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