Allied Radio Knight-Kit 83 Y 140 Manuel d'assemblage - Page 8

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ac on the 2.5 v a-c range. Therefore, a direct meter

reading in terms of decibels can be made only when

the meter is connected

across

a 500-ohm resistive

load.

Otherwise only relative db measurements

can

be obtained.

However,

in a large number of cases

relative measurements are appropriate, since reference

conditions are defined by other factors and only rela-

tive variations are important.

Frequency response to

3-db "fall-off? points is an example.

How to Measure Alternating Current

The

voltage

ranges

may

also

be used to

measure alternating current at power line frequencies

by simply inserting a known resistor (of low resistance

and sufficient wattage to carry unknown

alternating

current) and measuring the voltage across it.

The

formula:

I (Amperes) = Sega

is used to find the alternating current flowing in this

circuit.

Equipment

Cathode

voltages are most often bias voltages,

and indicate

whether

a tube is operating over tbe

proper portion of its characteristic.

Screen

grid voltage measurements

can

give in-

dications of the functioning of the associated circuits.

Very low voltages

at these points may indicate a

leaky bypass

capacitor.

Abnormally

high voltages

might mean a faulty dropping resistor.

Plate

voltage

measurements

can

show

an open

plate

load

resistor

or transformer

by the complete

absence

of potential.

Low plate voltage indicates

higher conduction than a high plate potential.

If tbe

value of the plate load resistor has a higher resistance

than the meter impedance,

the meter reading will not

read the actual existing voltage, but a value which

is somewhat

lower.

The

suggestions

given under

"How to Measure D-C Voltages'' should be followed

in such cases,

grid

circuits,

the

grid leak

resistors

have

usually a rather high resistance.

The d-c voltages

are also of low magnitude making it rather difficult

to use a fairly high d-c voltage range to maintain a

high meter resistance.

Consequently

the

resulting readings

are not a

very reliable indication.

In clipper, power amplifier,

and oscillator circuits, grid voltage can be measured

with a 20,000 ohms per volt meter, with a minimum

error occurring since the grid leak resistance values

are

not very high and will not be affected

by the

meter resistance.

D-C voltages in ave or age circuits give important

clues of the operation of the receiver either when the

signal

is or is not received.

During alignment a

specified voltage has to be applied to such systems,

which can be checked by a VOM connected to that

line.

Also overloading, witb resulting stage blocking,

can be checked by voltage measurement

of the avec

and age systems.

The d-c voltmeter can also be used as an align-

ment indicator.

For this purpose,

connect the meter

across the plate load resistor of the detector and set

Function Switch until a deflection of the meter pointer

results.

Tben proceed

with the regular alignment

using the meter as a peak or dip indicator.