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This is getting difficult to talk about. The antenna stage, i.e. the
circuit between the antenna and the RF grid, is virtually the same in
both receivers. The plate circuit is not the same. In the S-85 the
broadcast band (540 to 1600) is not the same. In the S-85 the plate load
of the RF amplifier is an untuned 6800 ohm resistor R-29 and C-26, a
220uuf capacitor across it. This is capacitance coupled by C-10, to a
tuned circuit in the grid of the mixer tube. The antenna tank and this
tuned tank will have some selectivity but it will be lower than on the
other bands. On the second band there is a 6800 ohm resistor acting as
the plate load, also coupled to the grid of the mixer through C-10 with
a tuned circuit in the grid. Without the 220uuf cap the gain should be
higher. Both coils for the mixer grid seem to be wound on the same
former with separate trimmers.
The puzzler here is where is the ground return for the antenna stage
of the two lower bands? On the two top bands the coils are link coupled
with one side going to A2 and grounded through the link on the antenna
termimals. On the two lower bands there does not seem to be any sort of
ground return for an ballanced load. A2 does not go anywhere. True on
both the S-40B and S-85. If a single wire antenna is used the return is
via the ground terminal, A2 goes no where. Yet the receivers is
specified as using either a single wire or balanced antenna on all
bands. So how are the two grid coils on L3 connected for a balanced
antenna? We have discussed all sorts of illusory connections but I
still can't see how any of them work or how to prove that they work.
My suggestion of connecting a signal generator or other source (a
piece of wire) to A2 with A1 grounded is to see if there is any signal
that way.
Now, the original question was why do not the trimmers on the two
lowest bands do anything? First of all, is this correct, do neither
antenna or mixer trimmers work? Is it on both bands or only the antenna
stage? I will find the original post and see what was asked.
This may be a separate problem. Now, I don't want to go through the
entire long thread again (but will despite being lazy) but have we heard
from anyone with a working S-40B or S-85 about whether their trimmers
work or whether their receivers have reasonable response on the two low
bands? Hallicrafters made both receives for some time so they can't
have been totally inoperative on even one band let alone two.
The original poster (not sure of spelling Emmanuel?) seems to have
an adequate signal generator. Could he try feeding a signal into the
grid of the mixer tube or at the plate of the RF amplifier (using a
blocking capacitor) to see if there is response at that point.
If neither trimmer works its necessary to isolate where the problem
is (could be both places).
I have no definite suspicions at this point. The circuit is a
puzzle because as its shown it appears that it can't work. We have been
over the possibility that the schematic is wrong or that the receiver is
wired wrong (would never have worked so how did is escape from the
factory?) I still want to know what happens if a signal is injected into
the A2 terminal with the A1 terminal shorted to ground.

On 2/15/2025 3:03 PM, don Root wrote:

Richard , I am getting confused. ?you spoke of the antenna stage,? I
don’t see ?difference you mentioned between the 40B and the 85.

The next wording must be part of the mixer tuning. On the bama drawing,
C10 is active only on band 4,? and does nothing when on band 1, all as
we have been mentioning. Can you send us a snip of your drawing, it
might have a magic answer.

RE _C-10 will also couple on the other bands but likely, as suggested,
will be swamped by the LC or transformer coupling there. Not sure of
this analysis_.?? ?I cant see that Richard, and we spoke of the ?C62 in
the S-40B as likely doing the coupling for Band 1,2

Back to the Antenna tanks for band 1,2 ; we think my proposal way back
[see the sketch] has the answer. Have a look and see if it is right or
wrong. We asked but you did not comment at the time.

Emanuele still has the original problem, and has done many tests
already, and published images, and still has that very poor sensitivity
on the lower half of both lower bands; very frustrating.

the tank peaking for bands 1,2 is way too far off so AFIK he is not
concerned about all the antenna variables you might run into.

Maybe you can review past postings, and highlight specific errors one by
one.

*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Richard Knoppow
via groups.io
*Sent:* Saturday, February 15, 2025 4:54 PM
*To:* [email protected]
*Subject:* Re: [HallicraftersRadios] S-85 refurbish: I've got a problem
during the RF alignment of bands 1 and 2 - trimcaps don't peak!

I looked at both (S-40B and S-85) schematics again. The antenna
stage is different. The S-85 has a broad band plate load R-26, C-29
rather than a tuned circuit. The coupling appears to come through C-10
which feeds a tuned circuit in the grid of the mixer tube. …. ++++++++++
+++++++

Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998