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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