[TenTec] Omni VI BFO crystal replacement and alignment ?s...

[John Clifford]

Greetings,

I have an Omni VI/Option 1, #11A10614 that I am trying to align after
replacing the crystal oven with a Ten-Tec-supplied TXCO and ICM-supplied 5
ppm thermally-stable BFO crystals (Y1 and Y2 on the TX Audio Board -
Revision 'F' on my rig).

Replacement/installation on these parts was straightforward.  However, I
have some questions when it comes to alignment, and when it comes to crystal
values.

David Hammond mentions, in his post describing the procedure
(http://lists.contesting.com/_tentec/2001-June/020448.html) that his
original crystals were 9.000000 Mhz (LSB) and 9.003000 Mhz (USB), while the
first set of replacement crystals received from Ten-Tec had a USB crystal at
9.003300 Mhz.  David remarks that this differentiation between original and
replacement Y2 crystals was part of a mod (factory-approved or created?) to
reduce CW chirp, and he saw no need for it.  David goes on to mention that
he ordered new crystals from ICM for 9.000000 and 9.003000 Mhz, installed
them, and went happily on his way.

My crystals in my rig were 9.000000 and 9.003300 Mhz for LSB/USB, and there
was also an additional 16pF capacitor ('mystery cap') that was evidently
used as a padding capacitor to augment the trimmer caps for CW/USB/FSK,
mounted between C16 and C13 on the TX Audio Board.  There is silkscreen on
the board where this cap goes, and the mounting points for this capacitor
were etched with the board, so I am assuming that this was done by the
factory.  However, when I talked to Paul at Ten-Tec support, he told me that
the factory NEVER used USB crystals at .003300 for the Omni VI -- that is an
Omni V value -- and knows nothing about the use of this crystal for an Omni
VI USB BFO crystal nor do they know anything about any mods to reduce chirp
via the use of this crystal.  He also says that the additional 16pF padding
capacitor, while used on the Omni V (in a different version of the TX Audio
Board... the circuits are very similar), was never used for the USB crystal
on the VI and there was never a factory provision for it!  Hmmm... my board
sure looks factory, and has factory pads for both the USB and LSB crystals
(two, in fact, for the LSB crystal, one used by the factory with an 18pF
padding cap, and one extra space that is currently not used).  Also, the USB
crystal had Ten-Tec part #s on it.

When I looked at the underside of the board during crystal
removal/installation, there were NO signs of after-marked mods (rosin
'stains').  It looks like I am the first hacker to touch this board... but
not according to the factory.

Here's my problem... I can't swing Y2 enough to set the BFO freqs to the
correct frequencies in any of the Y2 modes (USB/CW/FSK Mark/FSK Space).  The
problem I am running into is that I evidently do not have enough capacitance
in the respective trimmer caps.  I tried removing the 'mystery cap' to see
if that made a difference, and it doesn't.

For USB, I should be able to get 9.003000 Mhz... the closest I can come to
is 9.003090 Mhz (lowest) and up to 9.004500 Mhz +/-.

For FSK Mark and Space, I should be able to get to 9.002125 (Mark) and
9.002295 (Space).  Both trimmer caps move the freq about 20 Hz at most, and
center around 9.000000 Mhz.

For CW transmit/TUNE, I should be able to set the BFO freq to 9.000400.  I
get a range of 9.002500 to 9.001890 Mhz.

Note that removing the 'mystery cap' did not make much of a difference in
what BFO freq I can set via Y2.

I did check to see that Y1 and Y2 were in the 'right' places... and they
appear to be.

Hints/suggestions/etc., would be appreciated.

EXTRA:

I also tried to align the master freq reference via the TXCO attached to the
logic board (I replaced the crystal oven with this).  There were no clear
instructions in the manual... so I tuned the radio to WWV, put it in CW
mode, and then zero-beat the carrier by adjusting the trim cap integral with
the TXCO.  The sidetone and the WWV carrier are within 1 Hz now.

According to Ten-Tec, this won't work... I have to set the TXCO to exactly
10 Mhz using a test point on the board.  Why? (Why won't this work?)

Thanks,

 - jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1

email: kd7kgx@arrl.net