New type bell In 756 won't ring.

[only1carra]

Hi
Just acquired a 756 push button 'phone with the resistor already fitted and the new type bell.

I'm very new to these vintage 'phones but I've converted a few 746 'phones over the last few weeks. I thought this one would be just as straightforward. I've fitted a new cord and the 'phone dials out and has a tone. Unfortunately the 'phone won't actually ring and I can't see any instructions anywhere for converting. Could anybody please help?

I have attached a photo to show how I have wired it, if anyone has a photo of one to compare, it would be handy.

Thanks guys.

[julie_m]

Set your meter to its highest AC volts range, measure across the bell coils and call the phone from your mobile. What do you get? Should be bursts of about 90 V. or thereabouts, in time with the ringing of other phones on the line.

[AndiiT]

Hi,
First remove the 3K3 resistor fitted to T4-T5 and replace with a strap, it's highly likely that the replacement coils are already the required 4K for use on the "new" system; adding the 3k3 resistor in series with a 4K bell coil will certainly stop it operating.

I have also just noticed that strap T17-T18 is missing, in your phone it looks like these connect to an auxiliary switch (Bell on/off perhaps?) make sure that either this strap is fitted (You can use the strap between T8 - T9, as long as you ensure that the RED of the line cord is connected to T8 direct) or the switch connections are in the normally closed position; Looking at the switch wiring it may have been originally wired for shared service (party line) working.



Regards
Andrew

[only1carra]

Cheers. Where would I find the on/off switch?
Thanks.

[AndiiT]

Hi again,
The Bell on/off facility would have been provided by an additional "Auxiliary" switch, which would be situated between the switch hook plungers and operated by an additional push button which protruded through the case top.
It appears that there is one of these fitted to your phone (this is evident by the Brown, Blue and Slate (grey) leads which are connected to T8, T17 and T15 respectively) this may have been used as a bell On/off switch and, if so, would be of the latching type.
More likely the switch is of the non-latching type and, as previously mentioned, also looking at the wiring of it if original, may have been for either shared service working or PBX recall facilities. In normal operation the switch is not required and can be left out of circuit.

If you are still experiencing ringing problems begin by strapping the phone as follows

T4 - T5 - T6
T16 - T17 - T18 - T19
RED of line cord to T8
WHITE of line cord to T18
BLUE of line cord to T5
GREEN of line cord to T15 (Spare)

After confirming the phone does ring when wired this way check the impedance of the bell coils, in your phone they will be marked either 1000 or 4000, possibly with the word ohms or the Greek "omega" sign after the number. If 1000 ohms remove strap T4 - T5 and replace with a 3K3 resistor, which, from your pictures, you already have currently fitted to the phone, if 4000 Ohms (4K) leave strap T4-T5 in place.

Regards
Andrew

[Chipmunk]

I would agree with Andrew, with one addition: That looks like a 79A bell, those are usually 1000 ohm, but in this case, yes, it looks like the missing strap T17-18 would be the issue

[AndiiT]

Originally Posted by only1carra

I can't see any instructions anywhere for converting.

Hi, I just noticed your above comment, the instructions for converting the 746 should work fine for your 756

Regards
Andrew