[TenTec] Omni VI 30m frequency jump fix

[Peter A. Klein]

Steve Ellington and TenTec list:

Some time back I reported that my Omni VI had a periodic frequency shift
on 30 meters.  Every 25 seconds or so, the received frequency would jump
about 100 Hz higher in pitch, and then slide back to where it had been.
Someone else mentioned having seen this tendency in other Omnis, and
Steve asked me to document how I fixed it.

I finally got to do the procedure that Ten-Tec service recommended, and
I'm happy to report that it did clear up the problem.  

As described by Ten-Tec service, the periodic frequency jump is due to a
crystal oscillator that has been adjusted a little too far to lock in
properly.  The fix is to tweak L19 on the "Xtal Osc./L.O. Mixer" board
1/4 turn in whichever direction eliminates the problem.

Warm the radio up for at least 1/2 hour.  Turn it off and remove the top
cover.  Looking down at the top of the Omni VI with the front panel
toward you, the oscillator board is in the left front quadrant of the
radio.  To get at the oscillator board, you have to remove the aluminum
cover on which are mounted two other boards.

Disconnect whatever connectors you have to so that the cover can swing
out of the way.  Draw a sketch of what you've disconnected with the
proper number/letter of the connector to facilitate getting everything
back together again.  The Ten-Tec tech said to "rotate the cover to the
right."  I found it easiest to flip the left edge of the cover up and
over to the right side of the radio.  He also said that I could leave
the radio on, but I was a bit nervous about having connectors flapping
about and possibly shorting to ground while I worked.

Once inside, you'll see a bank of crystals along one edge of the
oscillator board. Each crystal has a small slug-tuned coil next to it.
L19, the 30m coil, is the third from the right.  The diagram is on page
4-25 of the Omni VI manual, but the picture is upside down from the way
you're looking at the board.  Turn the coil 1/4 turn clockwise.  Replace
the cover, and reconnect the connectors, but don't screw in the screws
yet.  Turn the radio back on, warm it up for a couple of minutes, and
see if the problem has been fixed.  If not, remove the cover again, put
the slug back where it was, and turn it a 1/4 turn counterclockwise.

Counterclockwise did the trick for me.  I went a little more than 1/4
turn.  The radio now reads 10.000.04 when the carrier is tone-matched
with the CW sidetone in CW mode, or in SSB mode when the Omni's audio
tone-matches another receiver tuned to WWV in the AM mode.  So I now
have a 40 Hz error (BIG DEAL!), but the frequency is stable.  Next time
I'm in the radio, I may back the slug off 1/8 turn and see if it stays
fixed with less error.  For now, I'm happy.

The preceding is provided as a helpful hit for fellow Ten-Techies.  I am
not a professional repairman, nor do I play one on TV.  Batteries not
included.  Your mileage may vary.  Void where prohibited.  So there :-)

73,
Peter - KD7MW
---
* My opinions do not necessarily represent those of Seattle University.
* Newsreaders may have an incorrect return address. Use pklein@seattleu.edu

Peter A. Klein  (pklein@seattleu.edu)  :    -----==3==      ---      ---
Network Administrator, LAN/WAN/Novell  :   |    |  |  |    |   |    |   |
Seattle University, 296-5569           :  @|   @| @| @|   @|  @|   @|  @|


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