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World of Ham Radio 1997
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1997-02-01
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Modification for Putting BC-200XLT In AM Reception Mode
I have come up with a crude mod for the BC200xlt scanner to allow AM
reception in the 136 to 174 range. Why would I want AM in this range you
ask? well I attend several Air shows during the year and the Thunderbirds
use 140 mhz area fregs. As we all know the Uniden switches to FM in this
range, causeing very distorted reception (slope detection). Somebody else
may have already thought of this idea (you?) but here goes. Break the AIR
Trace between dual diode D31 and IC1 pin 8. Connect the center of a single
pole - double throw mini switch to the trace leading back to IC1 pin 8.
Connect one of the other poles to the trace leading back to D31. This is the
"Normal" mode(This just restores what you have cut). Now connect the other
pole to the VHI (VHF HI) line at a easy to locate point. This is the "AM"
mode. What will happen now is when AM is selected the VHF and VHF air band
will all be in the AM mode. UHF and others are not affected. When in the
normal mode everything is just like it always was. I have done this mode
over a year ago so it's still a cloudy in my memory. But it works like this:
The microprocessor sends a high to Q7 to select the AIR mode, by routing the
VHI signal around D31 by means of the switch the AM detector is reselected.
Normaly D31 blocks out the VHI signal allowing FM in the VHF HI range. I
hope my explaination of this mod is clear. It does take some delicate
surgery to do. I also hope I haven't duplicated anyone elses idea.
Thanks
Here is another am mod that I found
BC200AM.MOD
───────────────────────────────────────────────────────────────────────────
BC 200XLT
AM MODE IN THE 136-174 MHz BAND
If you ever attend air shows, this modification might be of interest to you.
The Thunderbirds and support crew use AM modeon many of their frequencies,
and we all know the Uniden switches to FM in this range, causing very
distorted slopedetection of an AM signal. This modification will let
youenable AM mode and give your BC 200XLT another feature thatlivens up what
many people call the most boring band inscanning!
MODIFICATION PROCEDURE
----------------------
1) Break the AIR Trace between dual diode D31 and IC1 pin 8.
Connect the center of a SPDT mini switch to the trace leading back to
IC1 pin 8.
2) Connect one of the other poles to the trace leading back to D31. This is
the "Normal" mode (This just restores what you have cut).
3) Now connect the other pole to the VHI (VHF HI) line at a easy to locate
point. This is the "AM" mode.
When AM is selected ONLY the air band will be switched, the other bands are
NOT affected in any way.
CONSTRUCTION TIP
----------------
If you use the micromini slide switch (RS# 275-409), and drill asmall hole
in the back of the unit, you'll be able use a pencilpoint to make the switch.
This way the carrying case can still be used.
HOW IT WORKS
------------
The microprocessor sends a high to Q7 to select the AIR mode, by routing the
VHI signal around D31 by means of the switch the AM detector is selected.
Normally D31 blocks out the VHI signal allowing FM in the VHF HI range.