(2) A schematic diagram showing all the components of a low-frequency
All components are
identified by a number, and the value of each component is listed. The
components are connected into three stages to form the oscillator
Resistors R31 through R34, and capacitor C12, are connected
Crystal Y1 provides
the input signal to this stage.
Resistors R26 through R30, and
capacitors C9 through C11, are connected with Q7 to form a voltage
A second emitter follower is used as the oscillator
output stage. This consists of Q6, R21 through R25, and C6 and C8. A
feedback link is connected from R23 to Y1. The procedure of numbering
the components and labeling the stages makes it easy for the technician
to locate the various components when he is making voltage and
By seeing where and how a component is
connected in a circuit, he can determine the function of the component.
In many cases, the technician can look at the schematic and determine
whether a specific component could, or could not, produce the trouble
symptoms that the equipment is producing.
e. Layout Diagrams.
A layout diagram is a drawing or photograph of the
chassis of an electronic unit, showing the positioning of the parts on the chassis.
Such a diagram is shown in figure 4-6. With this type of diagram, the technician
can physically locate and identify the various parts and observe their relative
Some layout diagrams also show and identify the test points that are
located on the chassis.
There are several types of charts available to help the technician when he is
troubleshooting the equipment.
He uses these charts to localize a trouble to a
stage and isolate a specific part.
a. Voltage Charts.
A voltage chart lists the ac and dc voltages at specific
points in the equipment.
The technician uses the voltage chart as a
guide to measure voltages when localizing a trouble to a stage.
Voltage charts provide all the special instructions that are required
to obtain accurate measurements. In figure 4-7, these instructions are
included as Notes. The equipment publication usually gives tolerances
for voltage measurements; as an example, voltage readings 10%.
readings that are not within tolerance are considered abnormal.
(2) Ac voltages. The voltages shown on the voltage chart are the voltages
required for normal operation.
In figure 4-7, Note 1 specifies that
tube voltmeter (VTVM). The 6.3v ac shown for each tube is the required
filament voltage, and the VTVM must be set to read ac volts. Note 3
explains where to place the meter probes to read this voltage.