BOOSTS 38 db! REJECTS 38db! ANY SELECTED FREQUENCY!
Set SELECT-O-JECT for REJECT, tune by ear and - presto! - an annoying heterodyne or other unwanted signal practically disappears without materially affecting the wanted signal? Set SELECT-O-JECT for BOOST, tune - and - presto! - a selected signal rises above background noise and interfering signals! Can also be used as audio oscillator having over 100 to 1 frequency range with a single rotation of the tuning knob! Excellent as a code practice oscillator! Effective on any frequency from 80 c.p.s. to 9,000 c.p.s.! This is the amazing circuit described in the November 1949 issue of QST, page 11. See your National dealer for details.
SOJ-1 for all receivers
SOJ-2 wired for the HRO-7, NC183 or NC-173
SOJ-3 wired for the NC-183D, NC-183B, HRO-50, HRO-60, NC-173B or NC-57B
Source: The Radio Amateur's Handbook from 1950
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