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563freqjump [2011/07/21 14:13] (current)
n5na created
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 +====== Omni VI Frequency Jump Fix ======
 +
 +**From the Ten-Tec Reflector August 13, 1998**
 +
 +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 from KD7MW
  

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