[TenTec] OMNI VI Chirp Report

[Paul Christensen]

> 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