Mike:
Yes, the 20-meter W1AW signal does not do justice to the Omni Six/Six Plus,
and I do not believe it is representative of most Omnis.
As you know the chirp is correctable, but a considerable amount of effort is
required to get there. Most of my notes are back at the office, but here's
a brief snapshot of what's involved with a late-generation Omni Six Plus.
Older units like my Omni Six, require much more work. Ten-Tec took the Omni
Six Plus in the right direction, they just didn't finish the job. BTW, do
not confuse this mod with another chirp abnormality that has been discussed
here recently. The issue there is noticed on receive and the two are not
related.
1) A new crystal is required on the 9MHz BFO Board. The new frequency for
the LSB crystal should be 9,000.500 kHz. I had mine made from JAN Crystals
in Ft. Myers. The BFO board requires alignment after the mod. The crystal
is now shifted slightly down in frequency instead of up. This type of VXO
really hates to be pulled up. This accounts for, and addresses a partial
manifestation of the chirp, step 2 below cures the rest.
2) The 9 MHz @ 2.4 kHz filter requires modification to widen its bandwidth.
Several alternatives exist. In the stock Omni, the transmitter CW BFO
frequency falls square on the filter skirt, exacerbating the chirp effect.
(See additional notes below).
3) The waveform mod requires changing several R/C time constants on the 9
MHz Crystal Board. Working with SMD devices here is no fun, but the result
is a beautiful, symmetrical CW waveform with absolutely no overshoot. Your
amplifier will love your Omni.
4) The thump mod can be completed at any time. It requires the simple
removal of one plug. That's it.
5) A 10K pot on the Control Board was once in standard production to allow
the user to adjust waveform rise and decay time. In later Omnis, its gone
and a fixed resistor is in its place. The good news is that the holes for
the pot are still on the newer PC boards.
Incidentally, the older Omni V while using nearly the same CW generation
scheme, does not exhibit the chirp problem. Here's why: The Omni V used a
fixed BFO CW offset of 9,000.600 kHz. This frequency falls well within the
9 MHz @ 2.4 kHz filter passband, not on the edge of the skirt like the Omni
Six/Six Plus. Here, the fixed offset is down at 9,000.400 kHz. It may not
seem like 200 Hz is much of a difference, but when you get to the filter's
edge, it makes all the difference in the world. (Ten-Tec take note of
this). Ten-Tec HAD to use this fixed offset for the BFO. Here's why: The
Omni's minimum **software adjustable** CW offset is 400 Hz. The Omni's
software and synthesizer make up for the difference on a CW offset that's
anything **greater** than 400 Hz. The OMNI V's CW offset was 600 Hz. No
adjustment to the CW offset was capable. Period.
Another manifestation of the chirp can be found while observing the waveform
on an oscilloscope. Those OMNIs that chirp on transmit also exhibit a
slightly truncated trailing edge on the CW waveform. Interestingly, the new
Omni Six Plus does not allow the user to make the waveform adjustment (at
least on the new SMD Control Board I received from them back in June).
Instead, a fixed resistor places the Omni at full "hard," where the visual
effect of the chirp is not seen as on the adjustable Omnis.....only heard.
As you can see, it's a lot of work to make the rig sound gorgeous. For me,
it was worth it. Others will undoubtedly disagree. I hope this helps.
-Paul, W9AC
-----Original Message-----
From: Michael Little <kb4kkl@worldnet.att.net>
To: paulc@mediaone.net <paulc@mediaone.net>
To: <tentec@contesting.com>
Date: Saturday, September 19, 1998 11:50 PM
Subject: fix for Omni
>Hi Paul,
>
>I am interested in your TX waveshaping and fix for the QSK thump for the
>Omni VI+. I think you mentioned these in a post to Jim Reid a week or so
>ago. I do not own an Omni at the moment but have been considering getting
>one after seeing your post.
> I have run Drakes since being licensed in 84 - mainly a B-line with a
>R-4C or T-4X pulled into service at repair time. I have a Century 22 for
>mobile. About 8 months ago, it seemed every rig I owned was ailing and
while
>I enjoy troubleshooting and repair, I also like operating - hi. I had never
>owned a brand new rig and thought the Omni VI+ might be a good candidate
for
>the workhorse rig. I was ready to make the purchase when I heard W1AW on 20
>mtrs. You know the rest of the story. I was stunned - I couldn't believe a
>new Omni VI+ was transmitting. I swapped 3 receivers out with various
filter
>combinations - still there. I asked some hams at work for their opinion.
>They all agreed, after listening from their QTHs, there was a distinct
>"whoop" to the audio. I started to post something on the reflector but
>decided against it. Anyway - I kissed the Omni VI+ good bye. I had also
>been following the 1000MP and that's what I bought. I'm sure the MP's no
>songbird either but I figured for the same money I might as well get more
>features (bells & whistles). I like it, though its a somewhat aesthetically
>strange rig. I really like the dual receive. Its got the INRAD 400 and 250
>filters plus the IF mod. I find the RX very acceptable - behind my Sherwood
>mod R-4C but not bad. The one thing that really gripes me
>with the 1000MP is the QSK thump(pop). It's pretty bad - especially with
the
>250 filters in. Sometimes I turn on the notch filter (centered) during TX
to
>get rid of it - defeats the purpose of QSK. The weird thing about my owning
>this rig is that I don't operate SSB - tried it a couple of times but just
>didn't do anything for me.
> Well, to the point - I would love to try an Omni VI+ for CW if I knew I
>could cure the chirp and remove any QSK thump. Can you share with me what
>you did on your Omni?
>
>Many thanks es 73,
>Mike KB4KKL
>KB4KKL@worldnet.att.net
>
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