Avc Action; S" Or Signal Intensity Meter; The Second Detector; (10) The Beat Frequency Oscillator - Hallicrafters SUPER SKYRIDER SX-28-A Instruction Book

(7) AVC ACTION
A double A VC system is used. The RF and mixer tubes
are operated by the broadly tuned carrier coming through
only three tuned IF circui:s. The final signal however
passes through six-tuned IF circuits. As a result, when
the signal is slightly detuned, the receiver output has
dropped considerably while the AVC action has dropped
but very little. This results in a reduction of between-
station noise and a more sharply defined aural tuning
action.
(8)
"S" OR SIGNAL INTENSITY METER
The apprpximate DB per S unit equivalent is 6 DB's.
As is known, a DB, or decibel, is a unit of change in
signal level and is defined as being the least detectable
change the average ear can appreciate when listening to
a single pitched tone. 3DB is the least change the ear
detects when listening to sounds varying in both ampli-
tude and pitch. By comparison, a variation of one S unit
on the meter will indicate a change of two detectable
steps in signal level. Quanti tati vel y, a DB gain or los~
is equal
to
20 log
,,(~:)
where El
=
input voltage and
E~ =
output voltage.
(9) THE SECOND DETECTOR
As will be noted, a diode type of second detector is
used in the Model SX-28-A. Its choice was prompted by
the fact that such a detector is capable of handling large
percentages of modulation with very little distortion.
This is due to the output of the diode being easily filtered
(IF Removed). In addition, the rectified output contains
a DC component which can be used for A VC purposes.
(10) THE BEAT FREQUENCY OSCILLATOR
The BFa is turned on with the switch below the band-
spread handwheel and adjusted by the skirted knob
directlv below the tone control. The BFa circuit, as will
~e
.seen by referring to Fig. ·13, is the well known
Hartley oscillator. It will
be
noticed that a plate drop-
ping resistor is used to compensate for plate voltage
variations. An increase in receiver voltage causes an
'increase in the plate current of the oscillator. This in- .
crease in turn causes the voltage drop across the resistor
to
increase, thus maintaining a more constant voltage
at the plate of the beat oscillator tube. A favorable ratio
of capacity to inductance is used. The fixed tank capacity
has been artifically aged by alternately exposing it to
very high and then low temperatures. In this manner
any residual strains of the component parts are removed
and the capacity of the condenser remains constant.
The BFa coil is permeability tuned which further re-
moves the possibility of drift which would occur should
a compression variable be used to resonate the circuit.
Proper location of the Beat Oscillator tube and its
associated components plus excellent shielding and me-
chanical rigidity do much to keep stray fields from being
established. Little BFa leakage is to be expected in the
Model SX-28-A so "tweets" or BFO harmOnICS will not
prove to be bothersome.-
(11)
THE AUDIO AMPLIFIER
The second or output stage of the audio amplifier in
the Model SX-28-A receiver uses two 6V6GT tubes connec-
ted in push-pull. These tubes ar~ driven by the 6SC7
double triode. One of the triode sections of the 6SC7
tube is used as the 'inverter to the 6V6GT tubes. A por-
tion of the signal from the plate circuit of the first 6SC7
,
triode is fed to the grid of the othet· 6SC7 triode Section,
thereby giving two output voltages' in opposite phase
suitable for exciting the push-pull 6V6GT output am-
plifier.
(12)
THE POWER SUPPLY
The power supply in the Model SX-28-A is quite normal
except that it supplies voltage for the 6V6GT output
tubes directly from the rectifier or before the filter system.
Voltage fluctuations in the receiver are greatly reduced-
increasing the audio output of the receiver and stabiliZing
the operation of all circuits.
The filter circuit consisting of a total of 60 mfds of
capacity plus an additional filter in the 6SC7 plate supply
and a
12
henry choke keep the hum level of the receiver
in excess of 60 DB below maximum output. The power
transformer is built to withstand continuous operation
at 2S0 degrees F but has been designed to run at approxi-
matelv 160 degrees F under normal conditions.
(13 )
Tubes:
SPECIFICATIONS
1~6AB7 1st RF Amplifier
1---6SK7 2nd RF Ampl ilier
16SA7 Mixer
1 _ .. 6SA 7 HF Oscillator
1 -6L 7 1st IF Amplifier Noise Li mi ter
1-6SK7 2nd IF Amplifier
1-6B8 2nd Detector and S meter tll he
1--6B8 A VC Amplifier
1-6AB7 Noise Amplifier
1-,-6H6 Noise Rectifier
1-6JS Beat Oscillator
1-6SC7 1st Audio Amplifier
2-6V6GT Push-Pull Output Amplifiers
1-SZ3 Rectifier
Power Consumption-at 117 volts-60 cycles-138 watts
Power Consumption-DC operation-18 amp. at 6 volts
or 108 watts
Power Output -8 watts undistorted
SensitivitY-Cfor 500 milliwatts output) varies between
the limits of 6 to 20 microvolts over the entire frequency
range of the receiver. 2 x 1000 x-
Selectivity-IF broad (high fidelity) 12 kc 36 kc
IF Sharp 4.1 kc 22 kc
Frequency Range RF-Note: These are the actual fre-
quencies covered corresponding to nominal figures in-
dicated on the front panel.
')')0
to
1,620 kilocycles
I.S to 3.1 megacycles
2.9 to S.9 megacycles
S.7S to 11.S megacycles
10.3 to 21.S megacycles
20.4 to 43 megacycles
Frequency response AF (audio filter out-broad IF-tone
control high)-70 to 3000 cycles ±
2~
DB
Speaker Output Impedances-SOOO and SOO ohms
Intermediate Frequency-4SS kc
Table cabinet dimensions-20Y2" long x 10" high x 14
3 ;"
deep .
Relay Rack dust cover dimensions-14:l(" deep x 17 3 ;;"
long x 8%" high
Panel dimensions-19" x 8%"
Chassis
dimen~ions-17%"
x 13Y2"
Weight-(unpacked)-~7)
lhs. packed 11,7 lhs.
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