Tektronix 2246 1Y, Service Manual

Page: 148 / 433

Maintenance—2246 1Y and 2246 Mod A Service
11. Check Individual Components
W A R N IN G
To avoid electric shock, always disconnect
the instrument from the ac power source
before removing or replacing components.
The following procedures describe methods of
checking individual components. Two-lead com ­
ponents that are soldered in place are most
accurately checked by first disconnecting one end
from the circuit board. This isolates the measure­
ment from the effects of the surrounding circuitry.
See Figure 9-1 for component value identification
and
Figure 9-2 for semiconductor lead
configurations.
fsA A /W W V V n
I CAUTION <
When checking semiconductors, observe the
static-sensitivity precautions given 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 substi­
tuting a known-good component. Flowever, be sure
that circuit conditions are not such that a replace­
ment transistor might also be damaged. If substitute
transistors are not available, use a dynam ic-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 em itter-to-base and
e m itter-to-collecto r voltages to determine whether
they are consistent 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
em itter-to-base voltage for a conducting silicon
transistor will normally range from 0.6 V to 0.8 V.
The e m itter-to-collecto r 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 voltm eter across the junction rather than
comparing two voltages taken with respect to
ground. If the form er method is used, both leads of
the voltm eter 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
em itter-to-base values given, either the junction is
reverse biased or the device is defective. Voltages
exceeding those given for typical e m itte r-to -
collector values could indicate either a nonsaturated
device operating normally or a defective (open-
circuited) transistor. If the device is conducting,
voltage 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.
5 CAUTION <
v w w w w r S
When checking em itter-to-base junctions, do
not use an ohmmeter range that has a high
internal current. High current may damage
the transistor. Reverse biasing the em itter-
to-base junction with a high current may de­
grade the current-transfer ratio (Beta) of the
transistor.
A transistor em itter-to-base junction also can be
checked for an open or shorted condition by
measuring the resistance between terminals with an
ohmmeter set to a range having a low internal
source current, such as the R X 1 k fl range. The
junction resistance should be very high in one direc­
tion and much lower when the meter leads are
reversed.
When troubleshooting a field-effect transistor (FET),
the voltage across its elements can be checked in
the same manner as previously described for other
transistors. Flowever, rem em ber that in the normal
depletion mode of operation, the gate-to-source
junction is reverse biased; in the enhanced mode,
the junction is forward biased.
INTEGRATED CIRCUITS. An integrated circuit (1C)
can be checked with a voltm eter, test oscilloscope,
or by direct substitution. A good understanding of
circuit operation is essential when troubleshooting a
circuit having 1C components. Use care when check­
ing voltages and waveforms around the 1C so that
adjacent leads are not shorted together. An 1C test
clip provides a convenient means of clipping a test
probe to an 1C.
6-9