Google+ Antique GPO Telephones & British Vintage Telephones: 2012

Tuesday, July 31, 2012

21A Microphone Replacements

The British 1960's 706 and 1970's/80's 746 telephones were made to last. I believe they were the last UK telephones that were designed so that they could be serviced and maintained for many years, not just replaced and thrown away.

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.

Tuesday, March 20, 2012

Changing the Microphone in a Bakelite Telephone

Old British Bakelite Telephones such as the 200 series and 300 series telephones used the same handset. This Bakelite handset was known as a model no 164 handset, and was used for over 30 years. It was the standard telephone handset in 1929 and was manufactured all the way through to the early 1960's.

Compared to today’s modern telephone handsets it has a very good ear piece (or receiver). Its only weakness is that the microphone (or transmitter) in the mouthpiece does not live up to today’s expectations of sound quality. This lack of sound quality is only noticeable to the people at the other end of the telephone call. When you are using one of these antique telephones the person at the far end will think you are calling them from the bottom of a well :-) This can be rectified by swapping the old carbon granule microphone out with a modern electronic (electret) microphone. Several makes of replacement electret microphones are available however they must be designed for use in old telephones.

The instructions below show you how this microphone change can be performed.

1) First remove the Bakelite mouthpiece from the handset. If you are unsure how to remove it safely, please see the details on our web page removal of a bakelite mouthpiece.

2) The old Carbon Granule Microphone should just lift out (it is not hard wired).

3) Use a small flat bladed screwdriver to remove the old microphones spring clips (see photographs below).

4 ) Attach two wires to the new electronic microphone ideally red and white wire should be used to make life easier when you fit it (see photograph below). Thankfully new electret microphones used in these vintage telephones are not polarity sensitive, so it does not matter which way round you connect the wires.

5) Connect the new red wire from the microphone to the old red wire inside the mouthpiece, and the new white wire to the old white wire inside the telephone mouthpiece.

6) Modern electronic microphones are smaller than the old carbon granule ones, so you will usually need to cut a small piece of foam to fill the gap to stop the new microphone from rattling around inside the old telephone handset.

7) Finally refit the Bakelite mouthpiece on your handset and test the Bakelite telephone.

Providing the rest of your antique telephone is in good order, the sound quality and usability of your old Bakelite Telephone should be significantly improved.

Tuesday, January 24, 2012

Fitting a 1960's or 1970's Vintage Wall Telephone (Part 1).

This is an explanation of how to physically fit, or safely remove British 1960's (model No 711) or 1970's-1980's (model 741) vintage wall telephones to a wall.

1) The fist step is to locate and unscrew (half way out) the telephone case retaining screw on the bottom of the wall telephone. See the pictures below.

2) Next carefully lift the case starting from the bottom of the telephone, as it hinges at the top (however keep the telephone case parallel to the base plate). This can be tricky if you have not done it before.

TIP: It helps if you hold the telephone switch hooks down and press gently down on the vintage telephone dial while lifting the case from the bottom. See the position of my hands in the diagram below.

3) The vintage wall phone case should now lift completely off. See diagrams of the telephone below.

4) The retaining screw for the separate metal T shaped wall fitting is a single bolt/screw at the bottom, however it is hidden behind where the curly handset cord enters the telephone. So you need to slide the telephone handset cable grommet to the right, as shown below.

5) You should now be able to unscrew the single bolt that holds the T shaped bar to the vintage telephone. As shown below.

6) The telephone is hinged a the top, so simply lift from the bottom as shown above.

7) With the T shaped bar separate fit this to your wall with the appropriate screws for the type of wall it needs to fix onto.

8) Slide the curly handset cable grommet back into its original position.

9) The 741 or 711 Telephone can then be hung onto the T bracket at the top, and the retaining bolt tightened at the bottom.

10) Refit the wall telephone case, making sure the cut out in the plastic dial surround ring is in line with the metal finger stop, see below.

Tip: Holding the switch hooks down and pressing on the dial usually makes fitting the case easier.

11) Finally tighten the case retaining screw at the bottom.

12) These instruction should help, either remove these old telephones from the wall or fit them to a new position.