[TRLog] RFI and keying

AE0Q V31RY v31ry@ix.netcom.com
Wed, 10 Sep 1997 16:48:49 -0600

N5NJ wrote:
> The Sprint starts, and I realize that since I bought an old 30L1 a
>few months back, I haven't operated on 20cw using computer keying since
>whenever I hit a function key, it keys up constantly.  Oh boy - another
>rough start to the Sprint.   So I end up running low power on 20m and have
>a terrible time getting used to the way TR works.  

  There is a very simple circuit that is almost guaranteed to keep RFI out
of the PC ports, especially the LPT (that's the most sensitive).  You can
use almost any inexpensive opto-coupler IC instead of the keying transistor,
and there is no DC connection from the LPT (or COM) port to the radio.  I've
built mine (and some for friends) right in the DB-9 or DB-25 connector shell
at the PC end.

  The schematic for both circuits (serial port and LPT port) is on my web
site.  I don't know if the Radio Shack part number specified is still
available, but almost any opto-coupler will work, just change the pinouts to
match.  I usually use 4N24 (TIL-103) parts because I have a box of them, but
any sold by Digi-Key or Mouser will also work.

Go to the CONTEST PHOTO and INFO page and the files are about half way down
the page.  The schematics are drawn with ASCII, nothing fancy.  The same
circuit works with TR, CT, LOGic, LOG-EQF and probably others that I haven't
tried.  It certainly has cleaned up my RFI to the PC!!


73  --  Glenn
"Remember, any tool can be the right tool!"   Red Green

     AE0Q / V31RY          ex: GM5BKC, ZB2WZ, SV0WY, WA0VPK
  v31ry@ix.netcom.com   --SOWP 5558-M, ARRL LM, QCWA LM, NCVA--

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