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R7000.23
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1992-09-15
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R7000.23
Connection to R-7000
As mentioned earlier, a DC voltage is present at the
R-7000 IF output connector and is used to power an ICOM
TV accessory. A direct connection between the R-7000
IF output connector and SM-220 would damage at least
one of these units.
To block the DC voltage, a 0.1 microfarad capacitor was
soldered inside the R-7000, between the IF output jack
and the adjacent jack labeled "spare". Connection
between the R-7000 and SM-220 is then made using a
short length of RG-58/U coaxial cable, with one end
plugged into the R-7000 "spare" jack, and the other end
plugged into the rear of the SM-220.
This leaves the original IF output jack undisturbed so
it can be used with the TV adaptor accessory.
Vertical Sensitivity
When I connected my modified SM-220 to my R7000, the
SM-220 would display only the strongest of signals. To
improve the display sensitivity, I inserted a 20 dB
gain RF amplifier between the R7000 and SM-220. The
amplifier was a spare Ameco PLF2 FET receiver preamp,
adjusted for 10.7 MHz, but other amplifiers can be
used, provided they have at least a 500 KHz bandwidth,
and sufficient gain, at 10.7 MHz.
False Readings - Images
The SM-220 circuit is like a superheterodyne receiver.
The panadaptor itself has a 455 KHz IF, and like other
superheterodyne receivers with a low IF frequency, is
prone to images.
An image is manifested as a false pip, which moves
across the screen as the receiver is tuned, and is 910
KHz (twice the IF) away from the actual signal.
The images discussed here are in the SM-220, not in the
receiver. Images are bothersome on the strongest sig-
nals, like those 300 watt paging transmitters that
saturate the county with RF, belching out out strange
digital noises or voices (now illegal to monitor).
Use With Other Receivers
The modified SM-220 can be used with other receivers
having a 10.7 MHz IF. In other receivers, one would
need to find the proper point in the IF stage (before
filtering takes place), install a connector, then wire
a DC blocking capacitor between the IF stage and con-
nector.