|
|
|
|
Off Line
Telephone Tester |
|
H
ere is a circuit of an off-line telephone tester which does not
require any telephone line for testing a telephone instrument. The
circuit is so simple that it can be easily assembled even by a
novice having very little knowledge of electronics. A telephone
line may be considered to be a source of some 50 volts DC with a
source impedance of about 1 kilo-ohm. During ringing, in place of
DC, an AC voltage of 70 to 80 volts (at 17 to 25 Hz) is present
across the telephone line. When the subscriber lifts the handset,
the same is sensed by the telephone exchange and the ringing AC
voltage is disconnected and DC is reconnected to the line. Lifting
of the handset from the telephone cradle results in shunting of
the line’s two wires by low impedance of the telephone instrument.
As a result, 50V DC level drops to about 12 volts across the
telephone instrument. During conversation, the audio gets
superimposed on this DC voltage. Since any DC supply can be used
for testing a telephone instrument, the same is derived here from
AC mains using step-down transformer X1. Middle point of the
transformer’s secondary has been used as common for the two
full-wave rectifiers—one comprising diodes D1 and D2 together with
smoothing capacitor C1 and the other formed by diodes D3 and D4
along with filter capacitor C2. The former supplies about 12 volts
for the telephone instrument through primary of transformer X2
which thus simulates a source impedance, and a choke which blocks
AC audio signals present in the secondary of transformer X2. The
AF signal available in secondary of X2 is sufficiently strong to
directly drive a 32-ohm headset which is connected to the circuit
through headphone socket SK1 via rotary switch S2. During ringing,
a pulsating DC voltage from transformer X1 via rectifier diode D5,
push-to-on switch S3, and contact ‘B’ of rotary switch S2 is
applied across secondary of transformer X2. The boosted voltage
available across primary of transformer X2 is sufficient to drive
the ringer in the telephone instrument. Please avoid pressing of
switch S3 for more than a few seconds at a time to prevent damage
to the circuit due to high voltage across primary of transformer
X2. The circuit also incorporates a music IC (UM66) whose output
is connected to secondary of transformer X2 via switch S2 after
suitably boosting its output with the help of darlington
transistor pair T1 and T2. This output can be used to test the
audio section of any telephone instrument. After having assembled
the circuit satisfactorily, the following procedure may be
followed for testing a telephone instrument:
1. Connect the telephone to the terminals marked ‘To Telephone
Under Test’and switch on mains (switch S1).
2. To test the ringer portion, flip switch S2 to position ‘B’ and
press S3 for a moment. You should hear the ring in case the ringer
circuit of the telephone under test is working. Please ensure that
handset is on cradle during this test.
3. For testing the audio section, flip switch S1 to position ‘C’
and connect a headphone to socket SK1. Pick the telephone handset
and speak into its microphone. If audio section is working
satisfactorily, you should be able to hear your speach via the
headphone. If you dial a number, you should be able to hear the
pulse clicks or pulse tone in the headphone, depending on whether
the telephone under test is functioning in pulse or tone mode. If
the telephone under test has a built-in musical hold facility, on
pressing the ‘hold’ button you should be able to hear the music.
Now flip switch S2 to position ‘A’. You should be able to hear
music generated by IC1 through earpiece of the handset of the
telephone under test, indicating propor functioning of the AF
amplifier section. The circuit can be assembled on a small piece
of veroboard. Try to mount the two transformers on opposite sides
of the board, displaced by 90 degrees. Always keep handy
multi-type modular plugs for testing various types of telephones.
Mount all switches, sockets and LEDs on the front of testing panel
|
|
Click
Here For Circuit Diagram
|
 |
|