Hi Everyone,
Recently I changed the 2 variable resistors (100 and 1K ohms) that are used
to null the carrier during SSB operation in the balanced modulator circuit
on my Omni VI from the stock single-turn circuit board pots to multi-turn
units. This was discussed on the reflector earlier in the year. Great mod.
The result was a carrier null that was more precisely tuned, more stable
over time and equal between USB and LSB. To adjust the null after surgery, I
removed all audio, fully reduced the rf power and keyed the transmitter with
a remote PTT switch. I adjusted looking for minimum forward power on a watt
meter in the 20 watt range using the forward power position and in the
calibrate mode. Doing it this way, I have achieved good results over the
years on many rigs. With no scope available you get creative.
With success on the Omni VI, I decided to repeat this mod on my Omni V.
After successful surgery and parts substitution, I attempted to null the
carrier but noticed that a good deep stable carrier null was achievable on
only one sideband at a time. A good deep null on USB resulted in an unstable
null on LSB with an observed carrier level that increased slightly but
noticeably in power output during the duration of the key-down. After
returning to receive momentarily and then re-keying the transmitter, the
same result repeated. This opposite sideband carrier "run-up" reached a
maximum level plateau after about 2-3 seconds. Adjusting to achieve a good
null on the opposite side band just reversed the situation. I should add
that this effect was observed earlier on this rig BEFORE switching the parts
so I don't think my modification caused or amplified the condition.
While the escalating carrier on the opposite sideband is very low in actual
power output (I estimate a fraction of a watt, maybe more) I would like to
achieve good balance between USB and LSB carrier suppression as I did on the
Omni VI. I have also replaced the balanced modulator chip CA3053 with no
improvement or change.
Any suggestions as to what is going on? Would a bad SSB filter in the MHz
IF be the cause? (I am using a 2.8 kHz INRAD in both rigs at the 9 MHz IF
but all audio reports on SSB Tx (USB and LSB) are outstanding. The Rx audio
is the best of any rig I have ever owned bar none. I love this rig. (using
the N4PY V.9 mod too.)
Is this worth pursuing or am I chasing ghosts?
Thanks for any input. Stumped for now.
Happy Holidays to all.
73 de N1LQ-Dave
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