Tektronix 442, Instruction Manual

Page: 77 / 170

Maintenance—442 Service
4. VISUAL CHECK: Visually check the portion of the
instrument in which the trouble is located. Many troubles
can be located by visible indications such as unsoldered
connections, broken wires, damaged circuit boards,
damaged components, etc.
5. ISOLATE TROUBLE TO A CIRCUIT. To isolate
trouble to a particular circuit, note the trouble symptom.
The symptom often identifies the circuit in which the
trouble is located. For example, poor focus indicates that
the crt circuit (including the high voltage supply) is
probably at fault. When trouble symptoms appear in more
than one circuit, check the power supplies, then check
affected circuits by taking voltage and waveform readings.
The power supplies are interdependent. All the low-
voltage power supplies depend on the —8 volt supply for
reference.
Table 5-1 lists outputs and tolerances of power
supplies. Voltages are measured between power supply
test points and ground. If a power supply voltage is within
listed tolerance, assume the supply is operating correctly.
A defective component elsewhere may appear as a power
supply problem and may also affect operation of suppor­
ting circuitry.
NOTE
Voltages and waveforms given on the diagrams are
not absolute and may vary slightly between in­
struments. Individual variations should be noted on
the schematic diagrams for future reference. To
obtain operating conditions similar to those used
when the listed readings were taken, refer to the
voltage and waveform setup procedures for each
diagram. Note the recommended test signals, front
panel control settings, and voltage and waveform
conditions. The 442 control settings required to
obtain the listed waveforms and voltages are located
adjacent to the waveform illustrations.
8. CHECK INDIVIDUAL COMPONENTS. Thefollow-
ing procedures describe methods of checking individual
components. Components that are soldered in place are
best checked by disconnecting one end. This isolates the
measurement from the effects of surrounding circuitry.
W A R N I N G
I
The power switch must be turned off before remov­
ing or replacing components to prevent electrical
shock or circuit damage.
Table 5-1
POWER SUPPLY TOLERANCE
Supply
Tolerance
- 8 V Set within 0.5%
+8 V
Within 3%
+100 V
Within 5%
Semiconductors. A good check of transistor operation
is actual performance under operating conditions. A
transistor can be most effectively checked by substituting
a new component for it (or one which has been checked
previously). 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 tester. Static-type testers are not recommended,
since they do not check operation under simulated
operating conditions.
Refer to the troubleshooting chart in the Diagrams
section for help in isolating malfunctioning circuitry.
However, not all problems are listed inthe chart. Continue
with this check list for cases not covered by the chart.
6. CHECK CIRCUIT BOARD INTERCONNECTIONS.
After the trouble has been isolated to a particular circuit,
check for loose or broken connections, improperly seated
transistors, and heat-damaged components.
7. CHECK VOLTAGES AND WAVEFORMS. Often
the defective component can be located by checking for
the correct voltage or waveform in the circuit. Typical
voltages are given on the diagrams. Waveforms are shown
at the left of the circuit diagram.
When troubleshooting transistors in the circuit with a
voltmeter, measure the emitter-to-base and emitter-to-
collector voltages to determine if the voltages are consis­
tent with normal circuit voltage. Voltages across a tran­
sistor vary with the type of device and its circuit function.
Some of these voltages are predictable. The emitter-to-
base voltage of a conducting silicon transistor will normal­
ly be 0.6 to 0.8 volt. The emitter-to-collector voltage of
saturated transistors is approximately 0.2 volt. Because
these values are small, the best way to check them is by
connecting the voltmeter across the junction and use a
sensitive voltmeter setting. This method is preferable to
comparing two voltages taken with respect to ground
(both leads of the voltmeter m ust be isolated from ground,
if this method is used). If values less than these are
obtained, either the device is short-circuited or no current
is flowing in the circuit. If values are in excess of the base-
emitter voltages given, the junction is back-biased or the
device is defective. Values in excess of those given for
5-8
REV A MAY 1980