[TenTec] Help with Omni VI+ please

[Ken Brown]

Hi John,

You need to be looking at TX AUDIO BOARD (81597) U4 is the balanced
modulator. R3 and R4 are the balance adjustments.  Don't start messing with
them right away, because I suspect something else is at fault.

You can see on connector 60 where the USB and LSB control lines come in to
select Y2 or Y1 crystal for the BFO. The mixer balance should not change
enough between. BFO frequencies of 9.000 MHz and 9.003 MHz to cause
excessive carrier output on one frequency and not on the other. There is
another cause of imbalance of the balanced modulator that is used
intentionally for CW or RTTY transmission. Diodes D5 and D6 and transistor
Q2 are used to unbalance the balanced modulator and let BFO carrier through
in CW and RTTY modes. If there is some kind of bridge between the LSB
control line and the RTTY T or CW T lines the modulator would also be
unbalanced when in LSB. I suggest you use a digital multimeter or an
oscilloscope (in DC input coupling mode) to monitor the voltage where D5,
D6 and R13 all connect together. You should see that voltage go high in CW
or RTTY transmit mode and not in any other mode. I suspect there is a fault
making it go high also in LSB transmit mode.

There is another possible cause of more carrier feedthrough on one sideband
and that is dis-symetry of the sideband selecting filter and possible
incorrect BFO frequency in one of the SSB modes. This would not likely
cause the carrier power levels you are reporting, so I doubt this is the
problem.

Let me know what you find with the voltage at D5, D6, R13.

Ken N6KB

On Wed, Jan 19, 2022 at 4:34 AM Radio AI2Q <ai2q at roadrunner.com> wrote:

> John, check your balanced modulator circuitry.
>
> Vy 73, AI2Q, Alex
>
>
> On 1/19/2022 8:59 AM, John wrote:
> >
> > My Omni 6+ has developed a problem.  Here's the scenario
> >
> >
> > All bands
> >
> > LSB only
> >
> > Any power level setting
> >
> >
> > Keying the rig (pushing PTT) in LSB results in RF out even though the
> > mic gain is all the way down.  The level of RF is about 10% of the
> > setting of the POWER OUT control (ie: set wide open, I see about 9
> > watts out when the mic is keyed).
> >
> > This does NOT happen when USB is selected.   Radio functions fine
> > otherwise.
> >
> >
> > Any suggestions on where to look to solve this ?
> >
> > Thanks! I need my beloved Omni back and running well!
> > :-)
> >
> > John  K5MO
> >
> >
> >
> >
> >
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>
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