Loudspeaker Crystal Radio

This is the speaker variant of my series-tuned crystal radio with an alternative detector.

Loudspeaker Crystal Radio
Replacing balanced armature headphones (impedance around a few hundred ohms) with a low impedance speaker required the use of an output transformer.

A power transformer, with its inherent thick conductors, appeared to be the ideal substitute for a low-loss output transformer.

Trials with transformers rated 230V/24V-2A, 230V/24V-5A and 230V/24V-15A proved the last one to be the best.

Series-tuned Loudspeaker 
Crystal Radio - Schematic
Aided by the perfect match offered by my 'Homebrew Horn Speaker' and 60' wire antenna, reception of the lone local 200kW 612kHz AM broadcast station, located 20km away, was excellent.

Performance was also quite good using the parallel-tuned configuration.

Parallel-tuned Loudspeaker
Crystal Radio - Schematic
Related post: Compact Loudspeaker Crystal Radio
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Alternative Detector for a Shunt-fed Crystal Radio

After significantly improving the performance of my first crystal radio using a series inductor, the focus was on a replacement for the OA5 which would give a higher output.

OA5 diode
The base-emitter and base-collector junctions of germanium audio transistors like AC130, OC74, AC127, 2N61, AC188 and AD162 were tried out. However, they were all only as good as the OA5.

Germanium transistors
As luck would have it, when the 2N61 was being tried out, an accidental short between its base and emitter leads resulted in a tremendous increase in signal strength. Hence the OA5 in the crystal radio was replaced with the base-emitter-shorted 2N61.

Using a wire antenna 60' long, the lone local 612 kHz, 200 kW AM broadcast station could now be heard even at a distance from the headphones! Headphone current measured using a 1mA FSD 60 Ω meter, was 725 μA.


Crystal Radio - Schematic
The other transistors too gave identical results with their base and emitter leads shorted. Results with the base and collector leads shorted trailed close behind.

A dedicated unit of equal performance was built using a 3" x 3" x 1½" electrical bakelite box as an enclosure.

Series-tuned Crystal Radio Enclosure
Screw terminals were attached on the rear for connecting the antenna, earth and headphones. 

Inside the Series-tuned Crystal Radio
Lugs on the screw terminals facilitated soldering of the transistor, moulded inductor and earth link. 

When subsequent trials on series-fed crystal radios showed a degradation in their performance,
it was concluded that germanium transistors, with base/emitter interconnection, outperform germanium diodes only in shunt-fed crystal radios.

Related post: Current-operated 'S' Meter for a Crystal Radio
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Harmonic Interference from a Crystal Radio

One day, in spite of my Minimalist Converter being disconnected, harmonics of the local 612 kHz broadcast station were still being received in the shack receiver.

That was indeed surprising. Then, after some thought, realisation dawned that the Crystal Radio on the shack table could be the cause.

This was confirmed by disappearance of signals when the the ground connection to the crystal radio was removed.
Crystal Radio - Schematic
Then, on a feeling that the minimalist converter would display identical behaviour, its output was next connected to ground.

Minimalist Converter - Output grounded
The signals returned.

It was my first-hand experience of harmonic interference!
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Series-tuning for a Crystal Radio

Adding series-tuning was an afterthought to improve the performance of my first crystal radio.

Ferri Loopstick & PVC Gang Condenser
The loopstick (90 turns of Litz wire, close-wound on a 2" length of  ¼" ferrite rod) and variable (a 500+500 pF PVC gang condenser) were wired on a bread board and connected in series with the
antenna lead-in.
Series-tuned Crystal Radio - Schematic
With the plates of the condenser nearly fully meshed, the only local broadcast station on 612 kHz was received with a considerable increase in volume.

However, both the loopstick and variable were too big for the crystal radio enclosure.

Trials, with a fixed 1000pf capacitor in series with available fixed inductors, were unsuccessful.

Ultimately, a 200 μH moulded inductor was found to work quite well, with the 60' wire antenna itself providing the capacitance.

Moulded Inductor
It was easy to solder it right at the antenna socket.

Moulded Inductor-tuned Crystal Radio - Schematic
Related post: My first Crystal Radio
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Minimalist Converter

A simple converter was homebrewed in order to listen to a strong 612 kHz local station using the 1.5 - 30 MHz shack receiver.

Simple Converter - Schematic
Since the 1.612 MHz received signal was noisy, a 2.5 mH RF choke was connected across the output - to no avail. Then, on an impulse, it was connected across the OA90 diode. The result was an enormous increase in signal strength.

Tuning up later, with the converter inadvertently switched off, an equally strong signal was found on 1.836 MHz.

A real effective tripler had been stumbled upon, making the oscillator redundant.

Minimalist Converter - Schematic
So out it went, leaving only diode and choke to do the job.

The result - a chance minimalist project!
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Simple Low-cost Demo Repeater

This project is based on 3 of my earlier projects - 'Coupling a commercial VHF handheld to an external antenna' , 'Fox Hunt Attenuator' 'UHF on a VHF Rig'.

Parts required are a couple of Rubber Ducky Antennas, suitable connectors, a single OA5 diode and a metal enclosure.
Demo Repeater
Also needed are a MF local oscillator, a VHF local oscillator, 2 VHF handhelds & a UHF handheld.

The schematics are as shown below.

For in-band operation the 600 kHz fundamental crystal oscillator output is mixed with the incoming signal.
In-band Demo Repeater - Schematic
For cross-band operation the 290 MHz 5th harmonic of the 58 MHz overtone oscillator is used.

Cross-band Demo Repeater - Schematic
In-band & cross-band repeater operation is easily demonstrated using the handheld rigs.

Down-link signals are quite strong inside the shack.

However the range of the demo repeater is yet to be checked.
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Foxhole Radio Detector Variants

In the absence of a foxhole radio, a quick way to test out a safety-razor-blade-detector was to use it in place of the OA5 in my first crystal radio.

My First Crystal Radio - Schematic
Detection was established, on the very first attempt, with a new (not blue!) blade and solid hookup wire for contact.  It was found that only printed areas of the blade were effective.

However, the local broadcast station sounded weaker with the blade than with the OA5. Results were equally bad with a pencil for contact.

A spare carbon brush of a mixer/grinder was then tried out. It worked fine on most parts of the blade's surface and the signal strength went up multifold, though not as strong as with the OA5.

Safety Razor Blade & Carbon Brush
The spring made it possible to vary the contact pressure for optimum signal.

A fixture was then homebrewed using junk-box parts. A piece of copper-clad board was used as the ground contact, enabling the blade to be moved around while adjusting the spring force to locate a 'hot spot'. A lock nut was provided to retain the setting.

Safety Razor Blade Detector
Trials confirmed easy/reliable/repeatable set-up and adjustment.

Other rusty/oxidized/plated steel parts were also tried out in place of the razor blade. The best of them all turned out to be a piece of broken hacksaw blade which even outdid the razor blade!

A 'pencil contact' was then homebrewed as a replacement for the carbon brush. Results were not that good and it was not as easy to locate a 'hot spot' with it as with the carbon brush.

Pencil with spring
A discarded extruded-carbon-block water filter was the inspiration for the next version. It served as the base cum detector contact. After removing the fabric cover, a self-tapping screw was used to secure one of the lugs on the block. The blade was lightly held in place using a rubber band.

Carbon block water filter
Connection to the blade was made by sliding the other lug under it whilst a wrap of electrical insulation tape isolated it from the carbon block. Results with this 'giant', yet simple, detector were unbelievably good.

The performance of these blade detectors was considerably enhanced by connecting a run-down button cell in series (~ 0.1V with negative terminal to carbon brush).

Button Cell & 'Ginger Cell'
Next, the button cell was replaced by a 'Ginger Cell' - a piece of ginger into which a brass screw (+ve terminal) and a steel one (-ve) were screwed in. Its open-circuit voltage of 0.5V dropped to around 0.1V in-circuit and it was as good as the button cell.

Later, a cat's whisker detector was made with a black oxidized-brass screw mounted on an old plastic pulley. 

Cat's Whisker Detector details
The copper-wire cat's whisker was soldered to the plated screw and the rectifying spot located by adjusting the screw.

Cat's Whisker Detector & Screw
It was tricky to adjust, but its performance was quite good.

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