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Re: [TenTec] OMNI VI Chirp Report

To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Subject: Re: [TenTec] OMNI VI Chirp Report
From: "Paul Christensen" <w9ac@arrl.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 17 Feb 2008 18:57:17 -0500
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Don,

> 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.

First, not every Omni VI/VI+ exhibits the chirp phenomenon -- either through 
evolutionary changes, or component tolerances that allowed some Omnis to 
sound just fine.

Next, check your data as it is not correct.   The TX CW BFO frequency was 
originally 9,000.400 kHz.  It is possible that it was later changed up in 
frequency, but not down.   You state that it was changed downward.  Check 
your source and get back to us.

The 2.4 kHz filter is centered at 9,001.500 kHz -- leaving a margin of error 
of just 100 Hz.  Depending on filter tolerances, I believe this contributed 
to much of the perceived chirp on most (but not all) Omni VI/VI+.  In fact, 
it was myself who appealed to George, W2VJN to produce a 2.8 kHz @ 9 MHz 
filter for the sole purpose of ensuring that the CW BFO frequency had a 100% 
probability of falling safely within the filter's passband.  But that was 
only part of the overall solution.  Some BFO crystals perform better than 
others and my original crystal performed poorly.  Add all this to the fact 
that the ALC circuit was modified in the VI+ generation and you have a 
transceiver that produced leading-edge clicks that exacerbate the chirp. 
Did the F/W fix some units?  Perhaps it did.  Was it 100% effective for 
everyone?  Absolutely not.  Again, the best approach is to look for the root 
of the problem and solve it there rather than cutely masking it.

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

I am not trying to provoke a fight, Don.  I just let the data speak for 
itself.  If you have a an Omni VI/VI+ without any clicks or chirps, then 
goody gum drops for you.  If you have done nothing to the ALC circuit, I 
would be highly suspect of the click situation, unless TT had made an 
affirmative hardware change that I am not aware of.   The ALC design change 
was so aggressive that I would find it difficult to believe that it was 
solved solely in F/W.

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

I'm on 40M right now.  Give me a frequency and I'll be happy to give it a 
listen.  If you have no chirp or click problems, I will give you an honest 
report.  But if you, I'll be equally honest.

73,

Paul, W9AC


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