Since I undertook restoring an S-20R recently I
researched this receiver and its relatives. What I found is
interesting: the same basic receiver was built in various
guises from 1939 to 1968!
The S-20R is sufficiently different from the S-20 and
earlier "Sky Champion" models to consider it a new model. It
was built from some time in 1939 until 1945. It was replaced
by the S-40. In the S-40 the RF and converter stages are
changed to more modern tubes to improve performance at the
highest frequencies and there were a few other changes (like
ferrite core IF transformers). There are at least three
versions of the S-40. The earliest ones were essentially
identical to the S-20R with exception of the afore mentioned
tube type change. After early production the loudspeaker was
changed from a field coil type to a permanent magnet type.
There was also a revision of the first audio stage to change
bias to "contact" bias instead of cathode bias, probably to
reduce distortion. The S-40A had some additional slight
revisions.
The S-40B was changed mostly, I think, to lower
production costs. The front panel was simplified and one
tube was eliminated. In the earlier receivers the half of
the 6H6 acts as a "gas gate" diode to prevent the AVC line
from becoming positive if a tube gets gassy. This was
eliminated. Half the 6H6 now became the detector/AVC and the
other half continued to be the noise limiter. The 6SQ7 was
replaced with a dual triode half of which is the first audio
and the other half is the BFO, the 6J5 BFO tube being
eliminated. Also, the RF transformers were changed so that
on the two lower bands the RF stage is series fed from the
antenna. That was undoubtedly a cost saving step plus there
should have been enough gain and selectivity for the
broadcast band and lower HF band with out the transformer
coupling.
The S-40B was replaced with the S-85, which is identical
electrically. This receiver has two large dials
symmetrically placed on the front panel requiring the
speaker to be relocated to the lid. The S-85 was replaced in
turn by the S-108. Again, the changes are mostly cosmetic.
This RX has a large slide-rule dial for the bandspread and,
again, the speaker is in the top, but the circuit is
identical to the S-40B. The S-108, and also I think the
S-85, have a separate band spread capacitor rather than the
second set of plates on the main capacitor. This was
probably done because both of these receivers have
calibrated band spread instead of just a logging dial as in
the earlier versions. Otherwise there is very little change.
While the various versions of these receivers can look
quite different a careful examination will find that the
layout and placement of the controls did not change from the
S-20R to the S-108. Thus we have a family of receivers that
lasted for some twenty-nine years, a pretty good record for
any design.


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Richard Knoppow
Los Angeles
WB6KBL