Qex Articles By Paul Protas

Fig 6—The Junk Box Special, a low-cost shortwave receiver.

circuit's stability for CW and SSB reception.

C7 is the throttle-capacitor REGEN ADJUST control. As C7's capacitance is increased, more RF energy is coupled from Q2's drain to ground (and therefore the ground end of L2); this eventually initiates an RF oscillation. RF choke L3 isolates Q2's RF feedback from the detected audio, allowingjust the audio signal to pass on to L4. L4 is the primary winding of a small 6.3 V filament transformer. This inductor allows the audio signal to be extracted from the detector without introducing additional series resistance in Q2's drain circuit. This helps minimize detector frequency drift.

Note that almost any small transformer with a 120 V winding can be used. You can also substitute vacuum-tube-television vertical-output transformers or the primary windings of vacuum-tube audio-output transformers. Alternatively, omit the transformer and connect CIO in series with a 1 or 2 kii resistor to Q2's source. This connection provides a lower audiooutput level, but it is adequate for headphone reception. The resistor method also eliminates any "howling" (audio oscillation) effects that sometimes occur when a large inductance is used to extract the audio.

A Zener diode holds the detector's supply voltage constant. This greatly reduces frequency drift for CW and SSB operation and allows the regeneration level to be set much closer to the oscillation threshold for AM reception.

The audio signal travels via CIO to the VOLUME control, R5. SW2, a singlepole, double-throw (center-off) toggle, selects one of two capacitors to reduce the audio bandwidth. An LM386 audio-amplifier IC provides adequate volume to drive headphones or a small speaker. I suggest using a socket with the LM386 so you can change this chip easily. (The original device used in my prototype was very noisy and prone to oscillate. I suspect that many of Radio Shack's LM386s have similar flaws.) R6 and capacitor C15 help prevent the IC from self-oscillating.

This receiver was built to be very portable. The enclosure is the bottom half of a 5?/8x7?/8x3'/2-inch metal box. A piece of 3/i-inch-thick wood was cut to fit the inside bottom of the box. Wood screws hold the wood to the metal; they also secure the circuit board and other components to the wood. To mount the film-can coil form, I drilled a small hole in its bottom and attached it to the wood using a small brass wood screw. The wood bottom not only makes construction much easier, but helps minimize any loading effects on the main tuning coil.

The regeneration throttle capacitor and main tuning capacitors are attached directly to the ground plane of the circuit board; their shafts extend through holes drilled in the box side that serves as the front panel. All other controls were mounted directly to the front panel. Plastic knobs were used for all the controls; they don't increase hand-capacitance effects. Because I

(B)

Photos A and B—The upper photo shows a top-front view and the lower photo a top-rear view of the Junk Box Special regen receiver.

Nov/Dec 1998 31

Supply Requirements:

* Use a 5V Zener diode for 6 V operation

Audio Output

Winding

Main Winding

Tickler Winding f to Drain of 02

n.c. = Not connected

Audio Output

Winding

Main Winding

Tickler Winding f from Q1

L1 occupies approx. 1/4" on the coil form. L2 3/4" and L3 approx. 3/16"

f to Drain of 02

L1 occupies approx. 1/4" on the coil form. L2 3/4" and L3 approx. 3/16"

Coil Chart:

Turns required for use with a 200 pF Tuning Capacitor, #20 copper wire and a 1.12" diameter Plug - In Coil Form (Antique Electronics Supply No. PC-211)

n.c. = Not connected

Frequency Range

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