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Maintenance
Figure 13. Block diagram
of WR-99A .
Caution:
See
"Safety
Precautions",
Page
2
~neral
Performance
of the WR-99A
depends
upon
the
qnality of the components employed. If it should be
necessary to replace any of the component parts, only
Rc..~ replacement parts or equivalents of those shown
:=:
the Replacement Parts List of this instruction book-
ott
should be used.
The chassis may be removed from the case by re-
moring two screws from the bottom of the front bezel,
removing the bezel by sliding it off the bottom and
lifting upward,
removing two screws from the back
of the case, and removing 14 screws from around the
edge of the front panel. Pull the panel and chassis
out of the case.
If any alignment
adjustments
are made, the line
yol.tage should be 117 volts, at 60 cps. If trouble is
encountered,
voltage readings
should
be taken and
compared
with the operating
voltages shown on the
schematic diagram. Conventional trouble-shooting
tech-
niques should be used to locate trouble.
p
=
rRONT PANtl.. />OJ
Circuit Desceiption and Operation
The WR-99A is built around a Colpitts-type variable-
frequency
oscillator
(VI)
which utilizes
an RCA-
6AF4-A. (See Figure 13). This oscillator is tunable by
means of capacitor sections CIA, CIB, CIC, and CID
over a band of frequencies from 20 to 260 Mc. This
band is divided into eight overlapping
rf ranges. On
ranges 1 through 5, capacitor sections CIC and CID
and sections CIA and CIB are paralleled. On ranges 6,
7, and 8, only sections CIB and CIC ar-e used.
Output from VI is taken from the grid and fed to
the grid of the modulator stage, V2. Any internal or
external modulation is mixed with the rf signal in this
stage. Output
from the V2 modulator
stage is fed
through capacitor C6 to the attenuator network.
Internal
crystal-controlled
calibrating
markers
are
generated
by a Miller-type
crystal
oscillator
stage,
V4A, which generates
lO·Mc harmonic
signals. One-
megacycle
calibrating
markers
are generated
by a
I-Mc Colpitts oscillator, V4B, which is locked to the
V4A osciUator in a lO·to-l ratio. These oscillators may
be switched out of operation from the front panel.
. 20 .
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