A Chance Crystal Radio Project

A vintage unmarked inductor/transformer had been lying neglected in my junkbox for decades and I had to resurrect it.

Resistance checks showed that 2 windings, with a tap each, are terminated in its 6 pins. One measured 70 Ω with a 5 Ω tap and the other less than 1 Ω. A threaded core, with a movement of 5 mm is situated at the top of the unit.

The 70 Ω winding appeared to be suitable for a crystal radio. It was wired up as per the schematic shown below.

Chance Crystal Radio - Schematic
As luck would have it, the local 612 kHz, 200 kW AM broadcast station, situated 20 km away
(as the crow flies), comes in real loud using a 60' wire antenna and balanced-armature phones. Headphone current, measured using a 1mA FSD 60 Ω meter, is 250 μA with the core fully in and 300 μA with the core retracted.

The capacitive wire antenna series-tunes the link winding to resonance at 612 kHz, while the self-capacitance of the detector winding parallel-tunes it to resonance at the same frequency, as though the inductor/transformer has been tailor-made for this project.

Chance Crystal Radio
A cosmetic plastic jar makes a good base/enclosure for this chance crystal radio.

Related post: Another Parallel-tuned Air-core Coil Crystal Radio
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Pseudo FM Crystal Radio

After several failed attempts to build an FM Crystal Radio with a diode detector, a dual-gate mosfet version was tried out but also in vain.

The mosfet used was a 3N187 from the junk box.

FM Crystal Radio - Schematic
Now at a dead end, powering it up with a rundown 1.5 V button cell was the only way out. And it sprang to life!

Pseudo FM Crystal Radio - Schematic
A number of local stations were received.

Pseudo FM Crystal Radio
Notwithstanding the problems of hand effects and oscillations, it was music to my ears.

Pseudo FM Crystal Radio - Final schematic
The 330 μH moulded inductor was a later addition that solved the oscillation problem.

Pseudo FM Crystal Radio - Inside view
The factors contributing to my failure in building a real FM Crystal Radio could be the distance of the transmitters (6 km away as the crow flies) and their low power rating (1 to 10 kW).
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