[TenTec] Improving Frequency Stability of Older Omni VI's
pywacker@fuse.net
pywacker@fuse.net
Sat, 2 Jun 2001 09:12:58 -0400
Dave, very excellent info. Being one here who is a slickler for stability I
also have used crystals from ICM and found them to be excellent. In years
past I used ICM crystals to improve the stability on a Yaesu 757 and also an
Icom 735.
One nice thing I have noticed with later TenTec's, the Jupiter and the
Pegasus appear to be pretty stable.
Pleasure meeting you at Dayton after so many years.
73 de KE4WY Jim
----- Original Message -----
From: "David Hammond" <dhhdeh@concentric.net>
To: "tentec-qth.net" <tentec@qth.net>; "tentec reflector"
<tentec@contesting.com>
Sent: Saturday, June 02, 2001 8:50 AM
Subject: [TenTec] Improving Frequency Stability of Older Omni VI's
>
> 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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