[TenTec] hiss hiding, was Omni C vs Corsair II

[DJ5IL@aol.com]

I operate mainly CW and love my Corsair II (especially for it's superb
QSK) - except for two annoyances:

1) The excessively noisy receiver as described by by Steve, N4LQ, and
Duane, N9DG. I already addressed the problem on this reflector (Subject:
"Noisy Corsair II", September 15, 2001) when I wrote:

"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
udible 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 ..."

Paul, K9OT, replied: "The Omni has the quietest reciever of any  radio
that I have used. When I got the Corsair II that was one of the first things
I noticed was the recieve noise ... If you find a fix I would like to know.",
and Jerry, K9GCI: "I read your post with great interest. I also have a
Corsair II with the same problem you have."

2) The doubly balanced mixer D13 on the "RF MIXER" board mixes the
received signal RF with the local oscillator LO and the output spectrum
contains the frequencies nfLO+-fRF for all uneven n (n=1,3,5...) with:

Vout(t)=2/pi*cos(2pifRFt)*cos(2pifLOt)-2/(3pi)*cos(2pifRFt)*cos(2pi3fLOt)+...

The second term results from mixing RF with the 3rd harmonic of LO
and it's amplitude is only 20*log((2/(3pi))/(2/pi))=9.5dB below the desired
mixing product. Now, 20m is the only band where the PTO (5.0-5.5MHz)
is directly used als the LO signal instead of being mixed with the xtal
oscillator signal. And It's 3rd harmonic (15.0-16.5 MHz) is really very
invonvenient because here in Europe we have very strong broadcasters
on the 49m-band and and as a result the mixing product can be heard
with the CII tuned to on 20m. For example: I receive the German
broadcasting station "Deutsche Welle" with my TS-870 on it's original
frequency 6075 KHz with S9+55dB. And I receive it  with my CII tuned
to 14025 KHz loud and clear with S7 ! From the difference of 67dB we
can estimate the attenuation of the 20m-bandpass-filter to be roughly
67dB-9.5dB=56.5dB on the 49m-band which is absolutely acceptable.

73
Karl, DJ5IL