adjust the main or band spread tuning. This
takes some practice to be able to tune in an
SSB signal. The BFO control can also be used
to increase the sensitivity of the receiver
when receiving AM signals by introducing
regeneration. When receiving a weak AM
signal, the BFO control may be advanced
slowly for the best signal reception. Advanc-
ing the control too far will result in oscillation.
The AGC, S-Meter and Mode Switch:
The I-F signal is coupled to the automatic
gain control (AGC) circuit, through a small
capacitance. It is rectified and amplified by
Q5, a 2N5232A transistor. Its output is a pos-
itive voltage that is dependent on received
signal strength. Stronger signals result in a
lower positive voltage. This voltage is fed to a
milliamp meter that indicates relative signal
strength. The other side of the meter is re-
turned to a positive voltage through the meter
zero setting potentiometer.
The output of the AGC amplifier is filtered
by a pi R-C network composed of C15, C16
and R23 producing a slow decay AGC voltage
that is fed back to the bases of the I-F tran-
sistors Q3 and Q4, reducing their gain on
strong signals.
The MODE switch has three positions. In the
AM mode, the AGC is fed back to the I-F as
described in the previous paragraph. In
STBY the positive bias is removed from both
I-F stages causing them to be biased to cut-
off. In the CW position a fixed positive bias is
applied to the I-F stages instead of the AGC
voltage, causing the stages to run at a high
gain regardless of signal strength. This bias
is set by R24.
The Detector and ANL:
Output from the third I-F ceramic filter is
also fed to a simple diode detector circuit
that utilizes a 1N191 crystal diode (D1). The
recovered audio is fed to the VOLUME control
and to the audio amplifier circuit. In noisy
conditions the ANL diode (D3) may be
switched in to clip any positive noise peaks.
The Audio Amplifier:
The audio amplifier consists of four transis-
tors, Q6 through Q9. Q6 is a small signal
NPN audio amplifier that directly drives Q7.
C23 limits high frequency noise. Q7 drives Q8
and Q9, complementary power transistors. Q8
is an NPN MPSU05 and Q9 is a PNP MP-
SU55. Diode D2 is a “Stabistor diode, which
has a stable forward voltage drop over various
currents and temperatures. It helps prevent
thermal runaway by the power output tran-
sistors. This diode is normally mounted in
close proximity to the output transistors so it
experiences their temperature environment.
The amplifier’s low impedance output is cou-
pled through C26 and the headphone jack to
drive the internal speaker.
The Power Supply:
Being one of Heathkit’s later kits, the power
cord is three wire and the transformer can be
wired for either 120 or 240 VAC power. A
transformer rated about 18 VAC CT provides
22 volts to the audio amplifier and 10 volts at
about 30 ma to the rest of the receiver. The
low-voltage full-wave power supply, fused at
0.75 amps, is conventional with one exception;
there are two transistors that act as zener
diodes between the output of the bridge recti-
fier and ground. These transistors (D9 and
D10) each have their collector lead clipped off
and the two emitter base junctions of the
transistors are connected in series. Normally
they are back biased, but when the voltage
reaches a point where the junctions go into
avalanche conduction they act as zener
diodes. This appears to be a protective circuit
to limit any power line spikes from reaching
the sensitive transistors.
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