Elenco Telephone Line Analyzer Kit User Manual

TELEPHONE LINE ANALYZER KIT

MODEL TT-400K

Assembly and Instruction Manual

Elenco Electronics, Inc.

Copyright © 1994 Elenco Electronics, Inc. Revised 2001 REV-C 753253

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INTRODUCTION

Are you planning to install a new telephone? Are you having a problem with a newly installed telephone? Then
the Model TT-400 Telephone Line Analyzer Kit will help you pinpoint exactly where the problem is. The TT-400
connects between your telephone and the telephone wall jack and performs the following four tests:

a) Line Test - Verifies the DC voltage to your telephone.
b) Ring Test - Verifies the AC ranging voltage to your telephone.
c) Loop Test - Verifies the condition of the telephone line from the telephone company central office to the

wall jack in your house.

d) Telephone Line Cord Test - Verifies the condition of your telephone cord (if detachable).

SPECIFICATIONS

DC LINE TEST

SWITCH POSITION: LINE / RING
LINE OK: FOR 1/3 OF FULL SCALE, Vin= 40VDC

FOR FULL SCALE, Vin= 120VDC

REVERSE POLARITY: IF PHONE LINE POLARITY IS REVERSED, LED WILL BE ON.
ACCURACY: 3% AT 40V DC

LOOP TEST

SWITCH POSITION: LOOP
LOOP OK: FOR 1/3 OF FULL SCALE, I

in

= 20mA.

FOR FULL SCALE, Iin= 60mA

ACCURACY: 3% AT 20mA

RING TEST

SWITCH POSITION: LINE / RING.
RING “?”: 40 Vrms TO 48 Vrms, 20Hz SUPERIMPOSED ON 48VDC.
RING OK: 48 Vrms TO 130 Vrms, 20Hz SUPERIMPOSED ON 48VDC.
THE REVERSE POLARITY (REV POL) LED WILL FLICKER DURING RING TEST.

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TELEPHONE OPERATION

The primary purpose of the telephone is to transmit
and receive voice signals allowing two people with
telephones to communicate with each other. To be
of practical value, the telephone must be connected
to a switching network capable of connecting each
telephone to many other telephones. To accomplish
this switching, each subscriber telephone is
connected to the telephone company’s Central
Office by two wires referred to as the local loop. A
simplified diagram of this connection is shown in
Figure 1. The Tip and Ring designation of the + and

-- leads comes from the days of the manual
switchboard. The tip of the plug the operator used
to connect telephones carried the + lead and the
ring immediately behind the tip carried the -- lead.

When a subscriber wishes to place a call, they merely pick up the telephone and a small current flows in the
local loop. This current picks a relay in the Central Office indicating that service is being requested. When the
Central Office is ready to accept the number being called, a dial tone is sent to the calling telephone. The dial
pulses or tones then signal to the Central Office the number of the telephone being called. A path is then
established to that telephone. This path may be a simple wire connection to a telephone connected to the same
Central Office or it may go via wire, microwave link, or satellite to a telephone connected to a distant Central
Office. To signal the incoming call a ringing signal is placed on the local loop of the called telephone. The
ringing signal is a 90VAC 20Hz signal superimposed on the 48VDC present on the local loop. A ringing tone is
also sent to the calling telephone. When the called party picks up the phone voice communication is
established.

THE ROTARY DIAL TELEPHONE

A simplified schematic diagram of the traditional rotary
dial telephone is shown in Figure 2. The major par ts
of this telephone are explained below. In the newer
electronic type telephones, many of the bulky parts of
this telephone are replaced by transistors, integrated
circuits and piezoelectric buzzers.

HOOK SWITCH

When the hook switch is open (on hook), no current flows in the local loop. The 48VDC from the battery in the
Central Office appears on the tip and ring input to the telephone set. When the receiver is lifted, the hook s witch
closes and a current of about 20 to 120mA flows in the local loop. The resistance of the local loop drops the
voltage of the telephone to about 6 volts. The current picks a relay in the Central Office which tells other
equipment there that service is being requested. When the Central Office is ready to accept the number being
called, a dial tone is sent to the calling telephone. The dial tone stops when the first digit is dialed.

DIALER

There are two types of dialers, pulse and tone.

Pulse Dialer

Pulse dialing is accomplished by the familiar rotary
dial shown in Figure 2. The dial is rotated to the
stop and then released. A spring in the dialer
returns the dial to its null position. As the dialer
returns, the dial switch (S2) opens and closes at a

Figure 1

Central Office

Switch

Network

Ringing

Signal

Generator

Current

Relay

Control

Signal

Generators

Ring (--)

Tip (+)

48VDC

Local
Loop

Subscriber

Telephone

Figure 2

Balance
Network

Receiver

Transmitter

Ringer

S1

S2

C1

L1

L2

R2

R1

V1

L3

C2

C3

R3

V2

Figure 3

Loop

Current

Space

60mS

0

23

Pulse Period
100mS

Mark
40mS

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fixed rate. This switch is in series with the hook switch. Opening the switch interrupts the current in the local
loop. A series of current pulses is thus sent out on the local loop as shown in Figure 3. The number of pulses
sent corresponds to the digit dialed. Dialing a 0 sends ten pulses.

The dial pulses are sent at a rate of 10 pulses per second (100mS between pulses). Each pulse consists of a
mark interval (loop current). In America the mark inter val is 40mS and the space interval is 60mS giving a
mark/space ratio of 40/60. In Europe the mark space ratio is usually 33/67.

Tone Dialer

Tone dialing is accomplished with a keyboard of 12 keys
arranged in 4 rows and 3 columns. As seen in Figure 4, low
frequencies of 697, 770, 852 and 941 are associated with rows
R1 through R4 and high frequencies of 1,209, 1,336 and
1,477Hz are associated with columns C1 through C3. To send
each digit, two frequencies are sent to the Central Office
simultaneously. For this reason, this method of dialing is
referred to as dual tone multifrequency (DTMF). The different
frequencies are generated by connecting a capacitor to
different taps of a transformer to establish a resonant circuit of
the correct frequency. Each of the 3 keys of row 1 is
mechanically connected to switch SR1. Similarly each of the
other rows and columns are connected to their corresponding
switch. Thus, pressing any key closes two switches and
generates two frequencies. Pressing a 6 for example, closes
switches SR2 and SC3 and generates 770 and 1,477Hz.

TRANSMITTER

The Transmitter consists of a metal diaphragm and a metal case insulated from each other
as shown in Figure 5. The case is filled with carbon granules. Wires are connected to the
case and the diaphragm and a current is put through the carbon granules. When you
speak into the transmitter, the sound waves of your voice strike the diaphragm and cause
it to vibrate. This causes the carbon granules to compress and expand. When
compressed, the resistance of the carbon granules is less than when expanded. The
change of resistance causes a corresponding change in the current. The current thus
varies in step with the sound waves of your voice. In a newer electronic type telephone
the carbon granule transmitter may be replaced by an electret or other type microphone.
An electret microphone is made up of a capacitor with a dielectric material which holds a
permanent electric charge. Sound waves striking a plate of this capacitor cause the plate
to vibrate and thus generate a small voltage across the capacitor. This voltage is amplified
by a field effect transistor (FET) mounted inside of the microphone. The signal from the
microphone is then amplified before being sent to the Central Office via the local loop.

RECEIVER

There are sever al diff erent types of receiv ers. In principle, they work the same
as the speakers in your radio and TV. The speaker consists of a small coil
attached to a diaphragm. The coil is mounted over a per manent magnet as
shown in Figure 6. Coil current in one direction causes the coil and diaphragm
to be repelled from the permanent magnet. Coil current in the other direction
causes the coil and diaphragm to be attracted to the permanent magnet. If a
current of audio frequency is sent through the coil, the diaphragm vibrates and
generates sound waves in step with the current. Thus, if the current from the
transmitter is sent through the coil, the sound produced will duplicate the
sound striking the transmitter.

Figure 4

C1

1,209

C2

1,336HzC31,477Hz

R1 697Hz

R2 770Hz
R3 852Hz

R4 941Hz

1

2

3

4

5

6

7

89

*

0#

SR1

SR2
SR3

SR4

SC3
SC2

SC1

Figure 5

Insulator

Case

Carbon

Granules

Current

Diaphragm

Coil

Diaphragm

Permanent

Magnet

Speaker

Frame

Figure 6

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RINGER

As shown in Figure 2, the ringer is connected across the tip and ring inputs
in series with a capacitor to block the 48VDC. The ringer consists of a
permanent magnet attached to an armature as shown in Figure 7. When
an alternating current of 20Hz is passed through the coils, the armature is
alternately attracted to one coil and then the other. The hammer attached
to the armature thus strikes one bell and then the other to produce the
ringing sound. In a newer electronic type telephone, the hammer and bell
ringer may be replaced by a piezoelectric buzzer used in an electronic
oscillator. A piezoelectric material changes its dimensions when a voltage
is applied. When the oscillator is running, it applies a 3 to 4kHz AC v oltage
to the piezoelectric buzzer. The buzzer changes its dimensions and
produces a 3 to 4kHz sound. The oscillator is turned on only during 1/2 of
each cycle of the 20Hz ring signal. Thus, when a ring signal is received,
the buzzer produces a 3 to 4kHz sound s witched on and off at a 20Hz rate.

INDUCTION COIL / BALANCE NETWORK

When transmitting and receiving is done over the same two wires, the problem arises that current from the
transmitter flows through the receiver. The speaker then hears his own voice from the receiver. This is called
sidetone. Too much sidetone may be objectionable to the speaker and cause him to speak too softly. A small
amount of sidetone is desirable to keep the telephone from sounding dead.

The induction coil and balance network limit the sidetone. The impedance of the balance network shown in
Figure 2 approximately matches the impedance of the balance network shown in Figure 2 approximately
matches the impedance of the local loop. Thus, about half of the current from the transmitter flows through L1
and the local loop and the other half flows through L2 and the balance network. The currents in L1 and L2
induce voltages in L3 of opposite polarity which limits the voltage across the receiver to an acceptable level.
When receiving a signal from the local loop, the currents in L1 and L2 induce voltages in L3 of the same polarity.
These voltages combine to drive the receiver. The newer electronic telephones perform this function
electronically.

Figure 7

Hammer

Bell

Armature

Permanent

Magnet

Coils