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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