as the RF Section) is assigned a particular series of num-
bers. Table 4-1 lists the assigned component numbers for
the various circuits,
Switch wafers are identified by counting from the first
wafer, located behind the detent section of the switch, towards
the last wafer. For example, the designation 2R printed by
a switch section on a schematic, identifies the switch section
as the rear side of the second wafer when counting back
from the switch detent section.
Resistor Color Code
The instrument contains a number of stable metal-film
resistors identified by their gray background color and color
coding. If a resistor has three significant figures and a
multiplier, it will be EIA color coded. If it has four signifi-
tracing. The widest color stripe identifies the first color of
cant figures and a multiplier, the value will normally be
the code. Power supply voltages can be identified by three
printed on the resistor. For example, a 332
resistor will
color stripes and the background. White background indi-
be color coded, but a 332.5 k resistor will have its value
cates a positive supply, and a tan background is used to
printed on the resistor body. The color-coding sequence is
indicate a negative supply. Table 4-2 shows the wiring color
shown in Fig. 4-1.
code for the power supply voltages used in the Type 491.
The color coding helps trace a wire from one point in the
instrument to another.
RF cables for the RF and IF sections are miniature coaxial
cables. Some of these cables have a lossy characteristic
and are identified with a white outside coating, The stand-
ard 50 ohm low-loss coaxial cables have a clear plastic
outside coating. Do not interchange the lossy type with
the standard 50 Ω t y p e w h e n t h e s e c o a x i a l c a b l e s a r e
replaced.
Removing and Replacing Assemblies
WARNING
Disconnect the i n s t r u m e n t f r o m the power source
before attempting repair and/or replacement of
any sub-assembly.
Circuit Board Assembly Removal or
Replacement
If a circuit board
assembly
is damaged beyond repair,
the entire assembly
including
all components should be
replaced. The board
assembly
part number is listed in the
Mechanical Parts List
and may
be ordered as directed.
used in this instrument.
In most cases the complete circuit board assembly should
be removed when components are to be replaced. This
Wiring Color Code
will allow a soldering-iron tip to be placed at the back
side ar bottom of the board to unsolder the component
The insulated wire used in the Type 491 is color-coded
leads and remove the damaged component. The new com-
according to the EIA standard color code to facilitate circuit
4-3
