A few weeks ago I managed to get hold of a mint conditioned Corsair II with
all filters. I am having so much fun with it that my TS-870 went into the
cabinet for a time out. The only weak spot is the extremely high noise level
of the otherwise good receiver section. I am quite sure that the noisy RX is
a general peculiarity of the CII and not just specific to my txcvr, because
it has been described also by others. For example by N1EU, who shows how to
add High Fidelity Line Out Audio to the Triton IV or Corsair and writes: "...
The op-amps in the Corsair have very mediocre audio performance and are very
noisy."
By playing with the controls we can at least roughly approximate the origin
of the main noise source. Refer to the schematics of the IF/AF Board #80984
and RF Mixer Board #80987 in the Corsair II manual for the following short
test:
Disconnect the antenna from your CII and listen to the audio with the
following settings: BP Filter out, PBT 0, Notch out, RF Gain 10, Attn. off
(pushed). With the AF control fully ccw (0) only a very weak hiss is audible
but when you advance the AF control to a setting of approx. 2 you should hear
substantial noise. Therefore this noise must be generated in front of the AF
control and U4D/U5 cannot be the origin. Now Advance the BP Filter control,
the noise diminishes and in full cw position (in) the only remnant is the
ringing caused by that noise. Therefore the noise must be generated in front
of the Filter (U3A/U3B/U4A/U4B). Leave the BP Filter in and vary the Notch
control. You will find a setting (around 3) where you can notch out most of
the remaining noise. Therefore the main noise source must be in fron of the
Notch Filter (U1C/U1A/U1B). Finally, pull the Attn. control. This has no
influence and the noise remains nearly constant, so it must be generated
somewhere behind C6 on the RF Mixer board. I do not think that the the main
noise can be generated by the mixers themselfs. So if there is no logical
flaw in my reasoning it follows that it is not generated by the OpAmps but by
any or all of the the IF amplifier stages.
The noise level of my CII is so high that it is able to mask the radio noise
on quiet bands - What a pity ! By this behaviour the most outstanding virtue
of the CII concept is nearly wasted, namely the very clean LO signal compared
to the usual synthesizers and hence only very little reciprocal mixing and an
exceptionally clean reception of weak signals. An analogy for that situation
would be a super high-end HiFi equipment, capable to reproduce the full
dynamic range of a CD but set up for listening in a subway station during
rush-hour ...
The big question is: Where does the main noise originate and how can it be
tamed ? Any comments are very much appreciated. Let us discuss that topic on
the reflector and maybe we can make a very good transceiver even better ...
73
Karl, DJ5IL
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