‘Retro-40 Instructions July 11, 2011
Page 5
‘Retro-40 ‘ Description
Note: The text which follows describes the original (75M) Retro version. For the 40M
version , the IF is changed from 6 MHz to 4.915 MHz, and tuned circuits and the output
low-pass filter values are changed. The operating frequency is in the 7.0-7.3 MHz range.
Inspirations:
I’ve long been intrigued by relatively simple voice radios. The BC-611/SCR-536 [at right] was widely used
during WW II as a platoon-level radio. It had a ‘bare-bones’ crystal-controlled superhet receiver and an
AM transmitter with 350 milliwatts of output power. It had an effective range of about a mile with its built-
in whip antenna. Although pressed into service in the postwar years by enterprising hams, its lack of
selectivity limited its usefulness under crowded band conditions. As I extrapolated from this radio’s
highly inefficient whip antenna to a full-sized dipole, I realized that the effective range would be upwards
of 50 miles. In my book, that’s a usable communications distance for keeping in touch with friends! That
served as a challenge and as the inspiration for this project.
Although AM has long been supplanted by more effective modes of communication, AM activity is largely
channelized into ‘watering hole’ frequencies
.
See
www.AMwindow.org
. It’s therefore pretty easy to find
activity on the air.
7290 kHz is the standard ‘hot spot’ for 40M; this may vary by Region.
Receiver:
The receiver front end is entirely conventional. A series L-C T-R switch and bandpass transformer feed the SA612 first
mixer. The SA612’s internal oscillator is used to save component count by eliminating the need for a separate local
oscillator (LO) transistor. The varicap diode-tuned LO covers about 50 kHz of the 75M band. It operates at approximately
2.1 MHz and the 1
st
mixer up-converts to the Intermediate Frequency (IF).
In the interest of tradition, I initially used 455 kHz IF transformer in an IF design. A double-tuned filter using these
transformers proved woefully inadequate in keeping close-by SSB stations out of the detector stage.
In light of the BC-
611’s lack of selectivity, this was largely a case of ‘reinventing the wheel’.
Rather than wrestle with more stages of
filtering or the use of ceramic filters, I reverted to a tried-and-true crystal filter. I chose an IF of 6.00 MHz to keep both the
filter bandwidth and the LO frequencies reasonable. With the filter values shown on the schematic, pass bandwidth is 3.5
kHz with adequate skirt characteristics. I found it necessary to ground the case of crystal Y3 to eliminate pickup of strong
AM signals from the 49M shortwave broadcast band.
The IF amplifier uses the so-called ‘cascode’ configuration. Although the MC1350 IC is often used here, it’s been around
for an uncomfortably-long number of years. Parts cost was a factor, too- the discrete cascade amp has a parts cost of
about 15 cents. This amplifier is terminated by R6, whose value was chosen as a compromise between stage gain and
consistently stable operation.
The following stage is a peak detector which serves to recover the modulation envelope from the received signal. Q3 is
an ‘infinite-impedance detector’- a JFET biased near cutoff.
I’d always wanted to say I’d used one- the phrase has a
nice ‘Hitchhiker’s Guide’ ring to it.
I found it more sensitive in practice than an unbiased 1N34 Germanium diode.
U3- an LM386, is a widely-used AF Amp IC, and provides plenty of speaker volume. The transistors around it serve
several functions, Receiver audio needs to be firmly shut off during transmit. The transistor switch in series with the
LM386 output is important. That IC’s output voltage is dependent on supply voltage. As the power supply ‘pulls’ on voice
peaks, it yields an unwanted feedback path to the speaker.
The audio output waveform is also peak-detected by D6 and C20 and applied back to the gate of Q4 as a form of
Automatic Gain Control (AGC). Without it, nearby ‘Big Gun’ stations are easily capable of saturating this amplifier stage,
yielding harsh-sounding clipped audio. The AGC threshold is nominally 6 volts peak-peak, depending on the individual
characteristics of Q4. With this feedback path in place, even the strongest of stations sounds fairly clean in the receiver.
courtesy of N6GCE
www.prc68.com
