Hello Richard.

...and thank you for taking the time to share your thoughts.

Indeed I am quite aware that the reason for the bypassing of one of the
RF stages was due to overload problems.
Perhaps when I wrote my posting I had not yet discovered that I was still
sniffing out two distinct problems, which I
subsequently have discovered.

First, some of the early SX-25s had two RF coils with two trimmers. For
whatever reason someone disconnected both my antenna coil and the first RF
coil. The antenna coil wiring on the "generic" schematic available
everywhere is correct but the 1st RF coil portion is not if you have an
early SX-25 with two RF coils.
True, the broadcast band (Band 1) skips over the first RF coil ostensibly to
avoid the overload problems. I own two other (later version) SX-25s and
neither even is fitted with the first RF coil "inside" the
antenna-to-oscillator chain. I have figured out the wiring and my SX-25s
works like gangbusters now.

The second problem...that my BAND 1 (broadcast band) was completely
dead. I traced that to a missing jumper between the oscillator coil and the
switch.

I am now experiementing with improving the Band 4 performance. Adding
the shorted coil of wire to the RF and oscilllator coils is one way to go
but I am not convinced that, with slight rewiring of the coil chain I can't
provide an easier method of alignment by adding its own padding capacitor
for Band 4. Incidentally, every SX-25 I have owned...including this early
version I am now working on...have had the additional inductive turns
already added. In one case the coils were all modified and sealed with
either wax or Q-dope and then became covered with sludge from being
submerged in a flood 60 years ago; so I know that SX-25 owners in the 1940s
and 1950s were aware of the problem with IF tracking on Band 4 way back
then. Adding the shorted turn to the coils only flattens out tracking from
the bottom of Band 4 to about 30-35 MC anyway, so I am not convinced I can't
come up with a better way. Any thoughts on adding a discreet padder will be
warmly welcomed!

Thanks again for your thoughts and keep the spirit of glow alive. I am
quite sure this antiquated, simple and humble technology will still be
working long after much (most?) of today's junk technology has failed
irrepairably.

My best,

W9STB
I have the same schematics as you do, a Hallicrafters
1945 handbook from BAMA and a Rider's summary. Both show
that Hallicrafters used an arrangement that is common in
both H and other receivers with two RF stages , that is
by-passing one stage for the broadcast band. I suppose its
possible that both RF stages were used in early receivers.
The reason for by-passing one stage is mainly to avoid
overloading from the very strong signals often encountered
from broadcast stations and another is to broaden out the RF
bandwidth since this RX also has a broad IF response for
good fidelity from BC stations. I think there are other H
receivers which had such changes. Does the band switch have
the requisite contacts for an RF transformer between the two
RF stages? There should also be a trimmer capacitor for the
coil or perhaps scars from one that was removed.
I am trying to think of ways to find out what was there
in case an old handbook can not be found.
Hallicrafters seems to have changSed model numbers when making
significant changes in design, i.e. S-40, S-40A, S-40B or S-20 and S-20R
(R presumably meaning "revised").


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Richard Knoppow
Los Angeles, CA, USA