I am wondering if others have received similar reports and if anyone has
changed the keying waveform on the OMNI VI.
Joe:
If you check the TT archives from about 10-12 years ago, you'll see plenty
of my posts on this subject. I spent about six months searching for the
problem and ultimately came up with a solution. In a nutshell, here were my
observations:
- Later Omni 6+ units are more problematic than earlier units;
- TT released an upgraded F/W chip that helps to mask the chirp (not
completely) but at the expense of QSK;
- Omnis manufactured in the early '90s and had not gone through the factory
upgrade seem to sound the best;
- The Omni 6+ and 6+ upgrade radically changed in ALC structure. The
leading edge is no longer adjustable with the wave-shape pot on the Omni's
Control Board;
- At some point in production, TT removed the wave-shape pot and used a
fixed voltage divider as the pot was then useless;
- The root cause is related to several simultaneous issues;
- Issue one is that TT uses a fixed CW carrier set point that rests on the
lower skirt of the 2.4 kHz @ 9 MHz filter. The fact that the 9 MHz CW
oscillator is forced to pass through the asymmetry of the filter is what is
causing a large part of the problem. Square waves (CW) passing through the
asymmetrical edge of a crystal filter results is very strange behavior. TT
needed to place the CW carrier below the filter edge in order to allow for
full ST frequency adjustment;
- Second issue: I found that by experimenting with BFO crystals of various
designed C load and manufacture, that I could control the quality of the
note. I ultimately settled on a crystal from JAN (my notes on this are
lost);
- Third issue: On the N1EU website, you will see a dual-speed ALC loop mod
that I created. The mod brings back symmetry to the CW waveform, while
eliminating any power spike at all power levels. The chirp is exacerbated
by the sharp leading edge rise time. Making the rise and fall times
symmetrical greatly reduces the audible chirping effect;
- Finally, I made a mod on my Omni VI+ that bypassed the 2.4 kHz filter when
in CW Tx mode.
To this day, I can detect an Omni VI+ on the air with near 100% accuracy.
Playing with F/W chips results in moving a problem around. The use of a
scope to detect the problem is but one dimension of a multi-dimensional
problem. The use of an external wideband receiver, PC sound card and FFT
waterfall display will also reveal some interesting results, including the
possibility of more than one tone at a level much lower in amplitude.
Paul, W9AC
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