Inside the Telephone

In this laboratory we will dismantle telephones and identify some of the major components. The telephones will also be reassembled. As you dismantle the phone remember that you will also be putting the phone back together so try not to lose any part.

After dismantling and reassembling a working telephone you will then get a broken telephone to repair. Don’t panic, the telephones have been deliberately broken in a way that can be fixed. As always, help will be available when you have difficulties.
 

Part 1: Names of Telephone Components
 

Telephone Anatomy

1A.) Review the name of each part of the phone body shown on the Figure below. Identify each part on your telephone. These terms will be used when referring to parts of the telephone in later sections of the laboratory.

1B.) Phone Body (front). This is the main part of the phone with the numbers and place to put handset.

1B.) Phone Body (back view). Handset (the part that you hold in your hand and talk into)





1C.) Handset (the part that you hold in your hand and talk into)





Part 2: Verification of Proper Operation
 

Basic Talking/Listening Test
1.) Working with a partner, attach your telephone to the mounting plate on the wooden board on your table. With this test board, the phones cannot dial numbers or ring in response to a dialed number. The connection allows talking back and forth only.

2.) Verify that you able to and hear each other.

3.) Verify that you are each able to hear a tone when the numbers on the number pad are pressed on either phone.

Dialing/Ringing Test
4.) Working with a partner, bring your telephones to the test station in the front of the room. If the stations are all being used proceed as directed by the instructor. Once you are at a spot, attach your telephone to a test station.

5.) Dial your partner’s number to verify that the telephone works. You should take turns sending and receiving a call. Please work quickly so other people in the laboratory can complete this test.
 
 
 
 
 

Part 3: Disassembly

In this section the telephone will be disassembled. The primary purpose is to give you an opportunity to examine the components and learn the function of each. Basic knowledge of how the telephone works is essential in fixing a telephone that is not working.
 

Handset

Your phone conversation begins and ends in the handset. The handset contains a microphone that converts your speech into an electric current that is transmitted to the person to whom you are speaking. The handset also contains a speaker that converts the current produced by the other person back into sound which you can hear. The handset is connected to the phone body with a flexible wire cord.

Disconnecting Handset from Telephone Body.

1.)Disconnect both ends of coil cord and set it aside. To do this, push down on plastic tab while pulling.




Examine Handset Speaker.

1.) Unscrew the ear piece of the handset.

2.) Tip the handset until the speaker falls out. There will be two wires connected to the speaker which will keep it from falling off completely. Examine the handset speaker. Try to see portions of the coil of wire used to make the speaker electromagnet.

3.) Examine the wires that connected to the speaker. These wires bring the current to the speaker from the main circuitry of the telephone.
 
 

What color are the wires that connect to the speaker?
 
 
 
 

WRITE YOUR ANSWER HERE:

4.) Remove the speaker from the handset by loosening (but not removing) the two screws on the speaker. Disconnect the wires and set the speaker aside.
 
 
 
 
 

Examine Handset Microphone.

1.) Unscrew the mouthpiece of the handset.

2.) Tip the handset until the microphone falls out. The microphone is a black and silver disk-like object. There are no wires directly connected to the microphone. The microphone used in these telephone is a carbon microphone originally invented by Thomas Edison.

3.) Pull out the gray or brown plastic holder for the microphone. Observe the wires that are connected from the handset jack to the microphone.
 
 

What color are the wires that go to the microphone?
 
 

ANSWER:
 
 
 
 
 
 

If there are no wires directly connected to the microphone, explain how electrical contact is made with the microphone?

ANSWER:
 

4.) Set the handset components aside for later use.
 
 
 

Phone Body

The next step is to open up the telephone to examine the internal components.
 

Removal of the Telephone Body Outer Plastic Case

1.) Examine the Phone Body. Can you guess how the plastic housing might be separated from the metal base? Write your answer below, but don’t do anything yet.
 
 

ANSWER:
 
 

2.) The phone is held together in a very sneaky way. The screws that must be loosened are hidden behind the number plate. To open the phone, follow this procedure:
 
 

3.) Remove the piece of plastic that protects the slip of paper with the telephone number on it. One side of the plastic cover has a small hole drilled through it. Push the cover to one side as far as possible to reveal the hole.
 
 

4.) Put a slotted screwdriver into the small gap created between the wall of the plastic and the body of the phone. In other words put the screwdriver blade into the slot right on top of this small hole. Push on the screwdriver to cause the clear plastic piece to bend up and out from under the body of the phone. Once the plastic is bent up, grab onto it or continue pushing with the screwdriver until it can be removed from the body of the telephone.






5.) Put the plastic piece in your parts tray.
 
 

6.) Once the plastic is removed, remove the slip of paper.
 
 

7.) Two screws are now visible. These screws hold the phone body onto the base.
 
 

8.) Unscrew both screws in the front of the phone. They will not actually come out. When they have been loosened as much as possible a faint clicking or banging noise will be heard each time you turn the screw one complete rotation.
 
 

9.) Remove the plastic front from the telephone. This may require pulling in several different spots until the body breaks free of the base. It may help to push on the cradle while pulling up on the body.








Disassembly of Base

The next step will be to disassemble some of the components from the base.
 
 

1.) Unclip the phone cord plug located at the bottom of the phone. This is accomplished by sliding the gray plug up and off the metal base. This plug is used to connect the handset to the body of the telephone.

2.) There are four wires going to the handset plug. Two connect to the handset speaker and two connect to the handset microphone. Which wires go to which?
 
 

ANSWER
 
 
 
 

Number Pad Removal

3.) Next the number pad will be removed. Obviously, this is used to dial the phone. Notice the thin metal plate around the numbers is used to protect the number pad circuit from a static electricity shock.
 
 

4.) Remove both screws from either side of the number pad. Set these aside.
 
 

5.) Remove the number pad. This may require wiggling the pad. There are small plastic bumps that fit into holes on either side of the screw holes. To remove the pad, these bumps must be dislodged from the holes.
 
 

6.) Being very careful to not disconnect any wires, flip the number pad over. Underneath the number pad is a circuit board that contains many different components to make the telephone work.
 

Electronics Components:
 

Name Description Function
Capacitor Blue, gray, or orange-brown disks Stores electric charge
Resistors Brown striped cylinders lying on side with several colored stripes Provide a specified resistance to the flow of electric current
Diodes Small clear glass cylinders with one stripe or black cylinder with white stripe One-way valve for flow of current 
Transformers Red or green coils of wire in a square metal frame Controls voltage levels
Transistors Small black cylinders with one flat side Amplifies or shut-offs flow of electric current
Integrated Circuit (Chips) Black rectangle with rows of metal pins on each side Perform special purpose functions such as electronic storage of phone numbers
Crystal Silver or gray metal canister about 1/8 inch thick x 1/2 inch wide x 3/4 inch high Used to create tones when dialing

 
 
 
 

Connection of Wires to the Circuit Board

1.)Observe the method used to connect the various wires to the circuit board. These are not permanent connections. The connection is made through a removable clip-like connector. There is a sample circuit board with all of the wires removed so you can see what the clips look like without wires.
 
 

2.) There is a right and wrong way to remove the wires. When removing the wire should be grasped by the metal part as shown in the figure. PULLING ON THE WIRE WILL BREAK IT AND RENDER YOUR PHONE INOPERABLE.
 
 

3.) Remove any one of the red wires by pulling on the METAL END of the wire with the pliers. It does not matter which red wire you remove. WE ARE ONLY GOING TO REMOVE ONE FOR PRACTICE.
 
 

4.) Replace the wire by slipping the metal end back into the SAME clip from which it was removed.





Connection to the external phone lines.

1.) The telephone must be connected to the lines that carry in the signal from the phone company exchange. Can you identify where the telephone connects to the external lines? What are the colors of the wires used to connect the external lines to the circuit board?
 
 

ANSWER
 
 

2.) When the phone is attached to the wall, an electrical connection must be made to the four wires carrying the telephone signal. Examine connector on the bottom of the base used to attach the telephone to the external lines. Examine also the connector on the test board used to connect your phone for the initial testing. Based on your examination explain how the electrical connection is made between the telephone and the external lines. Draw a sketch to aid in your explanation.
 
 

ANSWER and SKETCH
 
 
 
 
 
 
 
 
 

Cradle

The cradle serves two functions: it holds the handset and provides a means to activate the hook-mechanism. The hook-mechanism must be activated when answering a call or dialing a number.





1.) Explain what happens when the handset is down (the phone is hung up). In other words, what sequence of events takes place in hanging up the phone that causes the switch to change position? Draw a simple picture to aid in your explanation.

ANSWER
 
 
 
 
 
 

Switch

2.) The switch must somehow make electrical connections (connections between pieces of metal) when the handset is removed from the phone. One such connection activates the dial tone. Examine the switch. Can you locate where on the switch electrical connections are made when the handset is removed from the cradle? Draw an arrow on the figure below identifying these connections.

ANSWER






Removing the Cradle
 

3.) The next step will be removing the cradle. Examine the cradle. How do you think the cradle is removed? In other words, what must be loosened, pulled, or taken apart to take the cradle off the telephone?

ANSWER
 
 
 
 

4.) Remove the screws on either side of the cradle. Remove the cradle from the telephone.

5.) Separate the gray hook mechanism from the cradle. How did you do this?

ANSWER
 
 
 
 

6.) Now that the cradle and gray latch have been removed, the switch can be more easily examined. Find a way to make the hook switch change from ON to OFF position. Explain how you accomplished this. Would you say that changing the switch position requires a large force, a moderate force, or a low force?

ANSWER
 

The Bell

1.) Locate the electric bell on the telephone. The function of the electric bell is to make a ringing noise to alert the user that there is an incoming call.

2.) Examine the bell mechanism. Find a way to make the bell ring a single time by manipulating one of the components of the bell mechanism. (The bell mechanism is a sturdy device so you do not have to worry too much about breaking anything while handling it.) Describe how you were able to make the bell ring. Use the photograph of the bell mechanism to aid in your explanation.
 
 

ANSWER








Consider the bell mechanism to be made of the three subsystems shown below. Explain briefly the function of each system and how the three work together to produce the sound heard from the bell.

ANSWER

Review

1.) Before reassembling the telephone we will review the function of the main telephone components.

Fill in the Component side of the following table. Use the components from the following list:

Hook Mechanism, Wall Jack, Handset Jack, Circuit board, Metal Base, Electric Bell, Handset Microphone, Handset Speaker, cradle
 
 

Component Function
Holds telephone components
Provides means of determining when phone is hung up or off hook
Holds Handset
Connects telephone to the external telephone exchange
Connects handset to main body of the telephone
Holds electronics which perform functions of dialing, activating bell, sending and receiving signals
Alerts user that a call is being placed to the telephone
Converts sound waves into electrical signals
Converts electrical signals into sound waves

 
 

Part 4: Reassembly
 

The following procedure is recommended:

  • Cradle
  • Reassemble the cradle and disconnecting latch.
  • Replace the cradle assembly and tighten the screws.
  • Number Pad
  • Carefully flip the number pad back over and set it back in its original position.
  • Tighten the screws.
  • Telephone Body
  • Clip the phone cord plug back in its original position.
  • Replace the plastic front and tighten the screws.
  • Replace the slip of paper with the phone number on it and replace the plastic phone number protector.
  • Handset
  • Replace the microphone and screw the plastic mouthpiece back on the handset.
  • Replace the speaker and screw the plastic earpiece back on the handset.
  • Plug both ends of the phone cord back in.
  • Testing
  • Complete the talking/listening test with the wooden test board at your table.
  • Complete the Dialing/Ringing test with the test station at the front of the room.
  • Obtain TA or instructors signature to verify that the reassembled telephone works.

 

Part 5: Troubleshooting and Repair of a Broken Telephone
 
 

1.) Obtain a "Broken Telephone."

2.) What do you think would be a first step in fixing this phone?

ANSWER
 
 
 
 
 
 
 
 

3.) Test your phone using the wooden test station at your table to determine the symptoms of the problem. List these below:

ANSWER
 
 
 
 
 
 
 
 

4.) What may be some possible causes for the problem?

ANSWER
 
 
 
 
 
 

5.) Proceed with repair of the telephone. Describe the steps you went through to fix the telephone. What were some things that you tried? How did you narrow down the problem? What were the actual repairs? What advice would you give someone else who is trying to fix a similar telephone. Use the space below or another sheet.

© 2001 John J. Krupczak, Jr.
All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of the author is prohibited.