For those who may be interested, I posted a message a
few days ago about what seemed to be an AGC problem
with my Omni V.
I spoke to Don at Ten-Tec about it and he suggested
I check the noise level (with no ant connected)
between 40 meters and 20 meters, and between 30 meters
and 20 meters. He indicated that if there was an
appreciable difference, it could be the high-level
diode mixer on the band pass board. I checked it, and
there was indeed a very noticeable difference in noise
level between these bands. It was most apparent
between 40 and 20 (more noise on 40), but 30 had a
higher noise level too. 40 had seemed a bit noisy all
the time, but 40 IS noisy, so I hadn't paid too much
attention to it. I ordered a replacement (at a very
reasonable cost I might add) and installed it last
night.
The difference is amazing! Everything is working
like it should! The high noise level on 40 has gone!
Just the usual 40 meter junk now. There was some very
noticeable 'howling' with the 500hz filter in, and
that is nearly gone. I'm sure this is because of the
drop in internally generated noise.
I also noticed the s-meter seemed to either be
pegged or show low levels, there was hardly any time
when a signal would be mid-scale, for example. This
replacement smoothed it out as well. I'm sure whatever
garbage was getting through the malfunctioning mixer
was affecting the AGC and s-meter.
Passband Tuning Adjustment
In another previous post, I mentioned that the
Passband Tuning board in this Omni V had been switched
to one from a Paragon at some point. I compared the
schematics and noted the differences. I have since
found out that crystal Y1 is the same frequency in the
Paragon and the Omni V (at least as far as the
versions I have info on-my Omni manual shows it as
15.302.3, but it's actually 15.301.2). The only
difference is that the Omni is adjusted for greater
range.
Since I've not seen this explained elsewhere,
here's what I did to adjust the Passband tuning
control:
1. Find a good spot to connect the freq counter! The
manual doesn't say, so I tried several places. First
place I tried was the emitter of Q4. Bad spot! The
counter displayed the freq, and I readjusted
everything. After I disconnected the counter and
checked it on the air, it was off considerably.
Basically the counter/cable was pulling the
oscillator. The best place I found was the emitter of
Q7 (easiest place to make a connection is where R44
connects to the emitter of Q7).
2. Turn the Passband control to the extreme that
places the lowest voltage on the varactor. I have to
admit I can't remember which direction this is!
Basically turn it to whichever extreme show's the
highest freq on the counter.
3. Adjust C22 (small trimmer cap) for a frequency of
15.3023 MHz
4. Move the Passband Tuning control to Center.
5. Adjust R18 (trimmer pot) for a freq of 15.300 MHz
6. Repeat steps 3-5. Actually I didn't have to repeat
it, but it doesn't hurt!
Hope this information will be of help!
Anthony, NT4X
=====
NT4X
__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com
|