thanks to Paul, W9AC for sharing this information

   
The ALC design in the Omni Six Plus results in a very steep leading waveform edge.  The leading 
edge is less than 1 ms and results in very hard keying.  And sadly, the internal waveshape 
adjustment does nothing to affect the leading edge in the Plus version.

I had to modify the ALC circuit by changing values and adding components which resulted in rounded 
corners, symmetrical waveshape, and both the leading and trailing edges are now adjustable.


All changes are conducted on the Omni Six Plus 9 MHz Filter Board:

1) Change C47 from 0.01 to 4.7 uF @ 16 WVDC (or greater) tantalum, positive polarity to D22.

2) Remove the 2.2Meg resistor at R38 and replace with a network consisting of the following: 
0.22 uF film cap in place of R38.  Then, place a 10K resistor in series with C37 (existing 1uf).

Note that the mentioned resistors and 0.01 uF cap are all SMD which makes the mod somewhat of a 
pain.  However, the result will drastically improve the leading edge, while extending the ALC time 
constant.  The ALC extension is only noticeable when turning the power control down to zero and then 
back up again.  It takes the ALC about three seconds from full power CCW to restabilize.  This 
doesn't affect operating, but thought I would mention it.  After the mod, the leading and trailing 
edge should now show change as the CW waveform control is adjusted.

Please validate these components on your version of the 6+ schematic before proceeding.

-Paul, W9AC      


Update from Thomas, AC7A:
The keying mod is a bit tricky as it does involve changing surface mount components on the 9MHz filter 
board. I found that it was easier to use some discrete, through hole components instead of surface mounts. 
For C47, which is changed from a 0.01uF to a 4.7uF tantalum capacitor, I left the 0.01uF on the board and 
then formed the leads of a discrete 4.7uF tantalum to terminate across the 0.01uF. I then bent the 4.7 uF 
over so that it laid to the edge of the PC board. R38 is then removed and replaced with a 0.22uF chip cap. 
The chip cap was much larger in width so I set it on its side and soldered it in place. Last, C37, a through 
hole 1uF electrolytic capacitor has a 10k resistor added in series with it. I removed the electrolytic and 
soldered a 1uF ceramic in one hole, bent it upward, and soldered a 10k 1/4w resistor in the other hole. The 
open ends were connected together and soldered.

I think think this is a very worthwhile improvement to the Omni VI+.  This weekend I had access to a Tek 2465 
oscilloscope and set it up to trigger off the output of my CMOS II keyer. Even though the scope isn't a storage 
scope, I was able to hold the envelope in place when sending a series of high speed dits. What I observed was 
a nearly identical rise and fall time; both about 4 or 5 ms. And no matter where I set the power level the 
envelope was maintained.