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TM 11-6660-204-25
aid to the repairman, because most troubles either
(b) Be careful not to damage other leads by
result from, or produce, abnormal voltages. Voltage
pulling or pushing them out of the way.
measurements can be measured between two points
(c) D  O not allow drops of solder to fall into the
in a circuit, without circuit interruption. Complete
equipment, since they may cause short circuits.
information on normal operating voltages is given in
(d) A carelessly soldered connection may cre-
ate a new fault. It is very important to make well-
a. Unless otherwise specified, voltages are mea-
soldered joints, since a poorly soldered joint is one of
sured between the indicated point and ground.
the faults most difficult to find.
b. Always begin the measurements by setting the
(e) When a part is replaced, it must be placed
voltmeter on the highest range so that the voltmeter
exactly as the original was placed. Give particular
will not be overloaded. Then, if necessary, set the
attention to proper grounding when replacing a part.
voltmeter on a lower range.
Use the same ground or circuit-ground wire as in the
c. When checking cathode voltage, remember that
original wiring. Failure to observe these precautions
a reading can be obtained when the cathode resistor
may result in decreased gain or possibly in oscillation
is actually open; the resistance of the meter may act
of the circuit.
as a cathode resistor. Thus the cathode voltage may
d. Test Cables.
be approximately normal only as long as the voltme-
(1) All chassis and panels are of the plug-in type
ter is connected between the cathode and ground. An
and may be removed from the cabinet for testing.
Test cable W604 and power supply test cable W606
abnormally high plate voltage may indicate an open
cathode resistor.
permit the operation of the components when they
are removed from the cabinet. Test cables should be
4-5. Resistance Measurements
used only between those receptacles shown in figure
After a trouble has been localized to a specific stage
4-1. If test cables are connected in any other way,
or circuit, resistance measurements will often isolate
equipment may become damaged.
the specific defective component. Normal resistances
(2) Power supply test cable W606 is connected
are specified in figures FO-13 and FO-14. Resist-
between P401 mounted on the power supply chassis
(fig. 3-14) and jack J105 located at the rear of the
ance of coils, transformers and motors are given be-
low. Insure that one side of the component under
recorder. The power supply test cable is used for this
measurement is disconnected from the circuit to
connection only as indicated in figure 4-1. Test cable
eliminate error due to shunt resistances.
W604 is connected between the components as indi-
cated by figure 4-1. Five of these cables are needed to
4-6. Waveforms
maintain the components in operation when all are
removed from the cabinet at the same time.
Checking waveforms is another method of localizing
troubles. Waveforms may be observed on an oscillo-
4-4. Voltage Measurements
scope at the various test points shown along with
Voltage measurements are an almost indispensable
actual reference waveforms on figure FO-15.
Change 3

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