[TenTec] OMNI-VI problem

[Ken Brown]

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 at 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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