Voltage Measurements

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-

figures FO-13 and FO-14.

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.

4-2

Change 3