Ten-Tec 1253 Assembly And Instruction Manual page 64

Simple regeneratve detectors have very low-level audio output suitable only for high-
impedance earphones. Boostng such audio must be done with care – the job of the
Q4 preamp circuit. This 1253 design bufers the highly sensitve oscillator/detector
secton and uses the generous audio output capabilites of U1, the same TDA2611A
audio IC used in TEN-TEC transceivers.
The 8 volt regulator U2 plus Q5 provides stable operatng voltage for the oscillator,
varactor and bandswitching circuits, isolatng them from audio current peaks, which
would otherwise cause extreme frequency instability. Q5 is an "actve capacitance"
circuit which multplies the efect of C29.
Bandswitch Board: The 74HC4017 TTL decade counter IC was chosen over its
CMOS 4017 counterpart because its superior current-sourcing capabilites are
essental for frequency stability in the L-C tuned circuits. While the zener diode
holds Vcc for U1 to proper TTL supply voltage, R2 lifs device ground above common
ground to permit switching of about 8 volts through the L-C circuits. Minimal switch
debouncing is required and is accomplished by C1.
Notice the unused circuit board positions for C35 through C42. We provided
these in our original design in anticipation of a possible need to put a small
capacitance in parallel with an inductor for a desired frequency coverage.
Adding any such a capacitor LOWERS the frequency range and reduces the
total tuning range of R20.
For the convenience of all kit builders, the bandswitch board is manufactured
as a double-sided board with plated-through holes.
The audio IC is LM386; good stability is achieved by using
separate batteries for the detector/varactor circuits and the audio
circuit. We provide a good-looking front panel, and the kit builder
supplies knobs, cabinet, speaker, wire and other hardware items.
Attention Club Leaders, Instructors:
The T-KIT Model 1054 Receiver Kit
is an economical and simplified version
of this same 1253 receiver
with mechanical pushbutton switching of
4 bands between 5 and 15 MHz.
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