[TenTec] Omni VI Mystery

[Bob McGraw - K4TAX]

That is one source of the many birdies that is typically found in the Omni 
VI and Omni VI Plus series.  As one that used the radio on 10M as the IF for 
my EME equipment, the birdies at or near the noise floor were the biggest 
objection I had with the radio.

I did a lot of work in adding ferrite chokes and bypass caps and shielding 
inside the radio to reduce the 10M birdies.  In many cases I had to make new 
cables being longer lengths required to add ferrite chokes.  Also the 
physical position or routing of some of the cables, I found, would make 
things better or worse.

73
Bob, K4TAX




----- Original Message ----- 
From: "Steve Hunt" <steve at karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Saturday, August 02, 2014 7:30 AM
Subject: [TenTec] Omni VI Mystery


> Ever since I got my OmniVI I've been annoyed by a low level heterodyne 
> whose pitch is independent of the main tuning but which changes rapidly 
> with the PBT setting. There was some discussion of it on this list back in 
> 2006 when I first purchased the radio, but I never got to the bottom of 
> the problem. Having recently taken the covers off  to look at Jim Allen's 
> CW offset issue, this morning I thought I'd take another look at the 
> annoying heterodyne.
>
> It was clearly generated within the PBT board: removing the input to board 
> didn't affect the heterodyne; disconnecting the output removed it. And it 
> was being injected upstream of the IF filters rather than downstream.
>
> The audio pitch of the heterodyne changed frequency at a high rate as the 
> 15.300MHz PBT oscillator was varied - indicating it was probably related 
> to a high value harmonic of that oscillator. I still haven't worked out 
> exactly what was beating with what!
>
> The waveform looked fine leaving VCXO Q4, but was badly distorted and rich 
> in harmonics at the collector of buffer Q3. TT obviously recognised an 
> issue here as there is a LPF ( L4, L5, C12, C13 and C14) between this 
> point and the mixer.
>
> To cut a long story short, tacking a 220pF capacitor between the collector 
> of Q3 and GND cleaned up the waveform significantly without reducing the 
> drive to the mixer. And the heterodyne has disappeared!!!
>
> Anything much less than 220pF didn't fully remove the heterodyne; anything 
> much more than 220pF reduced the drive to the mixer. YMMV!
>
> I don't usually like this type of "patch"; I much prefer to fully 
> understand a problem and arrive at a properly-engineered solution; but in 
> this case I don't really have the time to pursue it further. During the 
> 2006 discussion, several OmniVI owners mentioned they had the same issue, 
> so I'm posting this in case it may help them.
>
> 73,
> Steve G3TXQ
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