[TenTec] OMNI-VI problem

[K8mp at aol.com]

Thanks Ken. Sounds like you have gone a round or two  with OMNI VI's in the 
past. I do have a scope so I can check those sigs you  mentioned.
 
To make sure we are on the same page: The  displayed freq is correct. When 
this failure occurred, the display did not  change. It read 7025 before and 
after.
 
By the way, I identified that station. It's an AM  broadcast station at 
1550. Also, while listening to it, I  noticed the radio is doing a slow drift, 
making me think the voltage to the  xtal oven has failed. I did some voltage 
checking around the  DC distubution board. All seemed OK.
I wonder if I lost a voltage regulator  somewhere.
 

 
In a message dated 10/4/2009 8:32:48 P.M. Eastern Daylight Time,  
ken.d.brown at hawaiiantel.net writes:

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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