Tony Berg writes...
> Various methods have been used to lower the Omni V's audio gain. If
> you go to www.tentecwiki.org and click on the site map and then
> 562audmod, you will find some info (which you may be aware of). A note
> at the bottom references R34 in the notch filter circuit being changed
> (to 100k) to compensate for decreased gain in the associated mod. With
> the mod, R34 can be left as is (33k) for reduced gain. I am using 10k
> for R34 in my Omni V for even less gain.
Closer inspection of the Wiki modification revealed why I missed it.
It was added a little over a month ago--after I gathered up
information from that source. I should have checked back before
heating up the soldering station.
The factory-approved mod for lowering audio gain in the Omni V is to
shunt the audio signal to ground through a resistor. The audio path
comes back from the AF gain pot and the resistive pad on the Upper Pot
board to the IF/AF board through Harness 55. That input connects to
the input from the monitor, and then becomes the input to the first
op-amp in the amplification of audio to speaker levels. The mod
involves inserting a 3.3K resistor from the monitor return line where
it meets R79 and C51, and tying the other end of the resistor to
ground.
I found that a 3.3K resistor helped but did not do enough. Perhaps it
would have been enough had I gone back to a lower resistor value at
R34 as Tony recommended above, but that would have required removing
the board. Instead, I used a 1K resistor, tying one leg to the leg of
R79 and the other leg to the ground field on the PCB. As mentioned in
the Wiki article, that can be done on the top of the board without
removing it. This had about the same effect as adding resistance to
the audio signal at the resistive pad on the Upper Pot board, and it's
MUCH easier to get to, requiring the remove only of the bottom cover
and the inner cover.
These are separate from the mods first recommended by W3TS. Those mods
are designed to reduce IF intermod in the passband and to slow down
the AGC a bit to damp pumping. Those mods are good, too.
Now, the audio amp starts to clip with S-9 signals (AGC on, of course)
at about 12 o'clock on the AF control, but the clipping is not really
objectionable until 1 or 2 o'clock. I was able to turn an S-2 signal
(over about S-1 band noise) up all the way, at which point it was
uncomfortably loud in headphones, and was just starting to clip. I
call that just about perfect.
To get the Omni audio any louder without clipping requires external
amplification, assuming I don't have a fault in my IF/AF board.
For the first time, I'm really happy with the SSB audio out of the
Omni V without external amplification. I liked it before, but only
when I just cracked the AF on and then amplified externally. The audio
amp seems like it is just being over-driven.
And now, for the first time, I have really be able to figure out how
to make the bandpass and fade controls work. I was able to increase
the bandpass control, lower the PBT control, which together tightened
the passband, and then mix it into unfiltered audio gradually using
the fade control. The result was natural-sounding voice with very
narrow filtering to go along with the mountains of IF and roofing
filters on my radio. And for the first time, backing the RF gain down
to lower static crashes and band noise and compensate by increasing AF
worked as I expected.
Thanks to all who provided advice.
Rick, KR9D
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