Rick,
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.
73, Tony W1OT
On 3/24/08, Rick Denney <rick@rickdenney.com> wrote:
> I spent the evening with the covers off the Omni V to try and address
> my audio clipping problem. For those who don't recall, the symptoms
> were clipping at fairly low listening levels by the time the AF knob
> is only about a quarter of the way around the dial. I've seen the
> clipping on the scope.
>
> So, I started poking around the IF/AF board, and eventually solved my
> problem. But I'm a hobbyist, and I would like to run my revelations
> past real experts.
>
> The IF/AF board produces line-level audio. This audio is clean on a
> scope, and it sounds good when I tapped off of it into a separate
> amplifier. This is the audio that goes to the line-out jack.
>
> Line-level audio is fed, via Wiring Harness 55, to the Upper Pot
> board. There, (assuming the Fade control is disengaged completely), it
> goes through two 10K resistors to a pot, the other end of which is
> connected (again, via 55) to the audio "input" back to the IF/AF
> board. The wiper goes to ground. Thus, when the pot is at minimum
> resistance it has the greatest attenuation of the audio.
>
> On returning from the Pot board, the audio chain is fed into the first
> of two audio amps that together drive the speaker. The output of the
> first amp goes to the tone control circuit, which shunts high
> frequencies (using a cap) to ground and acts as a treble cut, and then
> returns the audio to the IF/AF board. Harness 55 does this, too. On
> its return from the tone control, the audio is fed into the second
> audio amp, which drives the speaker and headphone outputs.
>
> It appears to me that the listening level is controlled by attenuating
> the audio input to to the amps using (in the case of the AF pot)
> simple resistance.
>
> I gently bent out the line-level audio output from the IF/AF board so
> that I could connect the harness without connecting the audio input to
> the AF pot circuit. Then, I experimented with various resistance
> values to add to the resistance pad in the AF pot circuit. By adding
> 200K of resistance (yes, that's a lot), I was able to get clean audio
> over about 3/4 of the pot's adjustment range, and then with only minor
> clipping above that, as viewed on a scope.
>
> There may be something faulty about the way the op-amps in the audio
> amplifier are being driven, and I may have just been masking a
> problem. On the other hand, there's probably a working limit to what a
> couple of IC op-amps can deliver, even the heat-sinked package in the
> Omni. I'm thinking it can deliver maybe 2 or 3 watts, which for
> playing radio should be plenty.
>
> I restored the connector pin and added the two 100K resistors in
> series to R1 on the Upper Pot board. I just pulled one pin of R1 out
> of the board, soldered a 100K resistor in the hole just vacated, and
> then soldered another 100K resistor across the free ends of those
> resistors.
>
> The listening level is plenty loud for strong signals, and adequate
> for weak signals, using the speaker. All of it is loud enough to cause
> permanent hearing loss in my Heil headphones. There was no adverse
> effect on the fade and bandpass tuning operation. The two added
> resistors are upstream from the fade control that increasingly mixes
> audio not bandpass-filtered with audio that is. That all still works
> as it should.
>
> (For Rick Williams: I restored all the mods you made, and that also
> helped, but not with the AF level issue. Those mods help avoid
> overdriving the IF amp on the IF/AF board.)
>
> Comments welcome.
>
> Rick, KR9D
>
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