Telephone Lab
Definitions:
Phone Body (the main part of the phone with the numbers and place to put handset)
Front View
Hook Mechanism
Cradle
Handset
Phone Body
Number
Pad
Ringer Volume
Adjustment
Plastic Phone
Number
Protector
Coil Cord
Back View
Ringer Volume
Adjustment
Wall Jack
Handset (the part that you hold in your hand and talk into)
Handset
Speaker End
Microphone
End
Handset
Jack
Procedure:
Disconnect both ends of coil cord and set it aside. To do this push down on plastic tab while pulling.
Plastic Tab
Handset
Jack
Handset
Unscrew the ear piece of the handset.
Examine the handset speaker.
Q1 What color are the wires that connect to the speaker?
Unscrew the mouthpiece of the handset.
Examine the handset microphone.
Q2 What color are the wires that go to the microphone?
Q3 Explain how electrical contact is made with the microphone.
Phone Body
The next step is to open up the telephone to examine the internal components.
1.) Examine the Phone Body. Can you guess how plastic housing might be separated from the metal base?
Write your answer below, but don’t do anything yet.
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:
2.) 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. 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.
Put the plastic piece in your parts tray.
Once the plastic is removed, remove the slip of paper.
Two screws are now visible. These screws hold the phone body onto the base.
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.
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.
The next step will be to disassemble some of the components from the base.
Before proceeding take a minute to examine the base in its assembled condition.
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.
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?” Why or how does this do that?
Remove both screws from either side of the number pad.
Pull out the number pad.
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.
Identify the following components and verify their location with the professor or t.a.
Electronics Components:
Name
Capacitor
Description
Blue, gray, or orange-brown
disks
Function
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
Red or green coils of wire in a
square metal frame
Small black cylinders with one
flat side
Black rectangle with rows of
metal pins on each side
One-way valve for flow of
current
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
Transformers
Transistors
Integrated
Circuit (Chips)
Crystal
Controls voltage levels
Amplifies or shut-offs flow of
electric current
Perform special purpose
functions such as electronic
storage of phone numbers
{Samples with circuit board removed from keypad}
While the phone is open, we will observe some important components.
Connection to the external phone lines.
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?
ANS__
The circuit board is also connected to the telephone handset.
What are the colors of the wires used to connect the circuit board to the handset?
ANS__
Correct
Proper placement of needle
nose for removing wires.
Incorrect
Improper placement of needle
nose for removing wire.
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.
Hook-Mechanism
Cradle
Switch
Blow-up of Switch after Cradle and
Hook-Mechanism were removed
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?
ANS__
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.
Electrical connections must also be made when the handset is in the “hung up” position. For example,
there must be a connection to the ringer mechanism so the phone can ring when a call comes in. Describe
where these connections are located on the switch.
ANS__
A spring is used to change the position of the switch when the handset is “picked up.” Locate the spring on
the switch (DO NOT ATTEMPT TO REMOVE THE SPRING. IT IS VERY DIFFICULT TO GET IT
BACK ON CORRECTLY). Now examine the sample spring. Describe the properties of springs which
makes them suitable for use in the switch.
ANS__
Removing the Cradle
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.
ANS___
Remove the screws on either side of the cradle. Remove the cradle from the telephone.
Separate the gray hook mechanism from the cradle. How did you do this?
ANS___
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?
ANS__
Can the entire switch mechanism be removed from the body of the telephone using the tools that you have
available? Why or Why not.
ANS__
The Bell
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.
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.
ANS__
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.
Bell
Hammer
Electromagnet
Electrical connections
How many wires are needed to provide the electrical signal used to activate the bell? What color are these
wires?
Volume adjustment.
Identify and examine the volume adjustment on the telephone. Explain how the volume adjustment
controls the loudness of the sound produced by the bell.
Volume Adjustment
ANS__
Removal of Bell Mechanism
The next step is the removal of the entire electric bell mechanism. The bell mechanism can be removed
from the phone by removing only TWO screws. Identify and remove these two screws. Remove the bell
mechanism from the telephone base. If you run into any unexpected problems seek assistance from the
instructor or a TA.
Review
Before reassembling the telephone we will review the function of the main telephone components.
Fill in the following table:
Component
Metal Base
Hook Mechanism
Function
Holds telephone components
Provides means of determining when phone is
hung up or off hook
Cradle
Wall Jack
Holds Handset
Connects telephone to the external telephone
exchange
Handset Jack
Connects handset to main body of the
telephone
Circuit Board
Holds electronics which perform functions of
dialing, activating bell, sending and receiving
signals
Electric Bell
Alerts user that a call is being placed to the
telephone
Handset Microphone
Handset Speaker
Converts sound waves into electrical signals
Converts electrical signals into sound waves
Reassemble
The following procedure is recommended:
Replace the bell mechanism
Set the bell and motor back in the phone.
Make sure that the white plastic tab for the volume control fits inside the hole in the clear plastic lever.
Align holes on the base and replace the screws.
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.