[Icom] IC-765 - QSK keying improvement

[Paul Playford]

While testing my QSK modified linear amplifier I noticed the average output
power dropped when switching from VOX cw operation to FULL BK-IN on my
IC-765.

(I use an external keyer, and was sending a continuous string of dots)

Investigation revealed that with full break in the trailing edge of the RF
envelope was not delayed by the same 8ms that the leading edge was (Cure for
the "missing first dot" problem with Icom mf/hf rigs driving linear
amplifiers QST Nov 1995 p84 & May 1996 p48), resulting in shortened (by 8ms)
cw elements, regardless of keying speed.

I added a .047 uF capacitor across the normally closed contacts of the FULL
BK-IN switch (open in QSK mode).  This modification allows the VOX circuitry
to add about 10ms to the trailing edge of the internal cw keying envelope
during FULL BK-IN operation.  The output RF envelope with FULL BK-IN enabled
now matches the RF envelope during VOX operation.  During normal VOX
operation this capacitor is shorted out (when the FULL BK-IN switch is in
the out position) so it will have no effect on the other operating modes.

1st)    Remove bottom cover of IC-765.
2nd)    Position radio bottom up with front toward you.  All work is in the
front left corner of the MAIN UNIT.
3rd)    Disconnect plug marked "20" from J2 of the MAIN UNIT.
4th)    Select a .047 uF disc ceramic capacitor, spread the leads so they
are in a straight line and trim the leads to an overall length of 3/4".
5th)    Carefully solder one lead to pin 1 of the VOX unit (ground) and the
other lead to pin 6 of J2.  The lead to pin 6 must be soldered at the end of
the pin closest to the printed circuit board to allow re-installation of
P20.
6th)    Re-install P20 and the bottom cover.


After successfully completing the above modification I found a similar
modification published in the Frankford Radio Club Newsletter - August,
1991.  Their modification requires hinging the front panel down and
installing a .022uf Mylar Capacitor to the two FULL BK-IN switch terminals
closest to the front panel.

Nothing like re-inventing the wheel.

de Paul, W8AEF

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