The only weakness of these vintage telephones was the carbon granule microphone (transmitter inset No 16) used inside the telephone mouthpiece. These old carbon granule microphones degraded over time, resulting in crackling and noise that usually gets worse as the telephone conversation goes on. Looking at the photograph below the carbon granule microphone is on the left with a silver top and black base.
BT realised this weakness and started fitting electronic (electret) microphones in the early 1980's. These modern microphones were called "Microphone Insets No 21A". They significantly improved the sound quality experienced by the person at the far end of the telephone call as well as being far more reliable. The red microphone on the right in the picture above is a 21A microphone.
Collectors of old telephones will often pay a premium for these "Transmitter Inset No 21A" so that they can replace the old carbon granule microphones in their vintage 746 telephones, in turn bringing the speech quality up to modern standards, and improving reliability.
Original 21A microphones are hard to find in good condition. Of the few available spare 21A microphones in circulation many have suffered from moisture seeping into the circuitry, which results in a faint hum or buzz during the call which can be very annoying on a long telephone call.
To overcome this shortage of good quality 21A microphones several enterprising British telephone restorers have introduced a brand new equivalent electret microphone.
These new electret microphones were originally designed and made outside of the UK as replacements for the carbon granule microphones used in old American 500 series telephones. These 500 series antique telephones were the American cousins of our British 700 series telephones. However because the American microphones were physically smaller than the UK counterparts these new microphones have been augmented with a British designed and manufactured front/top so that they now fit snugly into 700 series telephones.
The photograph below show the front/top of these new microphones (on the right) as compared with the original 21A microphone on the left.
As you can see from the rear photographs below the new microphone (on the right) has a smaller diameter, however the British manufactured front/top prevents the microphone from rattling around.
The procedure below explains how to fit these replacement microphones using only a small flat bladed screwdriver (takes about 5 mins).
Step 1: Unscrew the mouthpiece cover.
Step 2: Remove the old carbon granule microphone.
Step 3: Unscrew the two microphone terminals and gently remove the two spade connectors.
Step 4: Pull out the metal retaining ring (be careful these are sometimes sharp).
Step 5: Attach the two wire to the modern electret replacement microphone. It does not matter which way round you attach them as they are not polarity sensitive.
Step 6: Fit the new microphone into the mouthpiece as shown in the two photographs below.
Step 7: replace the screw on mouthpiece. Sometimes it is necessary to be firm with the last screw turn as the microphones have a crimp up area to make them a snug fit.
Your vintage/retro telephone should now have as good if not better speech quality than a modern land line telephone.