On Tue, 2008-09-02 at 18:08 -0400, Carter wrote:
> Marinus Loewensteijn wrote:
> > I am looking for some input into solving a (potential) problem that I
> > have with my Omni-C.
> >
> > history: I obtained this transceiver some months ago and it had been
> > sitting in a box for 28 years. Cosmetically immaculate but needed to
> > have all the 100 nF bypass capacitors replaced since these were all
> > cracked.
>
> And I thought I was the only one having that problem. <grin>
>
>
> > I cannot find instructions for adjusting the TX mixer - what is in
> > the circuit diagram in the manual uses a ring mixer in a IC package
> > and what I have is a discrete diode ring mixer with two adjustable
> > potentiometers.
Adjust the pots for minimum carrier at the output in transmit mode with
no audio input. Not even a microphone.
> >
> > Similarly the SSB exiter has an adjustable coil and I cannot find
> > details on adjustment of that either.
Could be a trap or simply a signal bandpass. If a trap its adjusted to
minimize some unwanted signal, otherwise its peaked on the desired
signal.
> >
> > My problem is that after the rebuild the audio is distorted. I wonder
> > if this is due to some fault in the microphone or in the SSB exiter /
> > TX mixer. CW works like a charm and output is fine too. Even when
> > transmitting on CW the current nicely increases / decreases with the
> > modulation, only it is garbled.
Could be an impedance mismatch between the microphone and the radio
(high impedance microphone and low impedance audio), could be the new
parts have raised the audio gain a lot. Maybe it needs far less
microphone gain. Most ham rigs don't need the gain of an amplified
(unless electret) microphone.
> >
> > I do use this rig mainly on CW so it is not a problem but I do like
> > to fix this part. Where do I start?
>
> I'm not an expert on this rig, so let me give a generic answer.
>
> How about putting the rig on a dummy load, then setting the power for
> about 10 watts out with a test tone connected to the mic input and then
> using an oscilloscope stage by stage to see where the distortion enters.
> Hopefully, others will have some info on how to adjust the 'mystery stages'.
>
> Good luck,
> Carter K8VT
>
It takes two tons to have an interesting amplitude display. Otherwise a
single tone has a constant envelope and isn't as good at exercising the
linearity of the stages. The trouble with using the scope with the
single tone and looking for waveform distortion is that a distortion of
the RF sinewaves may be automatically filtered by the transmitter
filtering and so not be a significant source of SSB distortion. The
envelope of the two tone signal can be more effective at showing
distortion.
73, Jerry, K0CQ
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