Jerry & Ken
I think your explanations get closest!
The pitch of the heterodyne moves so quickly with a slight rotation of the PBT
control that it feels like it has to be at least the third harmonic, or
probably a higher one, of the 15.3 MHz oscillator that is beating with
something else. The pitch doesn't vary with band, but it varies between USB, CW
and LSB, so it looks like the mixing product must be at 9MHz or 6.3MHz.
The RF gain control affects the level, and the narrower filters cut it out, so
it's getting in towards the front end of the signal chain.
When I've got a free moment I'll monitor the 15.3 MHz oscillator with my freq
counter and compare the shift with the shift in audio tone - that'll tell me
which harmonic it is. I'll also check all the grounding screws.
73,
Steve G3TXQ
----- Original Message -----
From: Dr. Gerald N. Johnson
To: tentec@contesting.com
Sent: Sunday, November 26, 2006 8:14 PM
Subject: Re: [TenTec] Omni VI first impressions
On Sun, 2006-11-26 at 10:03 -1000, Ken Brown wrote:
> Hi Steve,
>
>
> >
> > There was a consensus of replies that the variation of display
brightness, and the "key up thump", seem to be "features" that other Omni VIs
exhibit.
> >
> The variation in display brightness is a feature. Key up thump? Not in
> my Omni VI.
> > At warm-up several heterodynes, one after the other, drift quickly
through the pass band - all of them are non-tunable. Once the rig is stable, if
I'm left with a heterodyne in the pass band it can be tuned out with PBT
control. I don't mean "filtered out" with PBT; just a few degrees rotation of
the PBT control around its mid position and the heterodyne rapidly moves in
audio tone and out of the passband. If the PBT control is more than about 5
degrees away from its central position I never hear a heterodyne at all. I
guess this must be some funny mixing product involving the15.3 MHz PBT
oscillator.
> >
> A very interesting problem. Since it happens on all bands, I would guess
> that the 15.3 MHz PBT oscillator is generating a spurious signal that is
> getting right into the 9MHz IF, or even the 6.3 MHz IF. Come to think of
> it, suppose that the 15.3 MHz PBT oscillator is mixing with the 9 MHz
> BFO, and generating a signal at 6.3 MHz. Changing from USB to LSB would
> move the 9 MHz BFO, and the setting of the PBT required to hear the
> unwanted mix would also move. Does that happen?
>
> So then why would your 9 MHz BFO oscillator or 15.3 MHz PBT oscillator
> radiate, or conduct through undesired paths, more than most other
> peoples? I would look for loose connectors and board mounting screws
> around the TX Audio Board (which includes the BFO) and the Passband
> Tuning Board. What about the shield covers over the optional crystal
> filter access? Are the shields there, with all the screws in place?
>
> DE N6KB
>
When a tiny adjustment of the offset frequency moves the beat out of the
pass band it sounds to me like a harmonic beat. As if the oscillator has
too many harmonics from overdriving a mixer, or as you suggest board
grounding or shielding that isn't working as it should. Most boards
depend on mounting screws for circuit grounds and one of those loose (no
matter who made the radio) can give some weird results. And since the
pressure on the ground connections depends on a screw compressing a
plastic board material, that plastic material does what plastic means
and replaces the pressure. Back long ago when I was at Collins it was
verboten to depend on any plastic to keep pressure on a connection, even
really hard plastic like a Jones barrier strip mounting screw was not
allowed to be used for a ground lug.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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