[TenTec] Omni VI Mystery

[Steve Hunt]

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