Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 77)

Telephones

(from

page

39)

ments

are supplanting

the carbon

mike,

particularly in

electronic

tele-

phones,

so be

alert to

it.

The final

element

the

speech net-

work

itself. Because the

telephone is

a current operated

device,

it's

sensi-

tive

to all

resistive

influences

-in-

cluding those in the link itself. The

amount of current flowing

through

your

telephone depends on the re-

sistance of the wires

connecting

your

telephone

to the

exchange.

The

far-

ther

you

are from the

battery, the

higher

the line resistance,

and the less

current available to

you. This creates

the

effect that the current in

short

loops is

relatively high, tending

make

voices too loud, while in long

loops

the

current

is relatively

low,

forcing

users to

raise

their

voice.

counteract

this effect,

the tele-

phone

contains

automatic

compen-

sation

circuits that

adjust the

current

through

the

transmitter

and receiver

so that performance

on a very

long

line

is equal

to that on

a short line. In-

side the

traditional

telephone

instru-

ment

network

of coils, resistors,

and capacitors that

make

up the

equalization network.

At the heart of the this network is

variable resistor called a

varistor

that

works in

manner just

the opposite

that

of a

resistor. Larger

current

flowing

through

it, increases its

resis-

tance, while

smaller current,

reduces

its resistance. Placing

a pair of

varis-

tors

series

with

the

line,

causes the

negative- resistance

characteristics of

the

varistors

to stabilize

current

flow

and

minimize

the effects of line resis-

tance.

Should you

note

any

problems

with

this

network, replacement

of the

entire

network is recommended.

In the

electronic telephone, a spe-

cial circuit called an automatic gain

control, or agc, compensates for line

resistance

sampling

the

incoming

signal

and adjusting the gain of the

electronic amplifier accordingly.

Troubleshooting

the

network

and

amplifier

portion

of an electronic tel-

ephone is

similar to repairing

any

electronic

instrument. Voltage

mea-

surements

and signal tracing are a

must.

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Mitsubishi Electric MUX-25TV - E1 User Manual Page 77

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