[TenTec] Omni Vl Option 3 SSB TX Problem

[Bob McGraw K4TAX]

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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