Dismantling a Telephone No. 706
Let's take a telephone to bits -
Telephone No. 706
One of a series of articles where I dismantle a telephone to show its construction and pass my own opinionated comments on the design and choice of material.
The design drew inspiration from various telephones already in service from Ericsson, Siemens, and AT&T, and most definitely from the Automatic Electric AE80. In addition to the innovative case design, the internal circuitry made use of advances in technology to provide improved speech transmission. So everything about the 706 was new: new case design; new handset; new dial; new transmission circuitry; new cordage and block terminal.
I bought the two telephones above specifically for this project at the Chatteris sale earlier in the year. Between them they provide examples of different elements of design. 706s were produced in a hard-wired version and a printed circuit version. The wired version is called Mark I and the printed circuit version is Mark II, although both were produced simultaneously depending on the individual manufacturer's preference. The telephones as found had been recovered from site and left in store for maybe thirty years. They appeared to be simply very dirty and I expected they would look good once cleaned.
Let's turn the phones over and have a look at the bases which shows the difference in the two styles of construction very clearly.
The wired phone (above left) has a thermoplastic base made from toughened
polystyrene. This is a good choice of material being resilient and
virtually unbreakable. The hollow circular areas to the left are the
supports for the bell gongs. The circuit board phone (above right) has a pressed
steel base with a crackle paint finish. Again steel is a good strong
material and resistant to damage. The bases of six rivets are visible
which support the bell pillars and circuit board.
The feet are made from a synthetic rubber, usually called Neoprene, which does not harden as readily as natural rubber. So, despite their age, the feet of these two telephones are still fairly springy. Early versions of the 706 had domed feet. [ref 1] These proved unsatisfactory as the phone skidded about during dialling. Later models have the ringed feet like these which act like mini rubber suckers and help prevent skidding. [Ref 5]. The manufacturer’s markings show that the wired phone dates from 1964 and the circuit board one from 1965.
Taking off the lid
The wired telephone has the original chrome escutcheons around the cover
fixing screws, whereas the red phone has the optional carry handle.
Both the telephones have a blanking plate in the hole provided for a push
button switch. Remove the cover by loosening the captive screws. I
always approve of captive screws! To lift the cover away from the
body, hold the body still by placing your fingers in the dial with one hand.
Pull upwards and forwards on the cover with the other hand. If need be
the captive screws can be released by pulling upwards and turning
The number ring is clear and the numbers and coloured background are silk screened on the back. This has not proved very durable as the ring shifts slightly in use and tends to wear away the printing. The circuit board one is particularly badly affected. Later dials have a clear plastic finger plate. The numbers are printed behind it, where there is less chance of them wearing off.
With the cover turned over you can see (above) how the metal escutcheon and the carry handle are held in place. The escutcheon can be removed by levering gently with a screwdriver from above. The carry handle needs only light pressure with a screwdriver from below to avoid breaking its lug. The Post Office instructions recommends straightening the escutcheon lugs and you may want to do this before refitting the escutcheons as they can be reluctant to go back.
The button blanking plate can be removed by sliding the retaining spring out with a finger. The cases are now dismantled ready for cleaning. Reassembly is straightforward. The dial ring is a bit fiddly to get in position. First line up the lugs on the metal ring with the indentations in the plastic rings, then squeeze the two together. There is a cut-out to fit round the dial finger stop on the case, the dial ring and the steel fixing ring, which must be aligned on reassembly.
With the cover removed it's easier to compare the two models. There are similarities - all the circuit components are the same - and differences - their positions are different. At this stage it also became plain that the circuit board phone was in a sorry state. It looked as if it had been submerged at some time! Many of the steel parts are rusty - more than just storage in a damp place would cause.
I understand that some telephones released as scrap have had the capacitors removed. This is due to a belief that they contain poly-chlorinated biphenyls (PCBs). This is almost certainly true of the tubular paper capacitors used in Telephone 706 but is almost certainly not true of the polycarbonate capacitors used in later telephones. But, as long as they are not leaking they are safe.
The hookswitch mechanism is a natty piece of design. Firstly the polythene stubs which protrude at the top are designed to be noticeably smaller than the aperture so there is no danger of them sticking. The knife edge pivot eliminates the sliding peg designs of previous telephones, also aiding the non-stick design. The single bow spring is simple, cheap and unlikely ever to wear out.
On the side of the case support pillar is another neat feature of the 706. The spring clip on the green phone and the swinging lever on the red phone allow the hookswitch to be latched on-hook so that the phone can be serviced at a customer site without busying the line.
The switch itself is well-designed using a plastic comb to operate bifurcated contacts allowing consistent spring pressure (above left). The plastic cover keeps dust from falling between the contacts. The only difference in the two models is the mounting and terminals. They both use the same fixing holes on the case support pillar. You may think that the steel bracket on the printed circuit version is unnecessary. However, it stops any tendency for the cradle rest mechanism to rock the contact assembly and loosen its solder joints.
While we're looking at this portion of the 706's construction, notice that the case support pillar is pre-drilled with all the fixing holes that may be required for accessories. (see above right) This means no need for multiple variants in stockholding or on-site drilling. Everything bolts into place.
Dial and Mounting
The dial is held in this circular clamp arrangement and on many versions is shrouded with a clear polythene cover to keep out the dust (very early models did not have the dust cover). The small screw at the front of the dial needs to come right out, but the larger clamping screw at the back should only need to be loosened sufficiently to allow the clamp to be removed (below left). Once the cover was off I put the small screw back in to avoid losing it. The drying marks round the rim of the circuit board phone's dial lend credence to my belief that the phone had been underwater at some time.
Time to remove the dial cord. To save you writing them down, the colours go: Orange, Pink (toward centre), Brown, Slate, Blue. Early telephones 706 were fitted with a Dial No 12 with a metal finger plate, but the two here have a Dial No 21. Both types of dial have the cunning trigger action which you can see by winding the dial up and watching the pulsing mechanism as it dials out. As mentioned earlier, later models of Dial No 21 had a clear plastic finger plate in polycarbonate with numbers behind.
To get the finger plate off, the window must be removed - the Post Office called it a 'label protector'. The window is a push fit into the centre of the dial. On coloured dials the fit is fairly tight so a spring is provided which can push the window out when a small screwdriver is turned against it as shown above left (slide the screwdriver towards the dial centre and then twist). I've taken the label out so you can see the spring clearly. On the clear dials the fit is much slacker and the PO rubber sucker, Extractor No. 29 (see above right), should be used. Alternatively use a length of sticky tape held in the fingers. Press its centre onto the dial window, then pull evenly. The windows on coloured dials are such a tight fit that it's a real struggle to make the Extractor 29 rubber sucker work, so use the screwdriver technique. Next, undo the screw in the centre and lift the finger plate clear. Release the spring retaining the number ring and lift it clear (see below left).
If the finger stop needs to be cleaned you can remove its retaining screws see above right) and lift it out. I'm not going to take the dial any further apart.
Pictured above are all the bits from the dial. [Back row: polythene contact cover, main dial mechanism, circular clamp. Front row: inner number ring, finger plate, finger stop, window (label protector), finger stop screws, finger plate screw, support bracket screw, number ring retaining spring.
The Terminal Area
The sheer number of links between terminals is testimony to the thought that's gone into the circuit design. By rearranging the straps a multiplicity of different circuit types can be catered for: shared service, extensions plans, switchboard extensions, local battery working, and so on.
Dismantling the Base - Printed Circuit Version
Four cheese-headed screws hold the circuit board in place. Two captive 3BA screws either side of the steel bracket (on the left) and two 4BA screws towards the front behind the bell. The front ones only need to be loosened to release the circuit board. Similarly the bell gongs and dial mount can be taken off, by removing the 4BA screws, leaving the steel base. There is a plastic support for the circuit board at the back of the base. This prevents the board cracking under pressure from screwdrivers when the terminals are being tightened. Two other plastic pegs further forward provide support.
The circuit board is made from synthetic resin bonded paper, SRPBP, which was the natural choice for the time. Fibre glass boards were unusual and expensive at this time as well as being unnecessary for this application. The soldering appears to be done by hand, although very neatly. The board has lacquer coating over the track with only the solder lands exposed making it suitable for flow soldering. Terminals 11 to 18 are not soldered whereas they would have been if the board had been through a flow solder bath. The white deposits are the remains of resin flux as a result of age and dampness.
There's not much to clean on the circuit board. I brushed the terminals with a brass wire brush which took off a lot of the rust visible in earlier pictures. I rubbed the bell clapper with abrasive to smooth off the corrosion. The base had a similar brushing. Repainting didn't seem to be justified in my mind.
This telephone is a Mark 2A, as we saw earlier. This means that the bell is soldered in place. The earlier Mark 2 had the same bell as the wired version with a wire link between the two coils.
Dismantling the Base - Wired Version
Firstly the capacitor can be unclipped by levering the retaining clip as shown above. Similarly the transformer is clipped into place and can be released with a screwdriver. The cradle switch contacts simply unscrew. The bell unscrews from below.
Next, after taking a careful note of the wire colours, I unsoldered three
wires from the transformer and one wire from the capacitor. Two other
wires I cut to avoid damaging them where they were double soldered onto a
I took the feet out of both phones. They come out very easily by pressing from the inside. However, they are pigs to get back. I can see no advantage in washing them separately so I advise you not to bother!
The bell gongs are mounted slightly eccentrically. When refitting them they should be turned so that the clapper just fails to touch them when stationary. Waggle the armature with your fingers to make sure that the clapper hits the gong and then falls back without damping the ring. The two gongs are tuned slightly differently to give that characteristic jangling ring. The wired telephone has Bell-gongs No. 23 and 23A, the printed circuit telephone has Bell-gongs No. 24 and 24A. I'm not sure why there is a difference. I have a wired Mark 1 from two years later which has Bell-gongs No. 24 & 24A the same as the printed circuit Mark 2 here.
Contrast of Constructions Methods
The innovation is on the left of the circuit diagram with the automatic sensitivity regulator, the Regulator No 1. A detailed explanation is given in the POEEJ article [Ref 2]. Here is a simplistic description. The line current is passed through the resistor lamp (shown as a positive temperature coefficient resistor). This is the black painted glass bulb that looks like a lamp. As line current increases it provides an increasing potential difference across the back-to-back diode bridges - the red tubular item with nine leads. These diodes in turn partially shunt the receiver to reduce the sensitivity. At the time of design the public network was entirely Strowger based and current on short lines could reach 95 mA. Recent experiments [Ref 4] have shown that the regulator only provides a shunting effect at currents above 40 mA. On modern electronic exchanges where the line current is limited, it would have no effect.
Early regulators had clear glass lamps. Of course they glow a dull red as the line current passes through them. The story goes that nervous customers telephoning in the dark saw the phone glowing and dunked it in a bucket of water thinking it was on fire. Whether this is true or not, all later lamps were painted black.
The regulator can be reversed and as you can see from the circuit track layout, in that position it simply shorts out terminals B, C and D to maintain continuity. I don't know when it would be necessary to use the telephone in unregulated mode - maybe for local battery working.
Summary & Conclusion
The smooth lines of the case and handset were right for the modern era of the Fifties. The transmission was improved ready for the future explosion of subscriber trunk dialling which started the previous year. The 706 electrical circuit design made it the most versatile telephone yet developed in terms of the number of different ways the basic circuit could be configured - central battery, local battery, shared service, extension plan working and so on. The most frequently used variants - press to call/ recall and bell on/ off were included in the basic design.
However the days of the 706's supremacy were numbered. The younger and cheaper upstart, Telephone No. 746 came on the market only a few years later.
Anyway, happy 50th birthday in 2009, Telephone No 706!
Last revised: January 30, 2021