[TenTec] OMNI-VI problem

K8mp at aol.com K8mp at aol.com
Mon Oct 5 11:58:49 PDT 2009


Thanks for the good info Dan. I have avoided xmiting.  Figured I'd toast 
something.  I'm sure the swr is whacko, but I do have a  dummy load if I need 
to key the rig. Since it's messed up in both RX and TX, I  probably won't 
need to use xmit to repair it.
 
Later,  Joe
 
 
In a message dated 10/5/2009 1:18:59 P.M. Eastern Daylight Time,  
OTAKEBI at aol.com writes:

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,
Dan/N4VET


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 
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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