My earlier response was done while enjoying my coffee on the deck.
Now with schematic in front of me, here's my suggestions. First,
electrolytic capacitors are known to decrease in value as they age.
For this reason, due to the age of the radio, I'll suggest specific ones
to replace. All of these are located in the transmit audio path, and
found on the TX audio board.
C34, C35, C37, C38, C39, C41, C42, C43, C44, C45 - 1 mF @ 30V
C46 - 33 mf @16V
The description of the level increasing with MIC gain and SP are
consistent with the signal being generated in the early stage of the TX
audio. The multiple signals observed are typical of harmonics of the
less than pure primary oscillation. I would not suggest deviating
from the original design values either in MFD or voltage for these
capacitors. It is likely easier just to do a complete replacement of
these as opposed to finding out which one or ones are contributing to
the issue.
73
Bob, K4TAX
On 9/9/2019 6:23 AM, Barry Simpson wrote:
I posted the message below some weeks ago. Not having had a response, I am
repeating it again below with a few additional comments.
My Omni VI Option 3 has a problem on SSB transmit whereby there is a
permanent carrier about 1.25kHz from the actual carrier frequency and
within the filter passband. It was first brought to my notice by members of
my local 80m net who could hear it in the background of my transmission.
I have found by monitoring on another rig that it is the same on either USB
or LSB. There is a second carrier at 2.5kHz on the edge of the passband but
at a lower level than the 1.25 kHz signal. On FM transmit as seen on the
bandscope of my TS990, the unwanted signals can be seen on either side of
the FM carrier and do not stop at the 2.5kHz spurious signal. Rather there
are four or five spurious signals on either side at 1.25kHz intervals
gradually diminishing in level. Their level increases or decreases as the
mic gain is varied.
The spurious signals are there whether a microphone is connected or not.
However, with no microphone and the mic gain at minimum, they are very low
level but the 1.25kHz signal can be clearly seen on my TS990 bandscope and
heard with its wavering sound. It increases with the mic gain and if the
processor is turned on. With the microphone connected it gets even stronger.
I am not sure when it first started as I mainly use the Omni on CW. I have
done a lot of "fiddling" with the Omni including correcting the various
oscillator frequencies (I do have a good frequency counter), installing new
IF filters and, most recently, removing, cleaning and refitting most of the
connectors on the various boards.
I have checked and rechecked that all the connectors are properly seated
and I have also taken the top and bottom covers off as well as the plate
underneath covering the various boards in an endeavour to trace the source
but nothing I do affects it and there are no hand capacity effects on its
level. I cannot trace the source of the spurious signals and I am beginning
to wonder if they are in fact intermodulation products resulting from an IC
or other component failure.
At the moment I am completely stumped and any help or advice as to the
cause of the problem would be very much appreciated. Incidentally the noise
blanker does not work at all but I would not think that this is related to
the problem I have described.
Thanks in anticipation of some enlightenment.
Barry Simpson VK2BJ
welcomed.
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