The Southwestern Historical Quarterly, Volume 97, July 1993 - April, 1994 Page: 73
754 p. : ill. (some col.), maps, ports. ; 23 cm.View a full description of this periodical.
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The Breakthrough Breadboard
um-tube portable radio, disconnected the tube mixer circuits and tied
our transistor mixer's output to its IF amplifier's input transformer. We
retuned our LO to track 455 kHz (instead of 262 kHz) above the station
frequencies because the tube radio's IF operated at 455 kHz. After a few
adjustments, we found we had an acceptable transistor tuner, inasmuch
as this "hybrid" radio was able to pick up local radio stations as well as it
had with its tube type input/mixer circuit. This was in spite of the mis-
match between the transistor mixer's low impedance output and the
tube radio's high impedance IF transformer input. At least it worked
well enough to give us confidence that we had something which would
probably work in a transistor radio, once all the other radio circuit de-
signs were complete.
For a tuning capacitor on this breadboard model mixer circuit, we
used the smallest one available at the time, the one from the Emerson
vacuum-tube portable. It, like most radio tuning capacitors, actually had
two variable condenser (capacitor) sections, one for tuning the input
circuit to the station being received, and the other for tuning the LO, a
frequency always 262 kHz higher than the frequency of the station re-
ceived.
By Sunday evening Mark and I were ready, as was Roger, to try and
"marry up" the first transistor radio's two key circuits, the input/mixer
and the IF amplifier. There was no way to know for sure that we would
not encounter some new parasitic oscillations and heaven knows what
other weird goings on. Our Tuesday deadline was rapidly approaching,
so we had no choice that evening but to start the "wedding" immediate-
ly.
Ed had been working quietly alone and had come up with an audio
amplifier circuit design that was complete and ready to be transferred
from his breadboard to the yet-to-be-completed radio's breadboard. For
increased audio power output with minimum power drain on the radio's
battery, he used what is known as a Class B, push-pull arrangement of
two transistors as the output stage to drive the loudspeaker. This final
stage of a radio (or audio amplifier, TV, etc.), being the one which must
supply the power output, itself consumes more power from the power
supply (battery) than the other amplifier stages. Consequently, it is im-
portant to find ways to minimize the battery drain, while maintaining ad-
equate audio frequency (signal) current out to drive the speaker. The
audio output stage was coupled to the speaker through a specially con-
structed audio-frequency transformer which Ed had designed.
That Sunday evening, Roger took the RF/mixer stage which Mark and
I had designed and the audio amplifier circuit which Ed had designed
and began to combine them with his IF amplifier to build a complete,
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Texas State Historical Association. The Southwestern Historical Quarterly, Volume 97, July 1993 - April, 1994, periodical, 1994; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth117154/m1/101/: accessed May 2, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Texas State Historical Association.