I have an Omni 6 on the bench now so I can check the “gain” of the 9 MHz board
but showing a 7 dB net loss seems a bit high.
You can rune out the filter (which I have found to be bad in a few cases) by
removing it and putting a wire jumper across the active pins on the board. You
loose selectivity but you test the filter loss.
The filter can be marginal and not affect TX power much because the TX chain in
the Omni 6 has far more than enough gain to produce good output even if the
filter is marginal.
Gary
> On Jan 25, 2019, at 9:19 PM, Phil Erickson <phil.erickson@gmail.com> wrote:
>
> Hi all,
>
> [Sending this to the list instead of directly to Gary and Peter, the
> usual suspects, in case the results are generally useful.]
>
> In the process of buttoning up my repaired (mostly) Omni VI+, I noticed
> further trouble: the MDS as tested was much worse than expected - at least
> 12+ dB less sensitive, not 3 dB or so where I wouldn't care much. It's
> hard to ignore that much of a problem on the higher bands where background
> noise is lower. Using a spectrum analyzer, I've been tracing first through
> the BPF / front end board and then through the 9 MHz crystal filter board
> (#81782, surface mount version) to see where the trouble lies.
>
> Here, I'll stick to the 9 MHz filter board. My tests show that it has
> negative gain: the IF signal loses over 7 dB from input (connector 25) to
> output (connector 38). In fact, it was even worse at 11 dB overall loss,
> but I got 4 dB of that back by adjusting what turned out to be misaligned
> tuned circuits (C3 and C4, optimizing impedance match of the 15 kHz Y1
> "preselector" filter; C10 and C12, resonating the LC bandpass filter).
> These are the only adjustments on this board.
>
> The 7 dB attenuation is using the always-in-line 2.4 kHz bandpass filter
> - i.e. no narrower filters switched in through N-1 or N-2 buttons. The N-1
> and N-2 filter positions have gain jumpers to kick in some extra
> amplification for those paths, but that doesn't apply here because they are
> switched out. 7 dB is not enough loss to make me suspect that some diodes
> are blocking the path.
>
> So anyone know whether is it normal to have this much attenuation through
> the board? If not, what to suspect - has the TenTec 2.4 kHz filter gone
> too lossy, or is there perhaps something wrong in the Q1 amplifier or one
> of the passive stages beyond that?
>
> [Further datapoint: I know the 2.4 kHz filter is used for TX as well as
> RX and I get a full 90 watts out without trouble. Maybe this means the
> filter is not excessively lossy? Or perhaps there is some kind of ALC
> compensation somewhere that means the filter could still be bad and do OK.
> Not sure.]
>
> Still learning. This much attenuation makes for a cruddy RX noise figure
> so I can't imagine this is intended given the expected performance, but I
> could be wrong.
>
> 73
> Phil W1PJE
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