[TenTec] Improving Frequency Stability of Older Omni VI's

[David Hammond]

Hi Everyone,

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




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