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Improving Frequency Stability in Older Omni VI Transceivers

From the Ten-Tec Reflector June 2, 2001

My Omni VI vintage November 1996 has served me very well. During this time I had added the Option III Upgrade and a full compliment of INRAD filters including the 2.8 Khz filters at both IF's for SSB. I have also added the INRAD IF audio mod recently released by George at INRAD. All of these changes have produced significant improvements. I recommend them all without hesitation. The Omni VI is a great radio. But I always need to fiddle to improve things!

One thing has continued to bother me with the rig. Frequency stability in the face of thermal changes noted on SSB especially. It drifts as it heats and cools. Mind you not outside of the spec's of the radio but enough to bother me. So I set out on an attempt to improve thermal frequency stability in the Omni VI within my abilities to do the work and procure the necessary parts. I can report that improvements can be obtained with nominal cost and a little fiddling and soldering.

My first job was to research the reflector archives and to my luck I found discussions on this topic between Paul W9AC and Bill N4WC from a few years ago (December 29, 1998). It described Bill's attempt to stabilize the BFO circuit through improved parts substitutions and different values of caps and transistors. I also noticed from photographs of several later model Omni VI+ logic boards that the new TT units now come with a TCXO replacing the crystal oven circuit for the master reference oscillator crystal.

I called TT Service and Don P. was so very helpful in suggesting how I could retrofit my Omni VI with the TCXO mod which is mounted on a small circuit board on the left side wall of the logic board compartment. The TCXO mod was ordered (at a very reasonable cost) and subsequently installed in my radio without much difficulty. Suddenly the very touchy master frequency adjustment of C33 on the logic board is now replaced with the TCXO built in trimmer and it is much easier to set this frequency. A very elegant and effective mod. Glad to be rid of the crystal oven. It was great when new but unstable as it cooked out over the years.

But I still had noticeable drift with temperature variations in the rig. Mostly noted in the change of received tone on SSB. I now suspected the BFO circuit on board 81597. I first tried removing the internal speaker located directly over the board and allowed convection cooling up through the now vacant speaker hole and grill. I did not want to introduce a fan as some have done in this spot. Result: some improvement. Easy to do and reversible for the future.

Next I substituted ceramic trimmer capacitors (available from Mouser) for LSB and USB receive (trimmers C16 and C12) Result: Surprisingly NO noticeable improvement in stability. (It even got a bit worse!). Original trimmer cap types (red color) were ultimately reinstalled. I was disappointed.

I now suspected the general thermal stability of the 2 BFO crystals, Y1 @ 9.000Mhz and Y2 @ 9.003 Mhz.

I ordered replacements from TT and received them in short order. Before installing these replacement crystals in the BFO circuit I realized that TT was now substituting a new frequency for Y2, now 9.0033 Mhz (not the original 9.003 Mhz). I suspect this being done to improve the alleged CW chirp problem documented ad-nauseum on the reflector in the past. After installing these new crystals the BFO thermal stability was not any better than before.

My final attempt came with the help of ICM of Oklahoma City, a commercial crystal manufacturer whose ad is found in most every QST. After spec'ing out the requirements I was looking for and providing them with a sample of the original crystal from the BFO for matching physical size and load capacitance, I ordered two custom-made BFO crystals. I received a new Y1 @ 9.000000 and a new Y2 @ 9.003000. (I decided to go with the original Y2 frequency and not the newer one TT now uses). These were installed with no complication and after alignment have now been in the rig for three days.

Bottomline: ROCK SOLID BFO THERMAL STABILITY.

I have for now chosen not to proceed further with cap and transistor substitutions (Q16) as suggested by N4WC which might add even more to the BFO stability. I'll contemplate these changes later on.

The Omni VI and I are again in love and my fetish with stability has markedly improved. Cost of the ICM custom crystal order? About $45.00.

Please remember that all rigs are made to meet price points. Inclusion of better crystals at manufacture would undoubtedly raise the cost of production and make the rig uncompetitive with others from aboard. I mean no criticism of TT design or production. I just wanted to try to improve things a bit.

I decided to share this adventure with all of you just in case like me you like to fiddle and are seeking improvement on the temperature stability of an already fantastic radio. These are my results. Yours may vary.

P.S. If you do these mods YOU WILL NEED a good frequency counter (unless you are very clever in using alternate means of calibration).

73 de N1LQ-Dave


I have received several e-mails requesting info on the custom crystals that I used in the mod I posted this morning on the reflector.

Here are all the details that I have:

  • ICM Tel.# 405-236-3741– ask for sales department–about a 3 week lead time
  • My order/job number: 53017 (use this for a possible reference)
  • ICM Catalog Number: EX49AF32
  • ICM Description:
    • ICM HC-49/U 32PF (FX=3.500-9.999 Mhz) Crystal Frequency: 9.000000 MHz
    • ICM HC-49/U 32PF (FX=3.500-9.999 Mhz)Crystal Frequency: 9.003000 MHz

Regards,

N1LQ-Dave


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