Telephone Fault Finding

This is a simple information sheet on how to do simple fault finding using no sophisticated tools. Reference will have to be made to the relevant diagrams whilst using this page.  Every telephone cannot be covered in it's entirety, but all general faults will be mentioned.  This page assumes that the telephone did work and has now ceased to function correctly.  Telephones found in an unknown state can be faulted by the use of this page, but they may have more detailed problems outside the scope of simple faulting.

First things first - if a telephone doesn't work, replace it with a known working telephone to isolate the fault away from the line or socket!

Most faults are very simple.  On the 100, 200 & 300 type range the components generally do not go faulty.

Newly Converted Phone - No dial tone

  1. Disconnect the blue line cord wire.  Phone should have dial tone, be able to dial out but will get no incoming ring.
  2. Double check wiring and metal straps.
  3. Remove new installed rectifier 205 if fitted.

No Dial Tone

  1. Ensure that dial is in the home position.
  2. Check connections are good on handset cord and line cord.
  3. Check that switch hooks are functioning mechanically.
  4. On Bakelite and wooden phones ensure that the receiver has a diaphragm (metal plate) fitted under the earcap.
  5. Check that the non-impulse contacts on the dial are apart when the dial is normal.
  6. Listen on the receiver and with a screwdriver - short out transmitter.
  7. Listen on the receiver and with a screwdriver - short out the impulse contacts on the back of the dial (these are normally closed).
  8. With a screwdriver - short out the switch hook contacts one at a time (there are two on the Bakelite telephones - you cannot do this on a 746 telephone).

Noise whilst talking

  1. Listen on the receiver and blow gently into the mouthpiece - if you hear a sound like frying bacon then the microphone has gone - replace with the same type or upgrade to a modern electronic microphone.
  2. On 746 type telephones listen on the receiver and slowly move the switch hooks up and down.  If you hear a harsh crackling then the switch hooks are faulty.  These cannot be replaced, so it's a new circuit board - try spraying the switch with a general purpose switch cleaner, it may work.
  3. Clonking/knocking noise is handset moved on 700 type phones is normally the receiver.  This can only be replaced.
  4. General noise which worsens if handset is moved.  Possibly the handset cord - normally goes faulty where the cord meets the handset.  Pull the handset cord tight and test - if the noise goes then you have the culprit.  Remedy is replacement on 700 type phones, or cut back on cloth covered cords.

Bell does not ring

  1. Ensure that bell fixings screws are all present and that bell mechanism is free and that the clapper arm can swing from side to side.
  2. Ensure that bell gongs are not touching the bell clapper arm.
  3. On 700 type telephones, metal swarf can sometimes be attracted to the magnet and get into the mechanism.
  4. Bell ON/OFF switch operated.

Bell faint

  1. Bell gong fixing screws are loose and gongs have rotated and closed together.
  2. On 700 type telephones - paper wedged between clapper and bell gongs.
  3. On 300 types telephones, paper jammed in the paper tray.

Faint outgoing speech

  1. Generally the mouthpiece - on Bakelites check that microphone is seated OK.
  2. If the caller is unable to hear completely, but you can hear them, then check the dial contacts.  With the dial normal the two small contacts should be apart.
  3. On 200 and 300 telephones bad transmission is generally a faulty old transmitter.

Faint incoming speech

  1. Generally a faulty earpiece on 700 type telephones - replace.
  2. On Bakelites - remove the earpiece cover, then remove the earpiece diaphragm (by sliding the metal plate sideways) and check that there is no swarf on top of the pole faces inside the earpiece and that the plate is clean and not rusty.

Dial is slow to return or stiff

  1. The dial governor should be checked for dust and fluff.  Oil sparingly the bearings and the governor cup face sparingly.
  2. The dial centre spindle could be seizing.  Normally a strip down - try dropping a very small amount of oil into the central spindle first.
  3. The return spring could be broken - normally requires replacement.

    Warning - removing the dial return spring can be dangerous, as it is under tensions and is made of spring steel.

Dial does not return

  1. Dial return spring broken - requires complete strip down - purchase a replacement and either transplant spring or migrate dial parts to replacement dial.
  2. Finger stop bent and fouling on finger plate.

Loud clicking noise in ear whilst dialing out

  1. Usual cause is one of the two smaller springs on the dial - clean the contacts.  Test by shorting out the contacts one at a time.  One contact will disable the microphone and the other the receiver.

 

BACK Home page BT/GPO Telephones A - Z Index Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: November 22, 2007

FM