[TenTec] OMNI VI Chirp Report

[VE1BN]

Paul -

I understand T-T changed the transmit audio/BFO board crystal frequency 
adjustment from 9.000700 mHz to
9.000400 mHz.  Apparently this was done as part of eliminating the small 
chirp.

Moving the F/W chips around did the job for me even if you think moving them 
around is not the solution.

I don't have clicks or chirps.  Want to check with me?

73 -  Don,  ve1bn at eastlink.ca



----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: "N4YG" <n4yg at comcast.net>; "Discussion of Ten-Tec Equipment" 
<tentec at contesting.com>
Sent: Sunday, February 17, 2008 12:23 PM
Subject: Re: [TenTec] OMNI VI Chirp Report


>> 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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