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I just finished restoring a Knight-Kit Star Roamer shortwave
receiver. It's a mediocre performer, but fun to play with. Actually,
it sounds pretty good using headphones.
Anyhow, it has a "selectivity" control that allows reception of
sideband and Morse signals. After a two-minute analysis of the
schematic I concluded that this control makes the 6HR6 intermediate
frequency (IF) amplifier tube function as a tuned-grid/tuned-plate
(TGTP) oscillator. This certainly isn't a sophisticated
implementation of beat frequency oscillator (BFO), but is works,
albeit with lots of fine adjustment to the bandspread and,
surprisingly enough, the antenna trim capacitor.
QUESTION: What other sets from the past have used this technique to
implement an inexpensive, but marginally functional, BFO?
-Dave Drumheller, K3WQ
The advantage wasn't just "one less tube" but that it could improve
selectivity a tad.
It's regeneration, as seen in the regenerative receiver and in q-multipliers.
Positive feedback with a control to allow adjustment. On the verge of
oscillation, the selectivity gets very narrow (though lousy skirt
selectivity), which can be useful. Move it up a notch, and it goes
into oscillation, and you get your BFO.
Making an IF stage oscillate is one way. Far more common was a q-multiplier,
an extra stage that did the same thing, though it was connected to the plate
of the mixer. It was rarely used as a bfo there, it not quite being the right
place but also because it was usually an add-on for the purpose of
selectivity, and most cheap receivers did have a bfo.
Someone mentioned the Hallicrafters S-38. I seem to recall the company
also made a ham band only receiver that put an IF stage into regneration,
though that one may have still had a bfo.