Transistor Substitution and Replacement

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