Al and all,
I have recently been through the same problem with my OMNI 6+ and the INRAD
2.8khz filters. I had installed both 2.8KHZ INRAD filters in my OMNI last
year and they were working well until this past spring. At which time, I
noticed that a small amount of carrier was present in my transmitted signal
on USB . This was a particular problem because I use the OMNI 6+ to drive a 6
meter transverter and then a six meter amp, which wound up giving me 30 watts
of carrier and 100 watts of SSB !!!! ( a horrible signal to say the least).
Preliminary attempts to rebalance the carrier in the OMNI 6 resulted in the
ability to find a razor sharp null point, which unfortunately would not
stabilize for more than a few minutes, making it impossible to set the
carrier balance alignment. My initial discussions with Ten Tec suggested that
I replace the balanced modulator chip, which I accomplished. However, that
replacement resulted in no change in the circuit performance. A second
discussion with Ten Tec provided a very interesting comment when I mentioned
that I had replaced the stock Ten Tec 2.4 khz filters with the INRAD filters.
Ten Tec stated that their filters have 20db of carrier attenuation built into
the filter design which INRAD 's filters do not have !!!!
A quick call to George, W2VJN, owner of INRAD (and expert RF engineer !!!)
confirmed what Ten Tec had told me. Sure enough, replacing the INRAD filters
with the stock Ten Tec filters put the rig back in tip top shape and no
carrier on the USB transmitted signal !
However, using the Ten Tec filters defeated the purpose of the INRAD filters,
which is to improve the quality of the receiver audio. George, W2VJN, at
INRAD, offered to check both 2.8khz INRAD filters for me and provide an
analysis. He did this very promptly and could find no significant difference
between my 1 year old filters and a "test standard" that he maintains in his
shop. However, he suggested that since the most significant filter (audio
wise) was the 6MHZ filter and that the carrier suppression is driven in the
9mhz circuit, that I retain the 2.8Khz filter in the 6mhz slot and replace
the 2.8khz INRAD filter at 9mhz with a 2.4khz INRAD filter. This was
accomplished and the radio is working beautifully. The carrier balance
adjustment has a reasonable null point which is not razor sharp and has been
stable for several weeks now.
The OMNI 6+ receiver audio sounds great as I have the INRAD filters plus the
INRAD audio mod installed....approaching my Collins gear more and more.
While this narrative does not answer what was wrong with either the old
2.8khz - 9mhz filter, or what shifted in my OMNI 6+ to no longer tolerate the
INRAD 2.8khz filter, it did solve the problem very nicely.
Both INRAD and Ten Tec are to be commended on their technical support of
their customers. I hope that this information is helpful to all.
73,
Dave, K2DP
St. Louis, MO
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