Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 33)

CONNECTOR

MODULE

Lt

r..

3

0

GRD 0}

L2

0

LINE

ORD

RINGER TERMINAL BOARD

AND

"NETWORK"

CAPACITOR

RED

YEL

RED

BLK

GRN

1.0

m

YEL

HOOKSWITCH

. HANDSET

o)

BLK

YEL

BLU

o

ROTARY

DIALERS

WHT

BRN

D

GRN

RED

O,

I

WHT

WHT

o

o

BLK

SPLICE

RED

REG

0

BLK

RED

O

0

RED

WHT

ORN

TRANS

1

V(PULSE)

o

Fig.

5.

This is the

schematic

diagram of a traditional

telephone instrument. In-

cluded is the

terminal board nomenclature.

Integrated

circuits are normally

used with

electronic ringers

to detect

the ring voltage

and drive the

piezo-

electric element. The

Motorola

MC34017 is

typical

of electronic tone

ringer

chips. As Fig.

6

shows,

the

MC34017

is

a complete

ring

detector

circuit

and

oscillator in

a single 8 -pin

DIP package.

Power for

the chip is

taken from the

telephone line

by

rec-

tifying

the ac ring

signal using

an

in-

ternal full -wave

rectifier

and zener-

diode voltage

regulator.

To isolate the tone ringer from the

voice

part of the circuit, the chip uses

a series capacitor /resistor combina-

tion that

connects

to the line. The

ring

tone

is

set by

C2

and

R2, while

C3 and R3 form a filtering circuit

that

rejects false triggers.

In normal operation, Cl blocks dc

from entering the

IC,

but

when

ac ex-

ceeding 36

volts appears on the line,

current

passes through

it

and into the

chip's power supply.

Application of

power to the

internal relaxation oscil-

lator then

produces

a

tone that's buf-

fer fed to the piezoelectric device.

Failure of an electronic

tone ringer

can often be

traced to the transducer

or the

chip

itself. The circuit can

be

tested by securing the

hookswitch in

the off position

(on -hook)

and apply-

ing

a 60 -Hz signal

from

a

Variac,

ad-

justed to

60

volts

across

the input

line. The

Variac

provides a steady in-

put signal

that

allows

you to take

voltage measurements

and monitor

circuit conditions. Oscillator output

can be

monitored with

a signal trace

or substitute transducer. Make sure

all tests are performed

with

the phone

removed from

service.

Rotary Dialers

Most prevalent of telephone

prob-

lems

are

those that

occur

in the

dialer, especially in the

case of the

mechanical rotary dialer'. An

amaz-

ing

achievement of

mechanical

en-

gineering, the rotary

dial

is

an

intri-

cate assembly that

rapidly

opens and

closes a set of contacts connected

across the phone line to

produce a

string

of pulses that the

phone com-

pany

interprets

as calling numbers.

As you turn the dial, a

spring

winds

and an auxiliary set of shunt

spring

contacts are

activated.

The

shunt

springs

short the

handset

to prevent

the pulsing from

being

heard

in the

earphones.

When the dial is released,

a ratchet

engages a cam that

actuates

pulser contacts. Dial

speed is main-

tained

at a constant 10 pulses

per sec-

ond

by a

mechanical

governor.

The ratio

of time that the

pulser

contacts are open to being closed is

critical

for proper

dialer operation.

This ratio is

61 percent, or 0.61 -sec-

R1

Cl

Diode

Bridge

T p -VVr

Ring>

AC1

AC2

RC

SCR

Transient

-

Clamp

Oscillator

2

11`

Input

Current

Mirror

122

V

Tone

Frequency

Divider

Bias

_L

Push Pull

Output

Buffer

Warble

Frequency

Divider

Ref

N

o

RSl

N

Y

Threshold

Comparator

l

Piezo

O

Sound

T

Element

C3

Fig. 6. The MC34017,

a

complete ring detector circuit

and tone generator

in a sin-

gle

8-pin

DIP package, replaces the

bulky

electromenchanical

bell.

Say

You

Saw

It In Modern

Electronics

April

/986 /

MODERN ELECTRONICS / 37

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