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
~
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.
~
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.
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