[ARC5] SCR-183: Narrowing the Classic TRF SCR-229/-429

[David Stinson]

This morning, I checked into the local Texoma
Trader's AM Net with the mighty 5-watt
"Jimmy Doolittle" SCR-183.  The Raiders
actually carried the SCR-283 24 V version,
but hey- let's not quibble :-D.

The BC-229 receiver in the set is very broad.
Not because the engineers couldn't do better;
they were good engineers and knew what
they were doing.  The pilot-cranked Command
radios were likely to be a few KCs apart from
each other and the aircraft close enough to
each other that wide selectivity was an
asset to the set's successful operation.
However, this is a problem on our
crowded bands.

A simple and effective solution is to employ
carefully-adjusted regenerative feedback.
A wire from the Antenna post, loosely coupled
to the Detector tube at the back of the set will
provide a feedback path.  The level of regeneration
can be controlled with both the receiver Volume
(in MVC mode) control and the Antenna trimmer.
As with most regenerative circuits when
receiving AM, signal boost and narrower
bandwidth requires careful adjustment
before the point of oscillation.

As with all regens, if you QSY very much,
the regen tweeking will have to be repeated,
but in my case, if I'm listening to Shortwave
I just pull the wire from the Antenna post and
run the regular bandwidth with no issues.
Otherwise, when I want to use it on 75M AM,
I adjust the regen tweeking for 3885 KC
and it holds just fine for 3880-3885-3890 KC,
the most common AM freqs here.

Here is a demonstration from this morning of
the difference between regen and "stock."
A station from Arizona checked into the net.
He was weak and buried in Side-Bandit QRM.
After connecting the feedback wire, he came
up out of the weeds and the Side-Bandits
faded into the gathering darkness:
https://youtu.be/aB5t6R1MitU

This next video demonstrates the improvement
in selectivity possible.  This is the 3890 KC AM net
next to a 3885 KC QSO, 5 KC away.
https://youtu.be/ioOgaH9jnuQ

This should hold for the RU receiver as well.
When I refurb the GF/RU, I'll let you know.

GL OM ES 73 DE Dave AB5S


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