Crystal Radio Building Contest
Dan Bedford (resincored)
Triple Tuned Circuit Receiver configured for maximum flexibility.
To further pursue ideas developed
in the Danco Super I and to incorporate refinements gleaned from
members of the Yahoo Crystal Set Club.
The circuit consists of three tuned circuits that can be configured in several different packages.
Antenna coil L1 can be either parallel tuned with C1a. having variable ground coupling via C1b. and C1c. by means of S2 and S3, or series tuned with C1a. and C1b. and C1c. if required.
Coil L2, tuned with C2 is the detector circuit.
Coil L3, tuned with C3 is the trap circuit however S4a and S4b. can reverse the roles thereby placing the trap between the antenna circuit and the detector circuit.
The antenna can be connected in at the C1 frame for further flexibility.
A detector socket is placed on the front panel to allow for ‘plug in’ detectors.
Link Lk is to allow for the possibility of using detector bias.
R1. C4. provides for resistance / impedance matching.
S5a. and S5b. enables multiple
output impedance matching from voice coil to crystal phones. R2. C5
provides a current path for crystal phones.
All components except for the coils are mounted on the front panel. This enables ease of experimentation with coil types and forms.
Vintage tuning condensers (I refuse to call something made in the 20’s a capacitor) were used for C2 and C3. Vintage wireless buffs will note that the A.W.A. (Amalgamated Wireless Australasia) condensers are almost exact copies of Pilot Condensers. C2 has been silver plated. C3 is yet to be done.
A consequence of the silver plating is that the maximum capacity increased by about 45pF. Useful in enabling a smaller value of L to achieve resonance.
The three tuning knobs are also vintage parts. Not quite a matching set!
The expensive looking ‘English mahogany’ base board comes out of a tin and the ‘ebonite rods’ on which the coils are mounted are pine doweling sprayed black and varnished!
The coils are 660/46 litz laced with floss. Thanks Richard, I used to use fine dial cord. They are suspended on four 1/8” polystyrene tubes carefully chosen to reduce losses. Mind the nose Pinnochio, the hobby shop only had tube when I went to buy rod. The loss nonsense only occurred to me much later. I seem to have a considerable mental hysteresis these days.
Wiring is mostly solid copper sleeved with woven cloth: I always liked that old-fashioned ‘bus bar’ look.
Yet to be made is an acrylic
cover to keep out dust.
I would love to list all the dx I have achieved but I cannot. Living in a state capital, I am surrounded by ten transmitters lighting up the ether. However, I can easily separate all ten local stations including two smaller “community” type stations. In quite spaces, sky waves are fading in and out but I have yet to get positive I.D’s.
My best site for dx is our fishing club’s cabin in the New England area of N.E. New South Wales. Dx will have to wait for my next trip.
The ‘crispest’ reception is achieved with crystal phones, ‘loudest’ with a pair of s.p’s home built into ear protectors, and ‘hi.fi.’ quality is got with a pair of voice coil z, 27 year old Akai stereo headphones.
I’m not happy with the back
end matching but this set was built as a flexible test bed. The real
work is yet to begin.
I didn’t click to why the turns
of my coils had small “s” curves in them until all three had been wound.
I had been subjecting the wire to a 90 degree twist by my winding action.
More mental hysteresis.
Consequences of Entry.
As mentioned earlier, time allocated to crystal radio building comes at a price.
My daughter requires frequent help and guidance with her school homework and she has been somewhat displeased with my lack of attention. My son cannot understand why I would rather play with crystal sets than play with the latest video game!
I have a lady friend across
town, who complains bitterly about the amount of time, (or lack thereof)
that I devote to her. If she were ever to find out that I have ‘wasted
time’, building ‘a crystal set’ when I could have been taking her out,
the consequences are too horrible to think of. I don’t believe I have
told her any lies, perhaps just distorted the truth somewhat, about
Still it’s been fun!
Thanks to members of the various
forums for the inspiration and encouragement to ‘make time’ to experiment
again. Particular thanks to Richard O’Neill for assistance with a purchase
from the U.S. and to Brian Smith of ‘Brian’s Wireless Workshop’ www.users.bigpond.com/brianswireless/, for ideas on how to solve my moral
dilemma over the issue of whether or not to use diode bias.
The builder made his first crystal set at age ten. By age 15 there was so much wire in the air over 32 Elaroo St., that the local avian population issued a ‘Notice to Pilots’ and declared the area a ‘No Fly Zone’.
A Radio and T.V. Apprenticeship
followed and eventually led to some 28 years in laboratory electronics
and instrumentation. Still at it. Retirement seems to be getting further
away instead of closer!
To build such a large and efficient system that I am summonsed by the Broadcasting Control Authority for “sucking a hole” in some broadcasters radiation pattern!