> Here's a question for those who have owned both the VI and the VI+... I
> understand that the VI+ has a different TX audio board, and uses the
> crystals differently. Anyone who has/has had both care to comment? I'm
> particularly interested in whether the LSB crystal is used for CW transmit
> on the VI+. I also understand that there was a mod (maybe not factory
> approved) that changed which crystals were used for what modes... anyone
> know anything about that?
That's right. In the Omni Six Plus (not, the upgrade...but the real deal),
the TX BFO crystal switching scheme is different. The theory is that in CW,
the LSB crystal would require less of a frequency shift and therefore would
exhibit less "microchirping." While Ten-Tec was working on this, I was
independently and unknowingly making the same changes to my Omni Six (with
upgrade #3).
I found that shifting the LSB crystal made a beneficial difference, but in
my opinion, the root cause was in the weak type of BFO crystals used. I
had a pair of custom 9 MHz crystals made from JAN Crystals in Ft. Myers, FL
and that single change made the most difference in stabilizing the BFO
circuit and removing the microchirp. I should note here that my change and
the factory's change also requires modifications to the Control Board. The
Control Board directs which BFO crystal is shifted during transmit. One can
make this change to upgrade #3 Omnis by purchasing a replacement 564 9 MHz
BFO Board and the new Control Board from the factory....although you'll
still need better quality BFO crystals.
When Ten Tec created the Omni Six Plus (both the upgrade and the 564), they
tampered with the ALC circuit which resulted in a sharp leading edge to the
CW wave form ( I am assuming to kill any ALC overshoot over the full output
power range). I studied the schematics of the Omni Six, Six Plus, and Omni
Five, and created a new R/C time constant which restores the edge symmetry
without introducing overshoot. Ten Tec must have known about the change to
the leading edge: in later releases of the Omni Six Plus, the adjustable
waveform symmetry potentiometer is missing from the Control Board and is
replaced with two fixed voltage dividing resistors.
-Paul, W9AC
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