Maintenance-Type 422 AC-DC
After the defective circuit has been located, proceed
with steps 6 through 8 to locate the defective compo-
nent(s).
6. Check Circuit Board Interconnections. After the
trouble has been isolated to a particular circuit, check the
pin connectors on the circuit board for correct connection.
Figs. 4-8 through 4-17 show the correct connections for
each board.
The pin connectors used in this instrument also provide
a convenient means of circuit isolation. For example, a
short in a power supply can be isolated to the power supply
itself by disconnecting the pin connectors for that voltage
at the remaining boards.
7. Check Voltages and Waveforms. Often the defective
component can be located by checking for the correct volt
age or waveform in the circuit. Typical voltages and
waveforms are given on the diagrams.
NOTE
Voltages and waveforms given on the diagrams are
not absolute and may vary slightly between instru
ments. To obtain operating conditions similar to
those used to take these readings, see the first dia
gram page.
8. Check Individual Components. The following proce
dures describe methods of checking individual components
in the Type 422. Components which are soldered in place
are best checked by disconnecting one end. This isolates the
measurement from the effects of surrounding circuitry.
A. 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 which has
been checked previously. However, be sure that circuit con
ditions are not such that a replacement transistor might also
be damaged. If substitute transistors are not available, use a
dynamic tester (such as Tektronix Type 575). Static-type
testers are not recommended, since they do not check
operation under simulated operating conditions.
CAUTION
The POWER switch must be turned off before
removing or replacing transistors.
B. DIODES. A diode can be checked for an open or
shorted condition by measuring the resistance between
terminals. With 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 meter
leads are reversed.
CAUTION
Do not use an ohmmeter scale that has a high internal
current. High currents may damage the diode. Do not
measure tunnel diodes with an ohmmeter; use a
dynamic tester (such as a Tektronix Type 575
Transistor-Curve Tracer).
C. RESISTORS. Check the resistors with an ohmmeter.
See the Electrical Parts List for the tolerance of the resis
tors used in this instrument. Resistors normally do not need
to be replaced unless the measured value varies widely from
the specified value.
D. 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 (roll-off).
E. CAPACITORS. A leaky or shorted capacitor can
best be detected by checking resistance with an ohmmeter
on the highest scale. Do not exceed the voltage rating of the
capacitor. The resistance reading should be high after initial
charge of the capacitor. An open capacitor can best be
detected with a capacitance meter or by checking whether
the capacitor passes AC signals.
9. Repair and Readjust the Circuit. If any defective
parts are located, follow the replacement procedures given
in this section. Be sure to check the performance of any
circuit that has been repaired or that has had any electrical
components replaced.
Battery-Pack Troubleshooting. The Type 422 with AC-
DC Power Supply provides about four hours of continuous
operation from a fully charged battery pack. The actual
operating time may vary due to the age of the batteries,
number of charge-discharge cycles, signal applied, INTEN
SITY control setting and other factors. If the batteries do
not provide the normal operating time, first completely
charge the batteries for the recommended 16-hour charge
period (see Battery Charging in Section 2 for complete
information on charging). If possible, charge the battery
pack in an ambient temperature between +15°C and
+25°C. This allows the battery pack to attain its maximum
charge and also will minimize the chances that the battery
pack protection thermal cutout will open because of excess
temperature (see Section 2 for more information). Measure
the potential across the interconnecting banana jacks of the
power pack. A fully charged battery pack should measure
about 24 volts. If the battery pack measures below 22 volts
either the battery charger circuit is inoperative or the bat
tery pack is defective. The battery charger circuit can be
checked for operation using the procedure given in step 7
of the Calibration procedure. If the battery charger is
working correctly and the voltage reads low, the battery
pack is probably defective.
If the battery pack measures about 24 volts but the
POWER light begins to blink too soon when operating from
the batteries, either the battery pack has lost its charge
retaining capacity or the low-batteries indicator circuit is
defective. First be sure that the batteries have been charged
properly for the recommended charge period. Then, op
erate the instrument from the batteries until the POWER
light begins to blink. Now, measure the potential across the
4-8
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