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World of Ham Radio 1997
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WOHR97_AmSoft_(1997-02-01).iso
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am1700.doc
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1997-02-01
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49 lines
Tweak AM rcvr 4 1700KHz
The AM broadcast band is expanding up to 1700KHz soon, and some AM
radios can be adjusted to accomodate this new spectrum. The
radio should have vague calibrations, as the calibration
would otherwise need to be changed. We're talking about analog tuned
radios, like cheap portables or clock radios.
Practice on a radio you don't care about before you attempt a good radio!
Step 1: You'll need a 1700KHz signal source, perferrably AM
modulated with a tone. Also tunable to the range 530 to
1710KHZ.
Step 2: See how far the radio will tune before you touch any
adjustments. Maybe it already is capable of getting 1700KHz.
If not...
Step 3: You need to identify the purpose of all trimmers and
adjustments in the radio. Assuming no RF stage, look for: antenna
trimmer cap, oscillator trimmer cap, oscillator tank coil, Antenna
loopstick (hopefully, the wire is wound on a slidable paper
coilform), and the IF coils.
Step 4: Mark, with a fine point magic marker, the orginal positions
of all cap and coil adjustments, so you can return them to their
orginal setting if this procedure doesn't work out. Look to see if
the osc and ant trimmer caps are currently set at about their
midpoints, not at or near minimum capacitance. If they are near or
at minimum, stop. This procedure then won't work with that radio.
Step 5: Set the signal source to 530 KHz, and check that the radio
can tune that in, and then see how far the radio can tune the upper
end. Also tune in radio stations of known frequency and zerobeat the
generator, as a check on the generator's accuracy. Now find a
station at or near the top end of the band and "walk" it doun your
dial a little bit by making small adjustments of the oscillator
trimmer (turn for less capacitance on the trimmer).
Tune to the bottom of the dial, and adjust the osc coil to bring back
the bottom of the band (530). Repeate this process a few
times, as you walk the station at the top of the band down. Check to
see if you can receive your generator set to 1710KHz yet (Gives
the radio a little leeway for temperature and whatnot drift over
time).
Step 6: Now that the radio can tune 530 to 1710KHz, you'll need to
tweak the antenna LC circuit. Tune a station near 620KHz, and slide
the antenna loopstick coil (avoid putting your fingers on the coil
itself, touch the ends of the coilform with fingernails or such)
for max signal strength. Now tune a station near 1480KHz, and tweak
the antenna trimmer cap. repeat process a few times.
You shouldn't need to touch the IF strip.
That should do it. Enjoy the new stations when they sign on.
Proceed at your own risk