[TenTec] Omni VI INRAD 2.8 KHz filters

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

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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