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
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