Bass "In-Out" Switch; Head Phone Jack; Summary Of Related Circuits; The 2 Stage Preselector - Hallicrafters SUPER SKYRIDER SX-28-A Instruction Book

ment of the BFa control which appears directly under
the TONE CONTROL the following procedure is sug-
gested. Set the BFa control to zero, now tune in a signal
either voice or code. If a code signal is received, only
the carrier or thump of the signal will be audible because
no beat note is present. Be sure that you have the signal
accurately resonated. Now, without retuning the re-
ceiver, rotate the BFa control until a beat note of the
desired pitch is obtained. You now have introduced a
beat note which differs from the IF frequency of the
receiver, namely 455 kc, by the frequency of the audible
signal. Variation of the BFa control will allow you
to
change the pitch, or frequency, of the oscillator which
~ill
prove to be of hdp under various conditions of
tnterference.
( 12) Directly under the fiFO control will be seen the BASS
IN-OUT Switch. With this switch in the BASS IN posi-
tion you will have normal audio fidelity. Placing the
switch in the BASS OUT position, the audio filter
C~
is inserted. The effect of this filter on the band of fre-
quencies passed is shown Fig. 11. This filter will contribute
greatly
to
the intelligibility of the received signal when
the receiver is operated in the advanced positions of
selectivity.
(13 ) The Head Phone Jack is connected to a tap on the out-
put transformer. The signal in the headphones is of the
proper volume for satisfactory communications recep-
tion. Since no direct current is present in the headphone
circuit crystal type phones can be used.
C
SUMMARY OF RELATED CIRCUITS
(1)
THE 2-ST AGE PRESELECTOR
The RF AMPLIFIER, or pre-selector, of the Model
SX-28-A SUPER SKYRIDER has 1-6AB7; 1-6SK7 tubes in
cascade on Bands 3, 4, 5, and 6. On Bands
1
and 2 more
than one stage is unnecessaty to obtain the required
image ratio and reduction of spurious interference. With
two RF stages using three pre-selection circuits, the band
width would be narrowed to such an extent that even
expanding the IF Amplifier to its utmost would still not
provide high-fidelity reception. The modern communi-
cations receiver requires two stages of preselection on the
higher frequencies to accomplish only one primary object
-satisfactory image rejection. . .
The Model SX-28-A has an image ration of 20 to 1 at 28
mc-350 to 1 at 14 me and a proportionately increasing
ratio as the frequency is lowered. While the two RF
stages are principally needed to obtain such image ratios
they also perform two other us\!ful functions-more fav-
orable signal to noise ratio and slightly increased selec-
tivity.
Examining the coil assembly will immediately show
how rigidly it is constructed and what care has been
taken to completely shield each section from the other.
The manner in which the RF and antenna coils are tuned
on bands 3, 4, 5, and 6 will be interesting. Rather than
push turns
to
compensate for variations in inductance,
each coil is permeability tuned. This results in exact
adjustment of inductance with improved tracking and
gain as the result. On Bands 1 and 2 the inductance of
the antenna coils is sufficiently large so that lead length
differences do not cause any noticeable inductance change.
(2)
THE OSCILLATOR AND CONVERTER
'A separate 6SA7 tube is used as the High Frequency
Oscillator in the Model SX-28-A SUPER SKYRIDER.
This tube proves desirable in this function because of its
very high value of transconductance which enables the
oscillator to operate with very little coupling to the coil.
This feature reduces the unfavorable effects of tube vari-
ations and voltage fluctuations on the tuned circuit. The
HF Oscillator is coupled to the 6SA 7 converter tube at
the Cathode Tap--a point where variations of operating
parameters of the converter tube will least affect the
6SA7 Oscillator. A 6SA7 tube is used in the Mixer Circuit
because tests indicated that changes in operating voltages
caused less reflection in the injector grid loading than
would occur in Q:lost converter tubes. Another feature in
favor of the 6SA 7 tube is that a negative loading is
- 7 -
applied to the tuned circuit feeding its control grid. This
characteristic improves the. gain and selectivity of the
tuned circuit which in turn improves the image and sig-
nal
to
noise ratio.
(3)
THE IF AMPLIFIER
The IF Amplifier of the Model SX-28-A was designed
with a view towards permanency of adjustment under
conditions oDf extreme changes in temperature and humid-
ity as well as unusual mechanical vibration.
The Lirst two IF Transformers arc perineability tUllcd.
In comparing this type of transformer with one having
compression mica tuning condensers, it must
be
remem-
bered that it takes many more turns of the adjusting
screw to cause the equivalent change in tuning of the
permeability tuned type. Hence a slight change in the
position of the screw will have negligible effect upon the
tuning. The adjusting screw is under spring tension
thereby making it impossible to turn under vibration.
The diode transformer is air-tuned with two variable'
condensers each with a lump capacity of 50 mmf and
variable of 50 mmf. These air trimmers are also under
spring tension so that they can withstand considerable
vibration. Being of the air tuned type, their capacity
change is negligible
wit~
w,ide changes in humidity.
Reference to the Schemattc wlll show that the IF trans-
formers are expanded in two steps-thereby enabling
medium or full reproduction of tIie higher frequencies
to be obtained.
(4 )
VARIABLE SELECTIVITY
Six ranges of selectivity are provided in the model
SX-28-A receiver. They are:
I-Broad IF-Cfor high fidelity reception)
2-Medium IF-(more selectivity-less highS)
3-Sharp IF-(reduces annoying interference-far less
highs) . .
4-Crystal Broad-(Similar to Sharp IF but cleaner
cutting of side bands)
5-Crystal Medium-(nextse!ectivity step to
#4-
gready increased sideband cutting-more pro-
no.unced crystal "Slot" for interference-very
li ttle highs present) .
6--Crystal Sharrlposition of extreme selectivity-
practically no sideband content-very pro-
nounced crystal "slot")
The graphic effects of the different steps of selectivity
on a signal are shown in Fig. 1 and Fig. 4.