Converting a UK GPO746 for US/Canada

[migette]

Hi just a quicky this time. I should of explained how the thermistor works, basically it is a variable resistor, not a fixed value one so one of those will not do. How they work, when ringing is applied to the ringer the bell rings,also when dialing out the pulses cause an emf to be set up and its that that causes the tinkling. A bias spring could prevent this by overcoming the magnetic effect. Sorry about being long winded,but can only explain it this way. Normal ringing is longer in duration, this is enough to cause the thermister to temp change value to a lesser value in ohms and so lets the bell ring. Spurious pulses during dialling do not persist long enough to cause the thermister to reduce the ohmage and therefore the bell does not become energised   no tinkling. Hope this explains sorry it took a long time. Any questions I will try and answer. Thermisters are made of carbon which unlike most coducting metals when experiaces heating up reduces ohmage Filaments in lamps increase resistance Tungsten when a light. Older carbon lamps showed the reverse, preventing them from being operated at high temps. The name thermister means heat resister. Hope this helps.  Peter

[Tribune]

Thanks Migette for the clarification. Makes much more sense to me now.

The GPO 706s and 746s used the GPO 1A-1 thermistor where needed, typically on party lines where interference could be a problem
http://www.telephonesuk.co.uk/miscellaneous.htm (towards bottom of page)
http://www.britishtelephones.com/t706options.htm (fig.7)

According to the limited web search I was able to do, the GPO 1A-1 thermistor has a cold resistance of 200,000 ohms +/- 20%, and a hot resistance (with 100mW) of 500 ohms.

[dsk]

Interesting, could one of these fit?
http://tinyurl.com/39s8wlz

dsk

[dsk]

The easiest may be to add a spring, a spring from a pen may do the job.

dsk

The spring is fixed to one of the terminals, and just pressed on to the ringer.

[Tribune]

Just came across this.

If your eBay-purchased from the UK has been adapted for use on the modern British Telecom network with a BT modular plug and you don't wish to mess with the phone's innards, you can always get a BT to RJ-11 adaptor, complete with ring capacitor built in:-

http://www.antiquetelephones.co.uk/contents/en-uk/p497.html

[gpo706]

I just recently converted 3 706's from the scrapbox to work, they all had the ringing capacitors ripped out of them and I assumed they would be useless without.

Having re-read Mark's article made me realise that the RC was these days in the BT main box now, so now I have another three orphans to take care off.

One is prettied up with a new green case and replacement green handset, the only thing I need to complete it is a green blanking button, which I was tipped off to here:

www.gpospares.co.uk

[GG]

Thermistors are components that decrease resistance as they heat up internally from the passage of current.  They will also produce the effect that the phone will start out by ringing quietly and then louder, kinda' like a Trimphone on the "ascending volume" setting. 

I used to use back-biased 60 - 75 volt (noncritical) Zener diodes for this, two of them, wired tail-to-tail together and then that assembly wired in series with the ringer at any available terminals in the phone. 

Zener diodes are also useful for extension exclusion: install a similar pair in series with the line (I don't recall the voltage rating I used for this; probably somewhere in the range of 24 volts) such that the first phone off-hook gets full line voltage that avalanches the diodes and loops the line for dial tone, but the 2nd phone attempting to go off-hook hasn't enough voltage to avalanche its diodes so it gets silence.  Definitely not for use in situations where a 2nd phone off hook may need to interrupt an existing conversation in order to make an emergency call, unless you also install a bypass switch and resistor (preferably automatically resettable by hanging up the handset, as on 500-series sets with exclusion key, or GPO 706-series with party line button) to enable bleeding enough voltage through the 2nd phone to activate the transmitter without causing the 1st phone to get excluded. 

[Owain]

Zener diodes are also useful for extension exclusion:.....   Definitely not for use in situations where a 2nd phone off hook may need to interrupt an existing conversation in order to make an emergency call, unless you also install a bypass switch and resistor (preferably automatically resettable by hanging up the handset, as on 500-series sets with exclusion key, or GPO 706-series with party line button)

It would be a 710 which had the option of having a latching button reset on going on-hook. I don't think a 706 could do that BICBW

Various forms of extension exclusion / line priority were provided on House Exchange System 3 and 4.

[twocvbloke]

I've had a good read through this thread on these conversions, and I have to add something myself that I haven't seen mentioned here...

If you get a hold of an original and unconverted GPO phone (as in, has loose wires or a BT-52A "Soap box" fitted on the end of the line cord), you can just use the phone as-is, no additional capacitors are necessary as most GPO phones up until the 1980s (when the PSTN standard came in) had ringing capacitors fitted, and most phones that have been converted, assuming the capacitors are still there and working, can be unconverted and rewired internally as per their original specifications...

At the BT-52A (which should have a strap between the A (White Wire) and Bell (Green wire) terminals, if not just jump them with a piece of wire), you just connect your Tip & Ring wires to the appropriate terminals, and then the phone should work off it's original hardware...

(though it seems that the Blue and Green wires can often be swapped round, so the Green is Earth and Blue is Ring, which gets confusing!!!)

A useful N-Diagram on the 746 in it's original, supplied configuration:
(PDF File link)

(Edited cos I made a slight mix up with the A & B wires being strapped to the Ring wire!!)

[GG]

Agreed; I've never had a problem converting a 706 or 746 to work in the US.  If I recall correctly it's just a matter of adding two straps on the terminal board.  This is for the PBX extension usage, where bell tinkle isn't a problem.  For bridged phones across a CO line (it's been so long since I didn't have a PBX:-), zener diodes or thermistors. 

Basically everything you need to know is in the schematic and it's easy to read. 

Now if a modern version is missing a capacitor, one can add one across two terminals on the terminal board.

What's maddening are the latter-day BT phones (and compatible ones such as Autophon Delta) with modular plugs and minus the ringing capacitor, and with the jacks soldered onto the circuit board and no terminal screws.  Almost always there are the expected tiny little drilled holes on the circuit board, where one can insert a capacitor (value range from 0.22 to 1.0 microfarad depending on how loud you want the bells or the tweeter to ring).  But the quality of the printed circuit boards is such that any excess heat can cause the traces to lift and the result looks a mess.  For which another workaround is an adaptor jack and an external terminal strip across which one can screw down a capacitor. 

[twocvbloke]

Regarding the BT-52A, I modified one I got off ebay some months ago with an RJ11 socket (sacrilege I know, but I used a "new" one, rather than one fitted to a phone!!), initially it was a 2-wire RJ11 socket (pic.2), taken from a broken modern phone or modem (I forget which), never used it for anything, I just did it as an act of boredom, but it could be an idea for those wishing to adapt their old GPO phones for use in the US, so long as you fix the BT-52A to something solid (even if it is with velcro so you can remove it with ease)...

The 2nd version (pic.3) was using a 4-wire RJ11, haven't a clue if I wired it up the right way round or not, but again it's just a boredom exercise, using the RJ11 socket from a broken Audioline phone, and it's wired using the original GPO wiring colour standard, so Green is Ringer, and Blue is Earth as per the N diagram (I'm just using a PSTN cord to demonstrate the modification), and you can see on the base (pic.4) how I cut the brown terminal block to fit the RJ11 socket, then glued it in place with a 2-part glue...

[GG]

Dude, that gets major points for clever!  Yes it modifies original hardware, but those terminal blocks are pretty easy to come by and I think they are even available as NOS spares somewhere, so stock up and mod freely. 

[twocvbloke]

Thanks, I forgot to show a picture of how well the RJ11 socket fits the BT-52A, there's no fouling of the lid (though that's the reason why I dug out a notch in the brown base), and with the blanking plastic cut away, the socket is easily accessible to plug in or unplug an RJ11 line cord (either from a wall socket, or to a phone)...

Looking at the early BT PSTN sockets (Jack 620A and 621A), what I made is kind of like the original Line Jack Units, and I suppose you could also carefully trim the lid of a BT-52A to make it a "captive" socket (so the plug can't be removed), like the pics below... 

[GG]

621-A looks like a very clever design.  Terminal strip internally, capacitor, probably can be wired to USA system easily enough.  What's the cost in Pounds?  And is there a source that'll ship to the US?  I'm not going to go modifying any more latter-day BT sets if there's a way to get those jacks here. 

[twocvbloke]

Those line jacks haven't been available since the 80's and the introduction of the standard LJU sockets (followed with the NTE5 Master socket), despite being nice and compact and a simple design... 

The nearest thing would be the LJU1 socket, which is tiny compared to an NTE5 or LJU2 or 3, with the LJU1 being just 5CMx5CMx2.5CM:

LJU1 Link

You could probably just get the LJU1/3A and add the capacitor to make it into a 2A (PBX-Master), as you don't really need the Surge arrestor and the Line Test resistor if you're using them to demo phones...