Emerson Keystone Figure 637, Service Manual

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Emerson Radio
2. TUNER — The r-f unit constitutes a separate
sub-chassis of the receiver. This sub-chassis con­
tains the r-f amplifier, converter, and oscillator
stages.
The channel switch, fine tuning control,
tuned circuits, and first video i-f transformer are
also contained on this chassis. Tuning and track­
ing adjustments for all twelve channels currently
in use are provided.
The tuner serves to select
and amplify the desired video and audio frequen­
cies and convert them to the carrier i-f frequencies
of 25.75 me. for video and 21.25 me. for audio.
No separation of these two intermediate frequen­
cies is made, and the complete signal is fed to the
first video i-f stage.
The tuner uses a rotary turret carrying individ­
ual coils for cach tuned circuit, for each channel
setting. A type 6AG5 (V 28) serves as the r-f
amplifier and a type 6J6 (V 27) as the converter
and oscillator. The r-f amplifier is a wideband,
tuned stage whose output is inductively coupled
to the converter (V 2 7 ).
The oscillator (V27B)
operates in a Colpitts type circuit. Individual
slugs provide for alignment of the oscillator on the
various channels. A variable-dielectric type of
condenser is used for fine tuning of the oscillator.
The output of the converter is conected to double­
tuned first i-f transformer ( T l ) .
The center-tapped primary (L I ) of the r-f coil
is designed to match a balanced 300-ohm line. The
secondary (L 2 ) is tuned by the input capacity of
V28 in series with the parallel combination of
trimmer A 14 and a 5 mmf. condenser. The output
of V28 is coupled to V27A by L3, which is tuned
by trimmer A15 and the output capacity of the
tube.
A 10K resistor loads L3 to provide the
required band pass.
The input capacity of V27A and trimmer A 16
tune the converter coil (L 4 ). The oscillator coil
(L 5 ) is wound on the same form with L3 and
L4, for inductive coupling. The initial oscillator
frequency is fixed bv permeability tuning of L5
and the preset .5-3 mmf. trimmer. The frequency
is varied by means of the TU N IN G control (3-5
mmf. trimmer) which consists of a spiral-shaped
dielectric disc rotating between fixed stator plates.
Figure 4-2 Schematic Diagram of Tuner
3. VIDEO SECTION— The video section consists
of the following sections: video i-f; video detector and
automatic gain control; video amplifier and d.c. re­
storer.
a. V ID E O I - F : Both the 25.75 me. video carrier
and 21.25 me. audio carrier are amplified by three
wide-band i-f stages. The four tuned cicruits are
peaked at different frequencies, forming a stagger-
tuned system of relatively flat overall response to
produce the required bandpass.
Self-resonant, slug-puned coils are used in the
i-f transformers. Two stagger-tuned i-f trans­
formers (T2, T 3) follow the overcoupled first i-f
( T l ) . T2 is provided with a 21.25 me. trap to
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Scanned by mbear2k - Jan 20121/17