Can anyone help identify this phone please?

[russell_w_b]

The case (at least) was made by E. K. Cole (same manufacturer as 'Ekco' radios). It bears the mark 'KKA'.

[mickash]

Hi Andy,

There may well be a date code stamped into the capacitor body, not the batch code on top of the cap.

The number 17 moulded into the case indicates it's made of ABS. Diakon mouldings are marked with a 12.

My Green GPO Field Trial 706 has the 3D arrows in the dial plate. It has a clear finger plate which I'm assuming is original.

I would think your phone is a non GPO version. The parts are probably all original to the phone but the original dial surround most likely would just have had numbers not numbers and letters.

Michael.

[andy1702]

I tend to agree with your thoughts there. It looks like they were just using up the bits they had to make a special type for someone. In the same box I also got a dial-less 746, which appears to have been jsed on some sort of military base if the screwed up bit of paper I found inside the receiver is to be believed! So perhaps these phones were on a private system of some kind.
As a matter of interest, what's the age of your green 706? I'm guessing it must be a later one.

[Pellseinydd]

Originally Posted by mickash

There may well be a date code stamped into the capacitor body, not the batch code on top of the cap.
The number 17 moulded into the case indicates it's made of ABS. Diakon mouldings are marked with a 12.
My Green GPO Field Trial 706 has the 3D arrows in the dial plate. It has a clear finger plate which I'm assuming is original.
I would think your phone is a non GPO version. The parts are probably all original to the phone but the original dial surround most likely would just have had numbers not numbers and letters.

The original dials on GPO Tele 746's had metal fingerplates and the silverish '3D' arrows on the plate beneath. Not long after introduction into service, the coloured finger wheels with the matching coloured plate beneath with arrows without the 3D effect(Dial No 21. The clear finger wheels were not introduced until the mid/late 1960's when the Tele 746 (which only ever had clear finger wheels on GPO models) were introduced. I remember seeing the first 706 coming out of the box in the Chester Telephone Area in 1959 - metal finger wheel (with '3D' arrows beneath) and the 'grebe' fabric covered cord - wasn't until sometime in 1960 that the curly PVC handset cords were introduced as the stocks of those with grebe cords were used up. My earliest issue of the N diagram ( N806) give the date of the change as Feb 1959 which is before they were introduced into service. The Post Office Telecommunications Journal has an article on the 'new phone' and mentions they are expected to become available in Spring 1959. It mentions 'Grebe cords are all a greyish colour on all colours of phone'. I've only managed to acquire one 1959 Tele 706 with original dial and grebe cord. See attached photo from the article - earliest one I can find in a GPO publication and just as I remember the first ones. Note the fabric covered 'grebe' cord and the dial with the '3D' arrow heads.

Ian
GW4BTW
CNet 0352 2345

[andy1702]

I think you've solved it there Ian. That looks spot on to me, except for the fact mine has the curly cord. I've also got an ABS case but a Diakon handset.

What threw me was that silvery paint on the back of the 3d arrows. It looks the same as on the 746 dials, so I thought it must have come immediately before the 746 came along. However if I've understood correctly then the flat printed colour coded arrows came between the two.

I must say this metal dial has a much better quality feel than the later plastic ones do.

Andy.

[winston_1]

Originally Posted by andy1702

I think I'll replace the resistor with the correct one and do a proper conversion with a new line cord.

There is no such thing as the correct one as it should not be there at all. The idea of a 3K3 resistor was dreamt up by someone who did not understand the concept of impedance relating to inductance. I never fit one as it does not do much.

[TonyDuell]

For inductive bells (as in the 746) you are probably right (the correct modification is to replace the bell with one with 4K DC resistance and no idea what inducance) but for the tone ringer (as used in a TRIMphone) the official change is to put a 3k3 resistor in series. Take a look at N8822, for example.

[russell_w_b]

A typical 59A bell impedance is 1550 +j2032, or a Z of 2556 at 25Hz. By fitting a 3k3 resistor you're dropping nearly as much voltage across the extra resistor as you are across the bell. The current through the bell coil is typically 15mA if a resistor is in circuit, as opposed to 35mA with the resistor strapped out. This is for one instrument.

The bell will work happily down to about 10mA but beyond that becomes a little indifferent.

[Nickthedentist]

Thanks for the maths, Russell. Useful to have hard figures rather than educated speculation.

N.

[russell_w_b]

Originally Posted by Nickthedentist

Thanks for the maths, Russell. Useful to have hard figures rather than educated speculation.

You're welcome, Nick. I have figures for one, two, three and four instruments connected - with and without additional resistors. I did some experimenting a while back with a 75V 25Hz PABX ringing generator.

I didn't mention the capacitor in the LJU but it goes without saying that its reactance cancels the inductive reactance of the bell coils, allowing the aforementioned current to flow. So with one instrument, the ringing load will be capacitive overall.

But in real terms on a practical line there may be 1000 Ohms or so line resistance and ringing conditions will be different to my 'bench' set-up. And there's a thing called 'ring trip', which, I believe, is like an overload setting on the exchange and will react to current spikes. Maybe one of the ex-GPO peeps would explain?

[andy1702]

Just to clear this up for non technical people (i.e. me) when converting a 746 etc to plug and socket, what should be done to make the correct conversion? Should the resistor be there or not? I know you're going to say change the bell coils, but they are not a readily available unit, so that's not often practical from a sourcing point of view. So what's the alternatives?

Andy.

[Nickthedentist]

Russell's calculation suggests it SHOULD be there, but Winston thinks it's un-necessary.

Suck it and see. The worst that can happen is that the REN would be too high with no R fitted, which means other phones on the line wouldn't ring properly.

N.

[russell_w_b]

I've managed so far without the resistor on all but one connected instrument, and I'm 6.8km from the exchange, as the electron flows. I have 1 x Ericsson 1071 (same gizzards as a 332), one 746 and one 741 (with 3k3 resistor). Before connection of the 741 I ran a 706 without the resistor. On top of which are an Ascom 'berkshire' electronic ringer and a Panasonic cordless two-wire.

I think the answer is to suck it and see. It may depend on your line-length, the robustness of any trip mechanism on your exchange, and the strength of ring required. I don't see the need to fit a resistor on a single instrument as it's what the line was designed for in the first place.

If you wish to take a more unorthodox approach it's possible to use the spare way in your standard BT plug to wire the bells of two instruments in series, as used to be done pre-liberalisation, so limiting the current AND getting it to do something useful instead of dissipating energy in a resistor. But you'd make the instruments unique to the house by doing so.

[Nickthedentist]

Great minds think alike

Originally Posted by Nickthedentist

Suck it and see.

Originally Posted by russell_w_b

I think the answer is to suck it and see.

[AndiiT]

Hi,
With reference to fitting a resistor to maintain the "correct" bell motor impedance, I believe that Andy's intention is to use his 700 type telephones on the extension ports of a Revelation PBX so the resistor probably won't matter that much as the ringing generator in the Revelation seems to be quite forgiving of high REN's, at least it is in my experience of that model of PBX (I have owned at least three Revalation's at different times and am using one for my phone and PBX/HES/etc. collection currently)

As an aside, I am using three of the extension ports on my Revelation to connect to a 3+12 PMBX cordless switchboard and found that I had to extend the "line length" by fitting a 300 ohm resistor in each leg of the appropriate exchange line feed to the 3+12 otherwise the line relay(s) in the switchboard would trip the Revelation ports to a busied out condition on the first ring.

Regards

Andrew

[andy1702]

I am indeed connecting phones to a Revelation. But as I've got some duplicates (about half a dozen ivory 746s) I'm also thinking about swapping or selling some, so I'd like to make sure they're converted properly so they will work on other people's systems / ordinary phone lines.

I've installed resistors in all the phones I've refurbished over the last few weeks and tested them both on my home extensions, which also have two other phones on them and also on the Revelation. I've not had any problems at all apart from finding occasional faulty units. So it seems that putting the resistor in certainly doesn't hurt anything.

[mickash]

Originally Posted by andy1702

As a matter of interest, what's the age of your green 706? I'm guessing it must be a later one.

Sorry Andy. The phone is stashed away so safely I can't find it at the moment.

[andy1702]

You must have got a filing system like mine!

[andy1702]

I've done the 'normal' plug conversion on this phone now, but there's a problem. When I lift the handset the dial tone is so faint in the speaker I can hardly hear it. It will dial other phones ok and I can ring in to it, but the speaker is so faint I can't hear anything from the calling phone. Also the calling phone doesn't seem to be able to hear anything coming from this phone's mic. The only thing on this phone that can be heard clearly (too clearly actually) are VERY loud clicks when the dial is returning.

I've tried changing the speaker in the handset, but the fault persists. Could it be that this phone needs some kind of special line to work correctly? Or could there be a faulty component inside?

Andy.

[Nickthedentist]

Post a pic of your internal wiring etc. so we can have a look.

It does sound like there's a fault but it could well be something trivial.

Nick.