Tektronix 2445A Service Manual page 152

Maintenance-2445A/2455A Service
A defective component elsewhere in the instrument can
create the appearance of a power-supply problem and
may also affect the operation of other circuits. Use the
power supply troubleshooting charts to aid in locating the
problem.
8. Check Circuit Board Interconnections
After the trouble has been isolated to a particular cir-
cuit, again check for loose or broken connections,
improperly seated semiconductors, and heat-damaged
components.
9. Check Voltages and Waveforms
Otten the defective component can be located by
checking circuit voltages or waveforms. Typical voltages
are listed on the schematic diagrams. Waveforms indicated
on the schematic diagrams by hexagonal.outlined numbers
are shown adjacent to the diagrams. Waveform test points
are shown on the circuit board illustrations.
NOTE
Voltages and waveforms indicated on the schematic
diagrams are not absolute and may vary slightly
between instruments. To establish operating condi-
tions similar to those used to obtain these readings,
see the voltage and waveform setup conditions
preceding the waveform illustrations.
Note the recommended test equipment, front-pane/
control settings, voltage and waveform conditions,
and cable-connection instructions. Any special con-
trol settings required to obtain a given waveform are
noted under the waveform illustration. Changes to
the control settings from the initial setup, other than
those noted, are not required.
10. Check Individual Components
The following procedures describe methods of checking
individual components. Two-lead components that are sol-
dered in place are most accurately checked by first discon-
necting one end from the circuit board. This isolates the
measurement from the effects of the surrounding circuitry.
See Figure 9-1 for component value identification and
Figure 9-2 for semiconductor lead configurations.
WARIIIIIG
I
To avoid electric shock, always disconnect the
instrument from the ac power source before remov-
ing or replacing components.
6-8
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When checking semiconductors, observe the static-
sensitivity precautions located at the beginning of
this section.
TRANSISTORS. A good check of a transistor is actual
performance under operating conditions. A transistor can
most effectively be checked by substituting a known-good
component. However, be sure that circuit conditions are
not such that a replacement transistor might also be
damaged. If substitute transistors are not available, use a
dynamic-type transistor checker for testing. Static-type
transistor checkers are not recommended, since they do
not check operation under simulated operating conditions.
When troubleshooting transistors in the circuit with a
voltmeter, measure both the emitter-to-base and emitter-
to-collector voltages to determine whether they are con-
sistent with normal circuit voltages. Voltages across a
transistor may vary with the type of device and its circuit
function.
Some of these voltages are predictable. The emitter-to-
base voltage for a conducting silicon transistor will nor-
mally range from 0.6 V to 0.8 V. The emitter-to-collector
voltage for a saturated transistor is about 0.2 V. Because
these values are small, the best way to check them is by
connecting a sensitive voltmeter across the junction rather
than comparing two voltages taken with respect to
ground. If the former method is used, both leads of the
voltmeter must be isolated from ground.
If voltage values measured are less that those just
given, either the device is shorted or no current is flowing
in the external circuit. If values exceed the emitter-to-base
values given, either the junction is reverse biased or the
device is defective. Voltages exceeding those given for
typical emitter-to-collector values could indicate either a
nonsaturated device operating normally or a defective
(open-eircuited) transistor. If the device is conducting, volt-
age will be developed across the resistors in series with it;
if open, no voltage will be developed across the resistors
unless current is being supplied by a parallel path.
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When checking emitter-to-base junctions, do not use
an ohmmeter range that has a high internal cuffent.
High current may damage the transistor. Reverse
biasing the emitter-to-base junction with a high
current may degrade the cuffent-transfer ratio (Beta)
of the transistor.