Transformer to change the output impedance from High to low to match whatever phones
I have around the workshop and to also see how they compare for sound level and quality.
Crystal Radio Mystery Receiver Mk4
Mk4 Medium Wave 'Mystery Coil' Radio with Built in Antenna Tuner and Output Matching Transformer. All Engraved Legends and Mitred Solid Oak Base.
Works Great and is selective. Was a nice project.
I have decided to call this the Jules Verne set.
Did you ever see the film '20000 Leagues Under the Sea'?
This reminds me in some way like the inside of Captain
Nimos Nautilus submarine with all its brass dials.
There is enough Naval brass on this set to sink a Battleship!!
This set has Aerial tuner built in plus adjustable impedance output
for any phones from 8 ohms to 5 K ohms.
The Aerial tuner coil can slide along the twin brass rail to get
a closer coupling to the main tuning coil . I used a Beryllium Copper
spring to make contact with the rails and it worked out well with smooth travel.
I have based this design on Dave Schmarder's #70 set.
I built this set for Alan Thomas.
HG Wells 'Time Machine Crystal Radio'
Please click on the photographs for a larger picture of this beautiful wireless receiver.
I based the circuit design of 'The Time Machine Radio' on my good friend Dave Schmarder's #20 set http://makearadio.com/crystal/20.php The Spider web coils on this radio are the all important heart of the radio and have a very high Q which is what we are really looking for to get the best selectivity and sensitivity from these simple radios.
To get the spacing correct I used my Myford lathe and a slitting saw blade mounted in the chuck. I turned the circular disc of HDPE by 40 degree on each cut which gave a total of 9 segments. This type of coil has to have an odd number of segments so that the winding does not end up at the same starting point. 9 segments seem to be the smallest number which make a tidy winding but any odd number ie 11 13 or 15 segments could be used.
The wire I used on this set was 220/46 Litz wire. This is 220 strands of number 46 gauge wire. Each strand of this wire is thinner than a human hair. The wire is also double silk covered.
I hand cut the dial pointer with a jewelers piercing saw from 22g brass sheet. This was screw fixed to main dial plate .Its the fine finishing touches that make this radio one of my more enjoyable projects.
I wanted to keep the two main Litz coils the same size physically which meant I had to add the extra inductance on a series coil. Also shown here is the Aerial terminal and its label. All labels were made on my Pantograph engraver using Gravoply professional engraving material. These are deeply engraved and can never wear off like dry transfer lettering.
I wanted to make the coils rotate on all axis so I could adjust the coupling in a number of ways, ie facing each other or at any angle to the horizontal or vertical. To do this I made a universal ball joint coupler for each coil. This was fixed to a brass shaft and connected to the handle adjusters on the base plate. Turning the handle would move the coils from the horizontal position to the vertical . I found that the best coupling was when the coils were facing each other and about 3 inches apart but I noticed that if coils were placed at ninety degrees to each other I was able to null out some adjacent stations or interference.
When coils were facing forward the set performed with much less over coupling for closer coil spacing. If coils were facing each other and less than 3 inches apart the set would become very over coupled and all stations would interfere with each other and the tuning became very broad.
This set has a Bogen T725 Transformer to match the headphones. Any phones can be used from High Impedance 4K ohms or down to 8 ohm types.
The selector switch was fabricated from brass 'Chicago studs' for the contacts these were insulated from the body of the switch with Nylon transistor mounting spacers as used on a type 2N3055 to mount to the headtsink The knob was from my local DIY store and was intended for a cupboard draw but I recoin it looked great on the switch.
The switch is spring loaded internally so that it makes a good contact on each stud.
This was a nice little engineering project on its own and I will be using this design on many more of my radios.
On the Back of the Radio is access door to the wiring. Note the little brass turned knob.
The 8 ohm headphone socket is mounted on the rear of the box.
The lovely grain of the Natural Oak wood shows up best on the back of the radio.
I didn't stain the wood finish on this radio it was just lightly oiled with Linseed oil which really brings out the grain detail.