Hi Joe,
I have not seen the problem you describe. I have perused the local
oscillator scheme of the Omni VI enough that I think I can make some
good guesses about what is happening.
The Omni VI has a 5.0 - 5.5 MHz synthesizer (actually it goes from
about 4.97 to 5.53 MHz) which is heterodyne converted, mixing with the
band select crystal oscillator, to become the local oscillator. The
frequency readout is not based on a frequency count of any of the
components that produce the LO, but rather is a based what the
synthesizer is being told to generate by the microprocessor, along with
some arithmetic which includes the crystal oscillator frequency and the
BFO frequency. The synthesizer does not actually have to be generating a
signal in order for the microprocessor to have a number to use in the
equation that calculates the displayed frequency. Since changing bands
does change what you actually receive, I am guessing that the band
selecting crystal oscillator is working. My guess is that the 5.0 - 5.5
MHz synthesizer is either not putting out, or it is not making it to the
XTAL OSC, LO MIXER BOARD. This could be caused by a loose or dirty
connection at connector 85 on either the 81599 or 81595 board.
Without the 5.0 - 5.5 MHz synthesizer, your LO is effectively just
the XTAL OSC leaking through the balanced mixer U1 on the LO MIXER BOARD
(balance is not perfect). My guess is that the signal you are hearing
when set to 160 meters is at either 7.300 MHz (or close to there) or it
is some harmonic of a signal that ends up near 7.300 MHz, such as 1460
kHz. There are other IMD products possible, and if your mixer is under
driven by lack of proper LO power, the unwanted mixer products are
probably worse than usual.
What test equipment do you have available? Can you look for the 5.0
- 5.5 MHz synthesizer output with a high impedance probe and a frequency
counter or an oldsillyscope at various places on the 81595 and 81599
boards? Or look for the final LO output from the 81595 board and see
what frequency it is or whether it changes.
Good luck and have fun,
73 DE N6KB
K8mp@aol.com wrote:
>
> Anyone seen an OMNI-VI do this?
>
> Symptoms:
> The rig appears to be stuck on the same QRG all the time. The freq display
> changes when you crank the tuning knob, but it's not actually changing the
> RX freq and I suspect it's the same with the TX freq. Changing the RIT
> has no effect either.
>
> If I change bands, the RX freq does change. For example, if I switch to
> 160, I hear what I believe is an AM broadcast station. However, the tuning
> knob still has no effect. I'd like to be able to ID that station, to find
> out
> its freq. That might give me a hint as to what has failed. Its audio is
> unintelligible because I can't tune to zero-beat.
>
> By the way, 160 is the only band that I can hear a signal. I suspect the
> other bands must be stuck on obscure frequencies.
>
> Master reset didn't help.
>
> Thanks, Joe, K8MP
>
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