BT 8755 will not pick up

[beko1987]

Hi guys.

Trying to get my 8755 into working order, and am having a problem with it.

If I plug it into a working socket, tested for the past year and fully working with an 8746, it will ring, but when I pick the receiver up, the line keeps on ringing. The bells stop on the 8755, and I get what sounds like fast pulses on the line. It does not answer the line.

Similarly, trying to dial out (1471) makes the line ring, although not the 8755. It will pulse out the numbers, but wont connect, I just get silence. The line keeps on ringing (extension to the kitchen phone, a modern sect, wired from a filtered master socket. The 8755 is plugged into the master socket, and works fine with an 8746. Making it ring the line as above makes the sect say 'call's on the id, and picking it up does nothing. If I tap the cradle on the 8755 I can hear it through the dect.

As far as I can see it is wired up correctly, as per the 8745 diagram, with the capacitor soldered onto the motherboard.

This is really confusing me, can anyone help? Can provide pictures and film footage if it helps...

Thanks

Sam

[OscarFoxtrot]

Should it be wired as Tele 756?
http://www.samhallas.co.uk/repositor.../0000/N856.pdf

The bell or ringing device should be 4000 ohms resistance. This can be rectified by either fitting the correct ringer or by inserting a 3.3K resistor in series with the ringer. The telephone should already be fitted with a Rectifier No. 205. If one is not fitted then it is recommended that one is fitted. If no dial is fitted then terminals T9 - T10 should be linked.
Remove strap T6 - T7
Remove strap T4 - T5
Insert strap T5 - T6
Red wire of line cord to T8
Blue wire of line cord to T6
Insert straps T16 - T17 - T18 - T19
White wire of line cord to T18
Green wire of line cord to T15
Insert a 3.3K resistor between T4 - T5
Insert a Rectifier No. 205 (if needed) between T1 and T2

[beko1987]

Hi OscarFoxtrot, thanks for the info.

I double checked the wiring tonight, and everything was almost as it should be. I replaced the strap between T4 and 5 with a resistor I pinched from one of my intercom handsets. Still no different.

I tried to dial my mobile tonight, and it did not connect. I hear (faintly) the pulses go down the line, but my mobile did not ring, and the handset was silent on the 8755. If I plug it into the socket without a filter on the end of it, I can hear the garbled mess of the homehub, pressing digits cuts through the garble as pressed, then nothing. Depressing the hook (putting the phone down) kills all noise, releasing it starts it again but still no dial tone. The phone rings, and picking up the receiver kills the ring on the phone, but the line (and all other phones) keep ringing.

Socket confirmed working with a Trimphone 766.

All the resistors/caps etc look ok, all wires are where they should be, and not broken.

Would wiring up a dial work? Sure I read somewhere that the push-buttons translate the signals into a rotary dial format. Got plenty spare with the intercom system. Would that be a helpful troubleshooting exercise or not? If I disconnect the dial completely, would I still/should I still get a dial tone? Problem elimination begins!

[AndiiT]

Hi,
Looking at you pictures it's difficult to see whether the Blue lead of the handset cord is connected to T3 (as it should be) or T4. Also one of your pictures shows what appears to be an Electrolytic capacitor across terminals T4 - T5 and not a Strap or 3K3 resistor.

It's highly likely that a 3k3 resistor won't be required in this model anyhow; you can check the stamp on the bell motor coils to confirm this - if they are 2x 500 Ohms a resistor is required, if 2x 2000 Ohms or a single 4K version a resistor is not required.

I have just noticed that the handset transmitter cords BOTH appear go to T2! (unless I am mistaken) the Green should go to T2 and the Blue to T3

Andrew

[Nickthedentist]

Originally Posted by AndiiT

Also one of your pictures shows what appears to be an Electrolytic capacitor across terminals T4 - T5 and not a Strap or 3K3 resistor.

I have just noticed that the handset transmitter cords BOTH appear go to T2! (unless I am mistaken) the Green should go to T2 and the Blue to T3

I agree

Get those sorted and you might have some luck.

Nick.

[beko1987]

Ah! It seems the parts liberated from the Pye handset are not correct. They made no difference, I shall take them out and check the above later once baby goes to sleep.

[beko1987]

Right, all wiring put back as above.

Blue handset cord moved from T4 to T3
Both random capacitors stripped out, replaced with link and it works! Thanks for the info guys.

Going to give it a good machine polish, get rid of all the marks and buy a wall bracket I think.

Now that's fixed I can go back to a car boot and find myself a new project!

[julie_m]

The first thing to try with a suspect phone should always be to wire it exactly as per the diagram. You don't know who's messed with it. Leave out anything optional such as extra switches, neons or watch receivers at first. They can always be dealt with once you have the basics sorted. Ringing first, answering and then dialling out.

The 10-key, loop disconnect push button unit on these phones has 5 wires like a dial, and is even wired in the same way as a dial.

[beko1987]

Thanks Derby, will keep it in mind. Sound advice which does work!

[julie_m]

Also, if you can get hold of a real dial, try that in place of the PBU (connect like colours for like). There's a possibility that a transistor could have failed, and this would break things. But we'll cross that bridge if and when we come to it.

If you don't have access to a suitable dial, short the pink and orange with a wire link and leave the blue, brown and slate grey disconnected; this will simulate a dial at rest, and should at least let you answer a call.

[Nickthedentist]

Glad it works.

Note that your bell coils are 2 x 2000 Ohms, so that resistor is NOT required, a strap is perfect.

Nick.

[AndiiT]

Originally Posted by Nickthedentist

...Note that your bell coils are 2 x 2000 Ohms...

Also the absence of C1/2, on the board would suggest that the bell motor coils are 4K as would the model designation 8755; All phones, of this type, prefixed with 8, were BT/GPO "wipe ups" which had been converted for use on the "new" plug and socket system.

Regards

Andrew

[beko1987]

Well, a very kind person on here donated a spare bracket for my 8755, and that arrived today.

It was missing it's fixing screw, however I found a coarse thread computer screw fitted perfectly and matched the thread. The semicircle missing from the circuit board was there before, handily!

Very pleased to have it complete now. Need to pick up a line extension cord and then I can wall mount it somewhere to show it off!

Thanks for your help everyone, especially AndiiT The machine polish is still to come however!

Regards

Sam

[AndiiT]

Originally Posted by beko1987

..... Need to pick up a line extension cord ...

Hi,
After BT converted to the "New" plug and socket system, where a 700 type wall mounted phone was to be fitted, it seemed to be common practice to mount a line jack around the same level as the height of the phone and provide a (very) short line cord to connect the telephone to the line jack.

I'm not sure if short line cords are still available these days though.

regards

Andrew

[beko1987]

The cord looks legit. Same style as my 8746, and the ends of the cord are crimped off. I presume the line jack was located right by the phone for wall mounted operations.

[AndiiT]

Hi,
As if to confirm what I mentioned about wall mounted 700 series phones I visited a customer today (quite coincidentally) with an install of that very sort, and took a picture!

Andrew

[Nickthedentist]

I've seen that too, but what a messy, roundabout way of doing things!

I even managed to sqeeze-in a hardwired broadband microfilter into mine.

[AndiiT]

I must admit I always thought that it was a bit odd to have a pluggable wall phone!

Looking through some old phone manuals a few days ago there was actually an extension plan created, after the introduction of the plug and socket system in 1981, which used the capacitor in a hard wired phone (with 4K bell coils) to provide the ringing feed for additional secondary line jacks; Which, I feel, would have been a better way of doing things if a 700 type wall phone was fitted.

Although the above system was developed/suggested I'm not altogether certain whether there were ever any installations which actually did adopt that method of wiring.

Andrew

[Nickthedentist]

Very interesting! I can see no practical reason not to have a "hybrid" installation, with most of the phones connected with the usual plugs/sockets, and the others hard wired with the bells in parallel and using the ringing cap in the BT NTE5 master socket.

With an ADSL "faceplate splitter", broadband can be accommodated without difficulty too.

Nick.

[AndiiT]

Nick,
In the days before the current plug and socket system there was at least one "hybrid" extension plan known as Plan 4 off 1A; Where a wall phone was used as the fixed bell of a Plan 4 installation which was the system that used jack plugs and sockets on portable telephones. No ADSL to worry about in those days though

Andrew