Jim, K9RJ said:
As you can see, the intermod and blocking characteristics are not very
good,
especially at close spacing.....
It is important to remember that the Collins 75A4 is a double conversion
receiver on all bands except 160 meters where it is a excellent single
conversion communications receiver. Even compared with some of the
current radios marketed nearly 50 years later the 75A4 is in a class by
itself on 160 meters. If you are doing 160 meter performance tests on
the 75A4 I would strongly recommend a real world A/B test with a top of
line FT-1000 or R-4C with all the Sherwood mods. (I have only done it
with a R-4C with some of the mods and the 75A4 won every time.)
There are operational qualities to the 75A4, equipped with a 250 Hz
Collins mechanical filter, where using the pass band tuning has a
quality of which I have not yet seen replicated in newer receivers. Let
me try to describe the auditory experience, i.e. the ability to roll
interference or unwanted signals into a deep canyon where they disappear
while the wanted weak 160 meter CW signal, normally tuned best around
300-400hz, is by itself. The mechanical connection of the PTO to the
pass band tuning knob of the 75A4 appeared to shift the pass band around
the wanted signal rather than shift the signal around the pass band
through a jungle of interfering signals. Some may call this a
physiological reaction and thus not a valid observation. It may not be
physics but it is a real world reaction to using a fine old receiver.
My advice, if you can find a Collins 75A4 for a reasonable price, buy it
and enjoy the experience on topband.
Herb Schoenbohm (ex-KV4FZ)
St. Croix, VI
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