Ramsey Electronics HR30 Instruction Manual - page 22
HR30
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counter’s busy humming action in the background, but not to a serious
degree. It drops out with a slight reduction of the RF gain. If you decided to
add such a counter connection to your receiver, remember that the coax as
well as the coupling capacitor become part of the oscillator circuit. Make the
coaxial cable connecting the counter to the receiver as short as possible. You
will experience a frequency change of several KHz when connecting or
disconnecting the counter. Also, this additional capacitance in the oscillator
circuit reduces the tuning range to approximately 100KHz, thus offering
another way of getting bandspread for your favorite frequencies. Finally, it
may be necessary to operate the counter on battery power only; connection of
an AC adapter might introduce considerable hum into the receiver.
THE LM386 AUDIO OUTPUT:
As already mentioned the LM386 is certainly capable of far more audio power
output than is given in this basic receiver circuit. It can only boost what it gets
directly from the SA602. There is certainly space on the HR-30 circuit board to
install an audio preamplifier (i.e. 2N3904, or minimum-gain LM386) in the
circuit trace between C8, the audiocoupling capacitor from pin 4 of the SA602
to the volume control. The following is an audio preamplifier idea submitted by
one of our customers. Ramsey Electronics in no way warrants that this
modification will achieve any particular ideal signal-to-noise ratio or audio
frequency response, but is does significantly boost the output of the SA602
reaching the LM386 audio amplifier IC. Instead of the traditional transistor
audio preamp circuit, an additional LM386 is used in its minimum gain
configuration (gain of 40), plus one electrolytic capacitor (1 to 10µF) to
achieve minimum parts and PC-board drilling. This use of two LM386 ICs
amplifies the audio signal of the SA602 by a factor of 1000.
This modification requires cutting the circuit trace leading from C8 to the
volume control, R3
TO
1
MEGOHM
NE602
PIN 7
COUNTER
27
pF