[TenTec] OMNI-VI problem

OTAKEBI at aol.com OTAKEBI at aol.com
Mon Oct 5 10:17:34 PDT 2009

I bet if you check SWR on transmit it will be off scale.
I would also get an AM station at times.
Check that PLL board and the small board next to it.
If I remember they are under the FM board.
Do not transmit with full power to check the SWR.
Good luck,
In a message dated 10/5/2009 9:36:43 A.M. Eastern Daylight Time,  
K8mp at aol.com writes:

Thanks Ken. Sounds like you have gone a round or two  with  OMNI VI's in 
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 

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 
making me think the voltage to the  xtal oven has failed. I did some  
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  
> changes when you  crank the tuning knob, but it's  not actually  changing 
> RX  freq and I suspect it's  the same with the TX freq.   Changing the  
> has  no effect either.
> If I change bands,  the RX freq  does change. For example, if I switch  
> 160, I  hear what  I believe is an AM broadcast station. However,  the 
> 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 
> 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 

> other bands must  be stuck on obscure  frequencies.
> Master reset didn't   help.
> Thanks, Joe, K8MP
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