Replacement Parts - Tektronix 7A18 Instruction Manual

2. Check Associated Equipment. Before proceeding
with troubleshooting of the 7A18, check that the equip-
ment used with this instrument is operating correctly. If
possible, substitute an amplifier unit known to be operating
correctly into the indicator unit and see if the problem
persists. Check that the inputs are properly connected and
that the interconnecting cables are not defective.
3. Visual Check. Visually check the portion of the
instrument in which the trouble is suspected. Many troubles
can be located by visual indications, such as unsoldered
connections, broken wires, damaged circuit boards,
damaged components,etc.
4. Check Instrument Performance. Check the cali-
bration of the unit, or the affected circuit by performing
Part | — Performance Check of Section 5. The apparent
trouble may only be a result of misadjustment and may be
corrected by calibration. Complete calibration instructions
are given in Part II of Section 5.
5. Check Voltages and Waveforms. Often the defective
component or stage can be located by checking for the
correct voltage or waveform in the circuit. Typical voltages
and waveforms are given on the diagrams; however, these
are not absolute and may vary slightly between instru-
ments. To obtain operating conditions similar to those used
to take these readings, see the instructions in the Diagrams
section.
6. Check Individual Components. The following
methods are provided for checking the individual compo-
nents in the 7A18. Components which are soldered in place
are best checked by disconnecting one end to isolate the
measurement from the effects of surrounding circuitry.
A. TRANSISTORS AND INTEGRATED CIRCUITS.
The best check of transistor and integrated circuit
operationis actual performance under operating conditions.
If a transistor or integrated circuit is suspected of being
defective, it can best be checked by substituting a com-
ponent known to be good; however, be sure that circuit
conditions are not such that a replacement might also be
damaged. If substitute transistors are not available, use a
dynamic tester (such as Tektronix Type 576). Static-type
testers may be used, but since they do not check operation
under simulated operating conditions some defects may go
unnoticed. Fig. 4-1 shows base pin and socket arrangements
of semiconductor devices. Be sure the poweris off before
attempting to removeor replaceany transistor or integrated
circuit.
REV. D, MAY, 1975
Maintenance—7A18/7A18N
Integrated circuits can be checked with a voltmeter, test
oscilloscope, or by direct substitution. A good under-
standing of the circuit description is essential to trouble-
shooting circuits using integrated circuits. Use care when
checking voltages and waveforms around the integrated
circuits so that adjacent leads are not shorted together. An
integrated-circuit test clip provides a convenient meansof
clipping a test probe to the 14- and 16-pin integrated cir-
cuits. This device also doubles as an integrated-circuit
extraction tool.
B. DIODES. A diode can be checked for an openorfor
a short circuit by measuring the resistance between termi-
nals with an ohmmeter set to the R X 1k scale. The diode
resistance should be very high in one direction and very low
when the meter leads are reversed. Do not check tunnel
diodes or back diodes with an ohmmeter.
Do not use an ohmmeterscale that has a high internal
current. High currents may damage the diodes.
C. RESISTORS. Check resistors with an ohmmeter.
Resistor tolerance is given in the Electrical Parts List.
Resistors normally do not need to be replaced unless the
measured value varies widely from the specified value.
D. CAPACITORS. A leaky or shorted capacitor can be
detected by checking resistance with an ohmmeter on the
highest scale. Use an ohmmeter which will not exceed the
voltage rating of the capacitor. The resistance reading
should behigh after initial charge of the capacitor. An open
capacitor can best be detected with a capacitance meter, or
by checking whether the capacitor passes ACsignals.
7. Repair and Readjust the Circuit. Special techniques
required to replace components in this unit are given under
Component Replacement. Be sure to check the perfor-
manceof any circuit that has been repaired or that has had
any electrical components replaced. Recalibration of the
affected circuit may be necessary.
REPLACEMENT PARTS
Standard Parts
All electrical and mechanical part replacements for the
7A18 can be obtained through your local Tektronix Field
Office or representative. However, many of the standard
electronic components can be obtained locally in less time
than is required to order them from Tektronix, Inc. Before
purchasing or ordering replacement parts, check the parts
lists for value, tolerance, rating, and description.
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