[TenTec] Omni VI INRAD 2.8 KHz filters
Rob Atkinson, K5UJ
k5uj at hotmail.com
Tue Nov 4 01:31:27 EST 2003
I was out of town all last week so am getting caught up reading old TT
email. About the Omni VI 2.8 Kc filter question, I have installed the inrad
2.8 KHz filters in the two IFs in my Omni VI/3. I have not yet done the
rest of the rx audio mod (waiting for a slow winter weekend). The other
optional filters will not change your tx bandwidth if you keep them in the
optional filter slots. Get the manual and spend some time going over the
schematics and you will see what's happening. Yes, the selectivity will be
worse with the 2.8s--you are putting in a wider filter. On rx, there is
some slight audio improvement but don't expect everyone to start sounding
like 10 KHz wide broadcast stations. For one thing, a lot of hams don't
transmit highs and lows to begin with. The biggest single rx audio
improvement I've done is to simply hook up a couple of big ten inch 8 ohm 10
watt speakers in parallel to make a 4 ohm load. That made the rig sound a
lot better compared to the sound coming from the small power supply speaker
(but for its size, it wasn't bad).
Installing the filters is a snap. the tricky part is that you will have to
re-adjust R3 and R4 on the balanced modulator board to renull the carrier.
You will have to at least run the rig into a dummy load, key it on LSB with
the mic disconnected and the PA Ic pot up a bit ( maybe around 9 o'clock)
and the mic gain pot up to 8 o'clock and find the carrier on a separate
receiver in the shack--a good ham band reciever if possible and adjust the
pots to null as much of the carrier as possible. Then repeat the process on
USB. You will have to go back and forth and spend some time getting it as
close to null as possible with a minimal power spike on tx because the stock
single turn pots are a pain for this so the null is real sharp plus the
filter's higher skirt is closer to the carrier. Some hams have replaced the
stock pots with 10 to 25 turn trimpots (I hope you have thru-hole boards)
which are available from Mouser for a few bucks each, to make it easier to
find a deep null.
For a company that is known for cw rigs, Ten Tec also makes great sounding
phone gear which I don't think they get enough credit for. I hear a lot of
great sounding Omni VIs, provided they have been installed correctly, and
the wider filter with a good mic and some high and low boost makes for a
nice improvement. It is important to tune up correctly (if running an amp),
watch the ALC and monitor your tx signal on a separate rx or better yet with
a scope especially right after changing the filters to make sure you're
sounding okay. Transmit on a quiet band like dead ten meters with full
power and have a ham a mile or two away listen and see if he can detect any
carrier coming through.
Someone else said the Omni VI was not originally designed for "hi fidelity"
(or whatever you want to call it) SSB and I agree. 2.9 KHz bandwidth is the
max you will ever get with it. That should be enough for anyone but if you
have not bought one yet and just want a good ragchew radio you're probably
better off with a newer Jupiter, Pegasus, or TS-870, all of which are newer
designs with dsp produced audio and they sound really good.
Rob Atkinson
K5UJ
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