amount of positive feedback that, in turn, creates the
regeneration needed to increase the gain and selectivity
of the detector circuit.
Circuit gain and regeneration is controlled by varying
the plate voltage of V1-A with potentiometer R3 and plate
resistor R3. Capacitor C5 bypasses any remaining RF
signal on V1-A’s plate to ground, while C3 couples the
detected audio frequency (AF) signal to the control grid of
V1-B. Resistor R5 provides a grid leak path as described
previously, and establishes a small reverse operating bias
on the control grid. V1-B acts as a small signal audio
amplifier with a gain of five. The amplified signal on the
plate is coupled to the volume control R6 by capacitor
C4.
From the volume control, the AF signal passes to the
audio amplifier module LN-1 that boosts it to speaker
volume. Earphone jack J2 is wired so that speaker SPK1 is
bypassed if an earphone is plugged in. Power is provided
by either a 12 volt battery (binding post J3) or an AC-to-DC power supply (jack J1). Diode D1 prevents current
from flowing back into the battery should an AC-to-DC
supply be plugged in at the same time as a battery.
Resistor R8 and capacitor C8 provide AC hum filtering
needed for the AC-to-DC power supply. Resistor R7 and
capacitor C6 provide additional AC hum filtering for the
more sensitive V1 circuits.
Construction
and Testing
Construction is divided into three stages, namely:
constructing the chassis on which the circuit will be built;
wiring the electronic circuit; and finally making the
spiderweb coil. Some of the construction techniques
employed may be new to readers. For instance, the
chassis requires basic wood working skills, and the circuit
28 May 2015
■ FIGURE 2. Hardboard sawing pattern.
■ FIGURE 3. Base chassis drilling template.
■ FIGURE 1. Schematic for
the retro regen radio.
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