Troubleshooting Techniques - Tektronix 7B92 Instruction Manual

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Troubleshooting Techniques
This troubleshooting procedure is arranged in an order
which checks the simple trouble possibilities before pro-
ceeding with extensive troubleshooting. The first few
checks ensure proper connection, operation, and calibra-
tion. If the trouble is not located by these checks, the
remaining steps aid in locating the defective component.
When the defective component is located, it should be
replaced following the replacement procedures given under
Corrective Maintenance.
1. Check Control Settings. I ncorrect control settings
can indicate a trouble that does not exist. If there is any
question about the correct function or operation of any
control, see the Operating Instructions.
2. Check Associated Equipment. 8efore proceeding
with troubleshooting of the 7892, check that the equip-
ment used with this instrument is operating correctly.
Check that the signal is properly connected and the probe
(if used) is not defective. The oscilloscope and vertical
plug-in unit can be checked for proper operation by
substituting another time-base unit that is known to be
operating properly (preferably another 7892 or similar
unit). If the trouble persists after substitution, the oscillo-
scope or vertical plug-in unit should be checked.
3. Check Instrument Calibration. Check the calibration
of this instrument, or the affected circuit if the trouble
exists in one circuit. The apparent trouble may only be a
result of misadjustment that can be corrected by calibra-
tion. Complete calibration instructions are given in the
Calibration section.
4. Visual Check. Visually check the portion of the
instrument in which the trouble is located. Many troubles
can be located by visual indications such as unsoldered
connections, broken wires, damaged components, etc.
5. Isolate Trouble to a Circuit. To isolate a trouble to a
particular circuit, note the trouble symptom. The symptom
often indicates the circuit in which the trouble is located.
For example, if stable triggering can be obtained in the I NT
position of the SOURCE switch and cannot be obtained in
the EXT or LINE positions, the External Trigger Preamp or
Trigger Source Switching circuits are probably at fault.
When the trouble symptoms appear, use the front-panel
controls and the CRT display to isolate the trouble to one
circuit. Keep the amplifier unit and oscilloscope in mind
when isolating the trouble. When trouble appears in more
than one circuit, check all affected circuits by taking
voltage and waveform measurements. Once the defective
circuit has been located, proceed with steps 6 and 7 to
locate the defective component(s).
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Maintenance-7B92 Service
6. Check Individual Components. The following pro-
cedures describe methods of checking individual com-
ponents in the 7892. Components that are soldered in place
are best checked by disconnecting one end. This isolates the
measurement from the effects of surround ing circuitry.
a. Relay. The eight pin relay used in the 7892 is
symmetrical and may be replaced in its socket facing
either direction. This relay, which is plugged into the
circuit board, may be removed and checked. Use an
ohmmeter to check the 600 ohm resistance. The relay
may also be actuated by placing the +15 volts across the
coil. The internal connections are printed on the body of
the relay.
b. Transistors. The best check of transistor opera-
tion is actual performance under operating conditions. If
a transistor is suspected of being defective, it can best be
checked by substituting a new component or one that
has been checked previously. However, be sure that
circuit conditions are not such that a replacement
transistor might also be damaged. If substitute transis-
tors are not available, use a dynamic tester (such as a
Tektronix Type 577 or 7CT1N Curve Tracer plug-in unit
and a 7000-Series Oscilloscope system.
c. I ntegrated Circuits. I ntegrated circuits should not
be replaced unless they are actually defective. The best
method for checking these devices is by direct substitu-
tion with a new component or one that is known to be
good. 8e sure that circuit conditions are not such that a
replacement component might be damaged.
d. Diodes. A diode can be checked for an open or
shorted condition by measuring the resistance between
terminals. Using an ohmmeter scale having an internal
source of between 800 millivolts and 3 volts, the
resistance should be very high in one direction and very
low when the leads are reversed.
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Do not use an ohmmeter scale that has a high internal
current. High currents may damage the diode.
e. Resistors. Resistors can be checked with an ohm-
meter. Check the Electrical Parts List for the tolerance
of the resistors used in this instrument. Resistors
normally do not need to be replaced unless the measured
value varies widely from the specified value.
f. Inductors. Check for open inductors by checking
continuity with an ohmmeter. Shorted or partially
shorted inductors can usually be found by checking the
waveform response when high-frequency signals are
passed through the circuit. Partial shorting often reduces
high-frequency response.
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