Tuning the PBT too quickly through the passband of the narrow filter is
my first guess too.
I don't know a lot about the Corsair, so there is a good chance I'm
about to say something that applies to Omni VI (which I am familiar
with) and does not apply to the Corsair. Beware!
When you use a narrow filter in the first IF things work differently
than when the only narrow filters are in the second and shiftable IF.
(I am guessing the Corsair has a second IF that can be shifted in
frequency using a Pass Band Tuning control, like the Omnis, based on
Lee's previous response to your question.)
When you have a real wide first IF filter, and a narrower filter
selected in the shiftable second IF, you can use the PBT (or IF Shift)
control to sort of select what audio tone you would like to hear, within
the limits of the first IF bandwidth. This is the way my old Kenwood
TS-440 worked. The first IF filter was broad as a barn door, so when
using a narrow filter in the second IF, the IF Shift control acts like
an audio tone preference selector. A Ten-Tec Omni will work the same way
as long as you have a wide filter in the first IF and a narrow filter in
the second IF. If both filters are narrow, the PBT (or IF Shift) control
adjustment can no longer be set to anywhere in a wide range to choose
the audio tone you like. It has to be set inside the narrow range that
works.
When the first IF has a narrow filter selected, then the second IF
bandpass has to coincident with the first IF bandpass in order for any
signals to get through both IFs and to the product detector. This
concept can be a little bit hard to understand, because the two IFs are
operating at completely different frequencies. How can the bandpasses of
the two IFs be coincident with each, other when they are not anywhere
near the same frequencies? One way to think of it is to imagine the part
of the received spectrum, the frequency of the signals coming in from
your antenna, you want to hear, that is passed by each IF filter.
Your main tuning knob moves the frequency that is passed by both IFs
simultaneously, by moving the first local oscillator. The PBT or IF
Shift control only moves what passes through the second IF. Only when
you get the second local oscillator (the one controlled by the PBT knob)
set so that both IF bandpasses converted by the local oscillators
corresponding to the same incoming frequency at the antenna input, will
you get signals turned into audio by the product detector.
The signal has to make it through both narrow IF filters for you to hear
it. To pass through both filters they must both be passing signals
coming from the same part of the spectrum at the antenna input.
Try adjusting the PBT control slowly for maximum noise. When you find
the right setting, the tone of the noise is the tone of signals you'll
be able to hear. Leave the PBT at that setting and adjust the main
tuning to find a signal.
DE N6KB
3fiy@radioadv.com wrote:
Gil, maybe you are tuning the PBT through the passband window too quickly
and missing the signal. The passband is pretty narrow with the 217 filter
and you may zip right through it. Find a good signal and tune the PBT
slowly and see if the signal pops out at some point.
Good luck and 73,
-Lee-
WA3FIY
On 27 Nov 2009 at 14:24, g p wrote:
Installed the 217 cw filter in my Corsair 1 but when switched to the
filter position the gain drops considerably and there is white noise,
I also tried a different 217 and the same thing lots of white noise
and only the stronger signals are heard, the weaker signals disappear
in the white noise and this is with the preamp on, also while in the
narrow mode the PBT does not function at all. This does not happens in
my Omni D-B or my Argosy. Any thoughts? Thanks, Gil
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