Vandalised GPO 2/764

[Jim_746]

I picked up a GPO keyphone (2/764 GNA 79/2) yesterday. I connected it up the phone line and it worked, I was also able to dial out using the cradle. However on opening up the phone I found that the original line cable had been cut out leaving the spade connectors in their terminals and that a 2 wire cable had been connected so the bells would never ring! I also found that the battery secondary No. 23 had been removed and the wires from the battery to the keypad had been cut, not only that but the previous owner had also completely disconnected the keypad! So I ask you how do I wire that key pad back up and can I power the keypad directly from the line. I have checked http://www.britishtelephones.com/t764.htm found the following:

POWER CIRCUIT STRAPS
To change between the different methods of battery charging, straps within the telephone must be moved. The telephone is supplied with the Battery strapping set to Line charging and the charging leads connected to T9 (yellow) and T19 (green).

Line charging - Terminals T26 - T27 are strapped together.
For 50v local power feed - connect power (see next paragraph) to the telephone, remove strap from T26 - T27 and strap across T27 - T28.
For 10v local power feed - connect power (see next paragraph) to the telephone, remove strap from T26 - T27 and strap across T28 - T29.

When using a local power (external) feed, move the yellow & green wires from T9 & T19 and terminate them on spare terminals (T11 - T15 are usually spare). Then connect the local power source to these terminals.


I suspect that the local power feed is if you wished to power the keypad from an external source and not for charging the battery. Am I in the right area?

Cheers,
Jim

[Dave Moll]

If what you show in your second picture are the leads from the vertical connector block at the left of the cradle support to the keypad, I'm not sure that I can give much assistance, as my 764 has wires soldered directly into the innards of the keypad.

Is there any chance of a photograph of the keypad itself, showing the terminals? Peering into the innards of my keypad, the solder tags do appear to have numbers, so it may be possible to map these to the terminals to which your leads attach.

Also, the likelihood is that the battery will need replacement. Assistance with this can be found in a link from the page referred to in your post.

[Jim_746]

Here are the requested photos.

[AndiiT]

Hi,
A "quick fix" for the non - ringing bell in your phone would be fit a strap between T17 - T18. this diagram may help with the wiring of the keypad, in the meantime I will try and dig out one of my push button telephones and see if I can identify where the wires should go for you - from the N digram I have enclosed I can see that some of the connections are as follows -

Orange of keypad to T8
Green of keypad to T19
Yellow of keypad to T9
Pink of keypad to T10
Brown of Keypad to T3
Blue of keypad to T2
Slate of Keypad to T1
Red of Keypad to Batt +ve
Black of Keypad to Batt -ve

The remaining six wires go to terminal block T26 - T31 but as the diagram does not show any colours this is where we need a phone to compare with.

The external power supply would have only been used where more than one phone was fitted at an installation or suitable line charging current was not available. I seem to recall an earlier thread where another forum member successfully converted a battery powered keypad phone of this type to work from a low voltage "wall wart" supply.

Regards
Andrew

[Jim_746]

I've now got the orange, pink, brown, blue, slate, green and yellow leads connected up but I cannot connect the black or red power leads as they have been cut off ! One thing I noticed was before the keypad was connected to the phone the receiver was very quiet and once I had connected it up it was as loud as my 746, does anyone know why?

Cheers,
Jim

[Dave Moll]

Sorry, I misunderstood your original picture. I now realise that you were showing the ends of the leads from the keypad, but it sounds as though Andrew has got your connections sorted out. I'm not clear from your description whether you still have the (snipped-off) original battery pack. If not, your simplest approach would be to follow the instruction given about acquiring new batteries and to wire them directly to the snipped-off leads to the keypad.

[AndiiT]

Originally Posted by model746

One thing I noticed was before the keypad was connected to the phone the receiver was very quiet and once I had connected it up it was as loud as my 746, does anyone know why?
Cheers,
Jim

This is due to the path that the speech current takes; with no keypad fitted there are no dial contacts and to maintain the correct speech path terminals T8 - T10 should be shorted together.
When this link is broken, such as with no dial assembly fitted, a small amount of transmitter current is fed via C1 and the gravity switch contacts 1,2 to the transmitter assembly.

With the link maintained the speech current travels through the transmitter assembly via the anti - sidetone network and via the induction coil, where a secondary winding on the coil provides speech current for the transmitter (via terminals 2, 3 and resistor R1)

More information about how a 700 type phone works can be found here

Andrew

[Jim_746]

I have had a look at the under side of the of the keypad to see if they suggest which terminals the leads might go. Does this suggestion seem anywhere near the correct wiring:
White, red band: 31
White: 30
Purple: 29
White, green band: 28
White, blue band: 27
Red: 26

Cheers,
Jim

[Chipmunk]

From my understanding, the 'local power supply' would have been used if the line current couldn't be used, but it would have charged the battery. It was usually connected via the extra cradle switch contacts so it would be disabled when the phone was off hook and only charged while on hook. I haven't got one of these at present to look at closely, but I *BELIEVE* the original battery was a 2 button cell NiCd, therefore a pair of Alkaline AA cells would work, with a couple of diodes in series to prevent them being charged, along with dropping the voltage by the required 0.6volts. I may be wrong about this though, but it's worth looking into as a possibility to get this phone working (button size NiCd's are now like hen's teeth thanks to ROHS legislation!)

[edit] Looking once again at the notes on the above-linked N diagram, yes, the power unit would have charged the battery only, and not powered the keypad directly.

I'd imagine a pair of AA alkalines would last at least a year, given the fact it was designed to be low powered and is only in use when the phone is off hook. (I think they take power when offhook but with the keypad idle as well as when actually dialling).