A few weeks ago I posted a yell for help about an S-20R
I bought at a local swap meet. While working this unit had
been modified and I wanted to get more information. This is
to thank those who helped and encouraged me. I was able to
get re-stringing information and restrung the band spread
dial. One important tip came from Phil Nelson who
recommended braided Dacron fishing line for dial cord. This
stuff works like a charm, is easy to find (any shop that
sells fishing equipment) and cheap. Doesn't stretch and is
not slippery.
I researched the S-20R and its successor, the
S-40/S40A/S-40B and picked up a lot of information about how
they were designed and how the design evolved over time. For
those interested I found that the S-20R had at least one
revision and the S-40/A at least four. The S-40B is a
re-design rather than a revision with many changes including
styling which lead me to believe it was done mostly to lower
the production cost in order to enable price competition.
I made one change in the S-20R and that was to change
the bias method of the first audio stage from cathode bias
to "contact" bias, as used in the S-40A. This reduced
distortion considerably especially on weak signals.
The performance is quite good. Consider that this was
an economy model and is at least sixty-five years old. Dial
calibration is surprizingly accurate on the first three
bands. I did some tweaking on the top band and improved it
but its not as good as on the lower frequency bands. I think
Hallicrafters was stretching things to get it up to 44 mhz.
Its quite sensitive and the AVC will handle very strong
local broadcast stations with the exception of a 50KW some
three miles from here. That works on a short antenna. It
also works pretty well on the ham bands and is more stable
than one would think, quite capable of receiving SSB signals
on 20 meters.
Since all the AM stations locally process the hell out
of their audio I tested for audio quality using a
Hewlett-Packard 606-A signal generator fed with a high
quality source. This generator has a quite high-fidelety
modulator in it. The audio quality of the RX is very good
considering its a single ended pentode and a small speaker.
I was lucky that this RX was in good cosmetic condition
so I didn't have to do much more than detail cleaning. It is
a neat receiver. Its sitting across the room glowing at me
right now.
I think I posted a tip about using tooth paste to clean
plastic dial windows. Here it is again in case I didn't.
Toothpaste with a cotton swab will remove cloudiness from
many plastic materials including cellulose acetate and
nitrate. It also works on scratched CDs.
Again, thank you to all those who helped with this
project.


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Richard Knoppow
Los Angeles
WB6KBL