Ramsey Electronics HR40 - Manual (Page 22)

HR40
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22
OTHER METHODS OF DIAL CALIBRATION:
A easy way of making your own dial is to paste a small rectangle of paper
behind the tuning dial knob. Then as you tune in different frequencies, pencil
in on the paper the received frequency. Or a simple logging scale may be
affixed behind the tuning knob on the front panel. A logging scale is simply a
set of numbers for adjusting a control. For example, 4 might represent 7.1MHz
and 5.5 being 7.12MHz, and so forth.
DIRECT FREQUENCY READOUT:
A novel advantage of the simple direct conversion receiver is that a general
purpose frequency counter may be used to give a direct readout of the
oscillator frequency. While you could experiment with an RF pickup loop or
tuned circuit and counter preamplifier to boost the oscillator’s tiny output to a
level suitable for the counter, the most reliable method is to wire a high-
impedance counter probe line directly to pin 7 of the SA602, terminated to a
rear panel frequency counter connection of your choice. This will consist of a
short piece of mini-coax to a 1 megohm resistor bridged by a 27 to 33pF
capacitor as shown below:
With the RF gain turned all the way up , it may be possible to hear the
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 100 KHz, 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 HR40 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