A 706, with a difference.

[Stephen Furley]

Telephone Lines, in Cheltenham, are rather like Phoneco; they've got some interesting stuff, on three floors of a building, but the website could be better.  They've got a large number of 'junk' 706 telephones, I think basically sold for parts; they're all missing dials, and some other parts as well.  They sell these on Ebay in lots of ten for 7.50 (pounds), buy it now.

When I was in the shop a few months ago I asked if I could buy just one, but if possible I wanted it to be grey, and a mark 1 model; the one with the plastic base, and conventional wiring rather than a pcb.

The mark 1 version is by far the less common, but as luck would have it they found one, near the top of one of the large crates full of the things.  Price 75 pence.  It was absolutely filthy, but at that price you can't complain.  The body was very badly yellowed, far worse than any other that I've seen, and it seemed to be a combination of ultra-violet yellowing and decades of tobacco smoke.  It was also covered in something black and sticky, tar possibly.  The line cord had been cut off at about a foot long, it was full of some sort of industrial dirt, and the transmitter seemed unlikely to function, judging by the sound it made when shaken; otherwise, it wasn't too bad!

I managed to clean most of the dirt off it long ago, and I have spare type 16 transmitters available.  It's still pretty yellowed, but looks a lot better than it did.  The reason I wanted a grey one was there was a grey dial blank on Ebay, which I managed to buy quite cheaply, and the reason that I wanted a Mark 1 version is that it contains a terminal for joining what would normally be the dial pulse contact wires, and three holes for storing the shunt contact wires; the mark 2 version needs to have an extra component, which I've never seen one of, fitted to provide this facility.

We now had a CB telephone, and it worked when tested on an exchange line, but this was not the aim of the exercise.  The 706 was the last standard GPO telephone to support local battery (LB) working, and a LB model was what I wanted.

Local battery working was pretty much obsolete by the time the 706 was introduced, and the GPO must have had very few of them installed, certainly, when the 746 was introduced a few years later they felt able to drop the option altogether.  There would have been some on private systems, but the only significant one was probably on the railway for point to point working between signalboxes.  However, the conversion was a fairly simple one to make; unlike some other models, such as the SC 1000, no special induction coil was required.  According to the N-diagram the parts needed, in addition to the dial-less CB conversion, would be:

Two extra straps fitted to the terminal block, I had some of these spare.

One Adaptor, Local Battery No.5.

One Generator, No. 26 AN or No. 26 AF, I managed to find one of each on Ebay.

One Block Terminal No. 35A, I found one of these, dis-used, in a service riser at work, and have since picked up another on Ebay.

One Box, Battery No.3.  If this is what I think it is then it's a varnished wooden thing, which I'm unlikely to find.

2 or 3 Cells, Dry R40, to fit in the above.  These haven't been made for many years.

Since I couldn't get the cells I didn't really need the battery box, a battery holder for three 'D' size alkaline cells, in a plastic box, would do.  Neither the battery box nor the generator were made to match the 706; they were both the same models used with the previous Bakelite 'phones.  If I should find an original battery box in future then I can always fit modern cells inside it.

The line cord is just an ordinary 4-conductor one, two wires being used for the line, and the other two for the battery supply.  The BT 35A is a six-wire junction box, used in place of the normal four-wire BT 52A.  The battery box and generator are both connected to the BT, rather than directly to the telephone, which is why a six-wire version is needed (only five are actually used) rather than a normal four-wire one, even though the line cord has only four wires.

The only remaining problem was the 'Adaptor, Local Battery No.5'.  This is a small device installed on the hookswitch frame of the telephone consisting of some additional switch contacts, in series with a coil.  After about a year of waiting, one of these finally turned up on Ebay recently.  If I can find one more of these then I've got all the parts that I need to set up a pair of LB 'phones, but for now I'll have to use it connected to my linesman's test set.

The extra switch contacts are a normally-closed pair that open to cut off the battery supply when the handset is placed on its rest, and if I've interpreted the circuit diagram correctly, the coil is to block the audio frequency signal from simply circulating within the transmitter circuit, and force it to go through the induction coil, and down the line.

The regulator has to be taken out of circuit by reversing it in its slot; I think this is to prevent it draining the battery.

The circuit is on the last page of N=806, available here:

http://www.samhallas.co.uk/repository/n_diagrams/0000/N806.pdf

I'll take some pictures sometime in the next couple of days.

[Stephen Furley]

I've now got this thing connected up.  Ringing works in both directions, and it receives ok from another set, but there's a problem with voice transmission.  The sound level in the set's own receiver is about what I would expect it to be, but it's very low when received at the other end of he line, only three metres away.  Increasing the Battery Voltage seems to make little difference.  I've had it connected to two different test sets, operating in local battery mode, and it's the same in both cases.  The two test sets operate correctly when connected to each other.  I'll have to look into it when I've got a minute.  Reversing the polarity of the battery, or the line, doesn't help.

Does anybody have experience with local battery sets?

[Phonesrfun]

Local battery sets are actually the most efficient design of them all for voice transmission.  Here is a very simplified drawing of a standard local battery circuit.  The battery is in series with the transmitter and the primary of the induction coil. and the receiver was typically in series with the secondary.  There are variations on this theme, and I have omitted the condenser and the hookswitch from this circuit.

Take a look at your local battery set and see if it is wired like this.  Do you have a link to the circuit for the phone in question?

Battery polarity is not important, and 1.5 or 3 volts should do just fine.

Probably what is happening is that the induction coil is seeing CO voltage on its secondary and getting saturated.  Remember that a local battery system did not have any common battery voltage on the two lines.  Only voice-level (milivolt) AC and AC ringing current.  By connecting the secondary directly to the telco line, you are probably swamping out the inductor and needing to overpower it by brute force by upping the voltage in the primary.

A couple of ways to test the transmission on the local battery set are:

Both of these tests would be with the local battery phone disconnected from the phone line.

1.   Short out or jumper the two line terminals, usually designated L1 and L2 here in the states.  With the phone off hook and talking in the transmitter, you should hear yourself loud and clear in the receiver.
2.   Instead of shorting out the L1 and L2 terminals, hook a regular phone, any phone will do, because you are using the second phone as basically an earphone.  Hook that second phone to the L1 and L2 terminals of the local battery phone.  Again, you should hear yourself loud and clear in the second phone.  You will, however not be able to hear the second phone in the first, unless it too was a properly functioning local battery set.

There are some simple circuits out there that allow you to effectively interface a local battery set on common battery lines that are not generally intrusive to the LB set.  Others have gotten wild and totally rewired their sets.  (Not recommended if you want to preserve the original phone)

[Phonesrfun]

Stephen:

I appologize, I did not fully read your last posts before I jumped to the conclusion that you had this connected to the Common battery CO.  I also see you have a schematic of the phone.

So, disregard what I just said, and I am going to study your information previously presented.  I am leaving what I just said posted, however, but I may revise it.

[Phonesrfun]

Stephen:

Having now actually read what you earlier posted, and reviewing the schematic, my only comment would be that with the hand generator hooked up and the capacitor shorting strap in place, maybe the switch in the generator is not opening, and thus maybe the gernerator winding is always across the line A and B.

I would imagine that the purpose of the regulator circuit serves the same function as the varisistors do in the WE 500 sets.  Perhaps they are not properly disabled and thus sucking up some of the supply battery voltage.  Try just jumpering points B, C, and D with alligator clip jumpers.

Try putting a jumper direct from T12 to the primary center tap at pin 2 of the inductor.  What happens then?

Sorry I can't be of more help

[Stephen Furley]

It's working now.  I was thinking about it earlier today, and had a feeling that it might me a problem with the generator switch, and now I've overhauled it the problem's gone away,  It looks rather odd to see a relatively modern 'phone like a 706 with a hand generator, but it was a configuration offered by the GPO, even if it would have seldom been installed.  All I need now is one more Adaptor, Local Battery No. 5 and I can have a working pair of them.

I'll try to get a picture of it tomorrow.

[Stephen Furley]

This is the local battery 706, the generator and the Block Terminal BT35A.  The generator looks really out of place, but this is the model which the GPO would have supplied for use with the 706 in the unlikely event that you'd needed a local battery one.  Note that the generator is almost as big as the 'phone, and probably weighs more.  The dial card, which says 'To call operator lift handset' is for CB use, and is not appropriate in this case.

All of the connections are made at the BT.  The brown cord is the standard line cord for a two-tone grey 706 or 746, but the connections are different.  The black cord is for the generator, the red and black loosely-twisted wires are the battery supply, and the bit of yellow and blue jumper wire is the line.  This should be proper cable of course, but I used the bit of jumper wire just for testing.

Inside the 706 the two sloping things are the hook switches, the left one is the normal one, and the right one, with the coil attached to it, is the Adaptor, Local Battery No.5, which I need another one of to be able to set up a pair of these; I have all the other parts needed.  On the back of the generator can be seen the switch which connects it to the line, and disconnects the telephone, when the handle is turned.

[McHeath]

That is really interesting, a modern style phone with a hand crank generator.  Does this phone work?  Wouldn't it need to be rewired to work on the modern system?  I'm guessing that this phone probably lived in some far away rural setting during it's active life.

[Stephen Furley]

That is really interesting, a modern style phone with a hand crank generator.  Does this phone work?  Wouldn't it need to be rewired to work on the modern system?  I'm guessing that this phone probably lived in some far away rural setting during it's active life.

McHeath,

I'm not sure what you mean by a 'modern' system.  Do you mean connected to a normal automatic exchange?  If so, then that's how this telephone spent most of it's life.  I converted it to local battery mode, but in doing so I did exactly what a GPO engineer would have done if he had received an order to install a local battery instrument with generator signalling from about 1960 onwards.  The GPO only stocked a few configurations of telephones, I think there was one with a recall switch already fitted, and at one time a dial-less CB version was also stocked, but this was discontinued sometime during the life of the 706 (1959-1967).  If a special model was needed then a standard model would be requisitioned, together with any necessary additional parts and these would be assembled by the engineer to produce the required configuration, and any parts not needed, e.g. the dial, would be recovered and returned to stores.  The way that I've set this up is just as a GPO engineer would have done it with a few minor exceptions:

1. The Block Terminal should be screwed to the wall.

2. The line connection should be via telephone cable, rather than the bit of old jumper wire which I've used for testing.

3. The battery should consist of three large R40 cells, in a wooden battery box, rather than three 'D' cells, connected by croc clips which I'm using at present.  I'm not sure what type of cable was used between the battery box and the block terminal; I suspect just ordinary telephone cable.

So, it previously worked in auto mode, it now works in local battery, generator signalling mode, and it would also work in several other modes to which it could be converted.  By the addition of a new line cord and a resistor, and with a few connections moved around on the terminals, it could also work on the current BT system, which uses a bell capacitor in the master socket, rather than one in each 'phone.  It needs to be connected either to a local battery switchboard or exchange (if you can find one), or directly to another local battery telephone.  Connected to an engineers set switched to local battery mode, with neither set supplying power to, nor drawing it from the line, it works fine.

If you mean would it work if connected to an exchange line as it is currently (forgive the pun) configured, then no.  It would ring, but it wouldn't take power from the line, it won't work without the local battery supply.

I looked up the parts specified for this particular conversion, obtained them from various sources, and assembled them as per the instructions in the N-Diagram.  I haven't put inappropriate parts together, though it certainly looks like it; if you had ordered a local battery generator signalling set from the time that the 706 was introduced, then this is what you would have got.  Presumably a more modern-looking generator was never introduced because the demand for them would have been so small by this time.  I doubt that even the existing model, as seen here was still in production, they would probably have obtained a reconditioned one from the stores which had been recovered from a previous installation.

The six wire block terminal (BT35A) would have been readily available new at that time as it was used with both the 706 and the 746 in a number of configurations, including various extension plans and certain types of shared service, which required a six conductor line cord.

Although the 746 replaced the 706 for most purposes in 1967, the 746 couuld not operate in local battery mode and so a refurbished 706, modified to the configuration seen here, was probably what would still have been supplied until BT stopped supplying 'phones for rental, sometime in the early '80s.  How many were actually installed, I don't know.  The N-Diagram to which I worked to make this conversion is revision 'H', dated 1976.  The local generator version is figure 7, on page 4.  All pages of this diagram are specified to be printed on white paper, 'Paper W' on the first page; this means that they can be used for new work, obsolete configurations which can no longer be installed for new work, but which may still be encountered in existion installations are printed on yellow paper.  This means that the generator signalling version was still current, at least in theory, in 1976.  When the last one was actually installed is anybody's guess.

The two slits, which look rather like eyes, in the strap between the two gongs, and the front notch in the metal bracket above the hook switches are the mounting points for the dial; the similar notch just behind it was for mounting a press-button switch if one was required.  All 706 telephones had these features, plus the wiring for a dial, even if no dial was originally fitted, for ease of later conversion to automatic mode if required.