[TenTec] Omni Vl Option 3 SSB TX Problem
Bob McGraw K4TAX
rmcgraw at blomand.net
Mon Sep 9 10:20:59 EDT 2019
Also check to see if the back panel mike/line input is connected to any external device. There is no switching between the front panel MIC input and the rear panel input. Only some degree of resistive isolation. In other words, both are active all of the time and the level is controlled by the MIC gain control.
Sent from my iPhone
> On Sep 9, 2019, at 8:58 AM, Gary Follett <dukeshifi at comcast.net> wrote:
> A 1.25 KHz square wave is being coupled into the microphone preamplifier. That is why you are seeing the Fourier spectrum of the 1.25 KHz square wave, which is modulating your SSB signal.
> If you leave the mike gain up but short the mike input connector to ground, do you lose the stray signal?
> The only sources for square waves in the radio would be in the various digital stages, which all produce square waves.
> I have to leave right now for a few hours or I would look myself but dig through the schematic to see if you can find any signals with a 1.25 K Hz fundamental.
> Do you have an oscilloscope?
>> On Sep 9, 2019, at 6:23 AM, Barry Simpson <vk2bj at optusnet.com.au> 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
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