to start with the power supply circuit, then proceed consecu-
tively from one circuit to the next.
Once the circuit has been isolated, refer to the Circuit
Description in section 3 for a description of the normal
circuit operation.
CAUTION
Use care when measuring voltages or waveforms.
The small size and high density of components
in this instrument establishes a condition such that
an inadvertent movement of the test probe or the
use of oversized probes may short-circuit between
components.
The pin connectors to the circuit boards provide a method
to isolate circuit resistance and voltages. Check circuit condi-
tions before disconnecting voltages to make certain bias
voltages are not removed, which may cause excessive over-
loads.
Transistor Substitution and Replacement
Transistors should not be replaced unless they are actually
defective. However, temporary substitution is often a con-
venient way to detect defective transistors.
Before substi-
tuting a transistor, it is recommended that circuit conditions
be checked to be certain that an exact replacement will not
be damaged. If a transistor is removed from its socket,
make certain it is replaced in the same socket in the same
position. Some transistors can be inserted incorrectly into
their socket. Fig. 4-21 shows the correct connections and
positions for the different types of transistors used in the Type
491.
In-Circuit Diode Checks
In-circuit diode checks may be performed with a voltmeter.
A comparison check of the voltages on each side of the
diode with the typical voltages listed on the diagram will
help isolate faulty diodes. Forward-to-back resistance ratios
on some diodes can be checked by referring to the schematic
and pulling appropriate transistors and square pin connectors
to remove low resistance loops around the diode.
Do not use an
internal current.
resistance ratios
CAUTION
ohmmeter scale that has a high
Do not check the forward-to-back
of tunnel diodes or mixer diodes.
4-19
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