UK '746' Keypad Problem

[andy1702]

Sooo... I bought this strange phone. It's from the UK, in a normal 746 case, but has an US style keypad fitted instead of a dial.

Well, it arrived yesterday and it doesn't work. I plugged it in and when I lifted the handset I was amazed to hear a train of dial pulses going out without me touching any of the keys. This must have pulsed 10 because it made my operator phone on my PBX ring, which normally rings by dialling zero. I hung up, gave it time to reset then picked up the handset again and got my PBXs correct internal dial tone. However when I pressed any of the keys nothing happened. No pulse and no tone either.

If I ring the phone from anotehr it rings ok, but when I pick it up I hear dial pulses again. This upsets my PBX, so I hang up and as soon as I replace the handset it rings again. This time I can pick it up ok and hear / speak to the caling phone ok. But still the keypad does absolutely nothing.

I've altered the terminals in the phone as necessary to make it compatible with UK plug & socket and checked the line A+B connections, which seem to be getting about 44v when on hook. The only other thing I've done is disconnected the optional button, all the terminals of which I've screwed together on the unused T15 (which is what I often do with 706/746 phones.

This is not a normal 746 because a BT issued phone would never have been called 746 if it didn't have a dial. Aparently this was made by a company called IBEX, who were making phones in 746 type cases long after normal BT 746 production had ended. So although it's called a 746 and uses a lot of 746 parts, it's not actually what we normally know of as a 746.

One interesting thing I've found is what appears to be a power connection going to the keypad circuit board, which has an empty jack socket of some sort on the end of it. There's a mark taht suggests this was stuck down inside the phone. Maybe I need to plug in some sort of battery to provide power to the keypad? there is no sign of anything like that ever being fitted though. Apart from the socket with nothing to plug into it that is!

If anyone could have a look over the attached photos and give me any pointers I'd be very grateful. This has completely stumped me. Why would it be pulsing anything when it has a * and a # on the keypad. Surely that should be DTMF, shouldn't it?

[andy1702]

Another photo.

[twocvbloke]

Could you get a better shot of that black component marked "EMP40-0038-001" and it's surroundings? If it's what I think it is (a rechargeable battery), then that may be the cause of the lack of activity, as I believe some of the original GPO push-button design dials used batteries in circuit to power the keypad electronics, and this recharged off the line voltage in the later models (earlier ones required the battery be replaced or recharged externally), if it is a battery, perhaps leaving it overnight on a powered line will help recharge it...

If not, then it's most likely that it is recharged through that 3.5mm jack, assuming the battery (if it is one) ain't dead of course...

[andy1702]

That black thing isn't a battery. That was my initial thought too, so I unclipped the cove and found what appears to be a long coil of fine wire covered in white tape (a bit like PTFE tape) under it.

The connections that go to that socket are marked with a tiny + and - on the corcuit board. So that and the fact they're red and black iis making me think something plugs in there. I have no idea what voltage though.

[twocvbloke]

Hmm, there goes that idea...

If the thing is meant to have a battery, and if it follows the GPO 764 dial standard, it could be that the jack is meant to have a battery pack connected to it, and the original battery packs in those phones were I believe 7.2v NiCad packs, so if you hooked 6v up to it (4xAA cells should do it) and it comes to life, that could be the answer, but I'd give the terminals on that jack a probing first to see if the phone is trying to power the socket as if to recharge a NiCad pack, don't want to unintentionally blow up some non-rechargables!!

[andy1702]

There doesn't seem to be any voltage to that jack. Bob Freshwater has these photos on his website and one of them shows a similar circuit with what appears to be something attached to it.
http://www.britishtelephones.com/ibex/t746.htm

I can't tell what that is from the photo, but it definitely looks like there's something there.

He also says the buttons are on a circular plate on these phones, but mine aren't. I might have to drop him a line.

The question is where the hell are those dial pulses comming from? It seems they happen when the phone is first plugged in then they stop and I just get dial tone. I wonder if it's a capacitor running down or something?

Andy.

[twocvbloke]

If the something you're referring to is the thing on the end of the black wire with one white stripe, that's another 3.5mm socket, but the sort you'd DIY solder onto the end of a wire, did try to find a picture, but I guess they're no longer made, they were quite common back in the day (broke enough of them myself!!), so still leads me to believe that there's supposed to be a battery on there on the end of a single pole 3.5mm plug...

[andy1702]

I think you're probably right. I just need to rig some batteries up to try it.

[Jack Ryan]

The ICs are:

MC14011 – Quad NAND
MC14419 - 2-Of-8 Keypad-To-Binary Encoder
MC14409 - Binary to phone pulse convertor

So it is definitely a LD dial and it probably needs power.

The # and * keys were often used for memory functions on LD push button dials.

Jack

[andy1702]

Thanks Jack. I've been a bit busy today so i've not got round to rigging up a battery yet. It's looking more and more like it needs power though. The strange part is there's no witness marks inside where a battery might have been.

Andy.

[twocvbloke]

They could have strapped the battery the hookswitch support with a bit of wire or a zip-tie to secure it, at a guess of course...