Russell,
Thanks for the links to comments about the Corsair audio.
I used a Corsair II that a friend had, and it was super on his favorite
mode, CW. It sounded fine to me on SSB as well.
That said, there are better audio chips that could replace the Audio IC
in the model. If that is too much conversion for you, simply change the
audio coupling capacitors to reduce the high frequency response to the hiss.
Particularly subject to failure is the series R and C network on the
audio IC output pin to ground, that is supposed to limit high frequency
response in older Audio ICs.
The first reference writer clearly did not understand that for a
communications / ham receiver use, you do NOT want "High Fidelity", but
you want communications audio punch. This is achieved by NOT passing
alll audio frequencies
but limiting the audio to the voice band 300 to 3 KHz. This was all
proved in Acoustics back in the 1930's. Typically, you want smaller
series coupling capacitors to alter the frequency range being passed to
the speaker.
And check that the aging speaker has not warped and thus is causing
distortion if it has become off center, or the cone detaches from the
edge mount.
The second reference has some useful suggestions to tailor transmit
audio as well as receive audio in Corsairs.
The other contributor to hiss is the age of electrolytic capacitors in
the Corsair or even Corsair II's if they have not been replaced in less
than 5 years. You may find lack of filtering by original electrolytics.
Any electrolytic such as used in ham radios has a limited lifetime. It
probably will degrade slowly enough that the failure will not be evident
until there is a large amount of hum or audio feedback, but it is worth
checking if the filtering is adequate.
The Corsair II's are one of the most desirable older Ten Tec radios.
That has been stated on this and other reflectors for many years.
Audio is very subjective to hams. You should try to listen to a radio
you are considering, with both speaker and good quality communications
headphones, to evaluate the audio fairly. Ensure you are not turning up
RF gain too high and thus adding noise during a test. Sometimes on
older radios, the correct RF gain is lower than where the S meter
operates as you would expect. I tend to give S meter reports by my ear,
as some rigs have a Scotch reading on the S meter. There were no clear
standards for S meters back in the day of the Corsair. It seemed to do
as well as other brands but it was not digital precision, since it was
an analog circuit.
I have seen hams who could not hear audio I could clearly hear on their
radio. At Field Days, folks often miss calls I hear and work, because
each of us has differing skills in "brain filtering" noise imposed upon
a signal.
Newer hams need to listen to HF bands for many hours and under differing
conditions to condition their ears to optimum "Communications Quality",
if they want to succeed with weak signal work.
-Stuart Rohre
K5KVH
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