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 most GPO telephones the main electrical components generally do not go faulty.  Dials, transmitters and handset cords do though.

If it's a newly converted telephone then double check your wiring first.

Newly Converted Phone - No dial tone

  1. Disconnect the blue and green line cord wires.  Phone should have dial tone, be able to dial out but will get no incoming ring.  If not check further.

  2. Double check all wiring and metal links/straps - then check again.

  3. On phones with Bakelite handsets, remove the earpiece and underneath you should find a black or yellow plate.  This will slide off revealing two screws.  Ensure these screws are tight and have a spring washer under them.

  4. Remove newly installed Rectifier No. 205 if fitted.

  5. To test the new line cord, firstly disconnect the line cord and connect the red and white wires together.  Plug into the line and call the line from a mobile phone.  If the call is engaged the cord is good.

No Dial Tone

  1. Ensure that dial is in the home position.

  2. On 706, 710 and 711 telephones - make sure that there is a small regulator card, that is plugged into the phone,  between the switch hooks and that it is inserted properly.

  3. Check connections are good on handset cord and line cord.

  4. Check that switch hooks are functioning mechanically.

  5. On Bakelite and wooden phones ensure that the receiver has a diaphragm (metal plate) fitted under the ear cap (the ear cap unscrews and the diaphragm slides sideways to remove).  After removing the plate, two screws will be revealed - ensure these are tight and that there is no bits of metal on the magnets.  Also check that the screws have spring washers.

  6. Check that the two non-impulse contacts on the rear of the dial, are apart when the dial is normal.

  7. Listen on the receiver (don't put to close to ear) and with a screwdriver - short out transmitter - if you get dial tone then transmitter is missing or faulty or the handset cord broken internally.

  8. Listen on the receiver (don't put to close to ear) and with a screwdriver - short out the impulse contacts on the back of the dial (these are normally closed) - you should loose dial tone when shorting one set of contacts.

  9. 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, dial a digit 0 and blow gently into the mouthpiece - if you hear a sound like frying bacon or a harsh noise coming back at you, then the transmitter is faulty - replace with the same type or upgrade to a modern electronic microphone.  Short out the transmitter terminals with a pair or pliers or a piece of wire - if the noise goes then the transmitter is faulty.

  2. On 746 type telephones listen on the receiver and very 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 as handset is 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 wriggle - if the noise comes on, then move the cord and see if the noise changes.  Remedy is replacement, or cut back on cloth covered cords.

Bell does not ring

  1. If the phone is converted then check that the resistor is installed or a metal link is fitted in place of the resistor.

  2. 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, hitting the gongs.

  3. Ensure that both bell gongs are not touching the bell clapper arm at the same time.

  4. On 700 type telephones, metal swarf can sometimes be attracted to the magnet and get into the mechanism.

  5. Bell ON/OFF switch operated.

Bell faint

  1. Bell gong fixing screws are loose and gongs have rotated and closed together.

  2. Bell clapper has dropped off - replacement of the whole bell assembly is required.

  3. On 700 type telephones - paper wedged between clapper and bell gongs.

  4. On 300 types telephones - paper jammed in the paper tray.

Faint outgoing speech

  1. Remove the transmitter and the phone should give no dial tone.  If you get dial tone then it is normally wrongly wired.

  2. Generally the mouthpiece - on Bakelite's check that transmitter is seated OK.

  3. 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.

  4. On 200 and 300 telephones bad transmission is generally a faulty old transmitter.

  5. On 700 types, a black cased, silver, metal topped transmitter, will generally be noisy.

Faint incoming speech

  1. Generally a faulty earpiece on 700 type telephones - replace (clue is normally a rattle from the earpiece).

  2. On Bakelite's - 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.

  3. On Bakelite's - also check that the two earpiece fixing screws have spring washers under them and are not loose.

Unable to dial out

  1. All Digital Voice lines and any VOIP lines supplied by other carriers, will not support dials.  You can buy a small converter that will convert the dial pulses to tones.  Click here if you have a dial telephone.

  2. Check that dial is running at the right speed - dial a digit 0 and as it returns say the following at normal speech speed - "one, one thousand and one".  The dial should reach it's stop as you finish.

  3. Check that dial is wired up correctly. 

  4. All GPO dials are wired as follows - looking at rear of dial and the contact set screws upper most, the dial is wired from left to right as Blue - Grey - Brown - Pink - Orange.

  5. On 300 types ensure that the dial is also connected to the terminals block inside the phone correctly as well.  Release the three screws that hold in the chassis.  Turn the chassis so you can see where the dial cord terminates.  With the bell gongs towards you the dial cord is wired, on the telephone base, from left to right, Blue - Grey - Brown - Pink - Orange.  If the Pink and Orange wires are linked with a metal strap - remove it.

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 (a dab of light oil on the end of a small screwdriver).

  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 - requires replacement.

    Warning - removing the dial return spring can be dangerous, as it is under tension and is made of spring steel.  Never remove the spring from the spring housing, the spring and housing should be removed and replaced as one!

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. Oil on the central spindle has gone hard - complete strip down to remove the central spindle.

  3. 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 by rubbing a flat screwdriver blade between the contacts.  Test by shorting out the contacts one at a time.  One contact will disable the microphone and the other the receiver.

  2. Dial wired incorrectly.
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Last revised: April 21, 2023