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Old 2nd Oct 2006, 3:49 pm   #1
Sparks
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Default BT type 746 intercom

Hello everybody,

I am new to this forum so I crave your indulgence until I'm settled in.

I have a number of old 746 phones, all converted to the plug-in standard and in perfect working order. Can anyone tell me if two or more of these phones can be connected back to back for intercom purposes but independent of a phone line ? If so, are wiring diagrams available and is battery/DC power necessary ?

Looking forward to your replies. Thankyou.

Sparks
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Old 2nd Oct 2006, 6:24 pm   #2
coil1234
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Default Re: BT type 746 intercom

Hello,
yes it is possible, a dc supply will be required for the speech circuit and an ac supply to ring the bells (it's an odd frequency 16 Hz I believe at about 50v other forum members will probably correct this).
Many years ago when an apprentice I built an electronic automatic exchange based on a design in 'ELEKTOR' magazine in the late 70's unfortunately I lost the magazine some time ago and only have circuits for the bits I modified and I'm not sure where they are!
It did work quite well and had a capacity of up to 9 lines, although I used a mains frequency derived ringing current so the bells didn't ring as they should, and of course only accepted dial phones.

Mike A
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Old 2nd Oct 2006, 7:03 pm   #3
Dave Moll
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Default Re: BT type 746 intercom

Quote:
Originally Posted by coil1234 View Post
yes it is possible, a dc supply will be required for the speech circuit and an ac supply to ring the bells (it's an odd frequency 16 Hz I believe at about 50v other forum members will probably correct this).
According to the specification at http://www.telephonesuk.co.uk/technical_info.htm, the DC supply should be 50V (but drops to approx. 10V off-hook) and the ringing signal is 75V at 25Hz.

What I am less sure about is how you would initiate the ringing signal to let the other extension(s) know that you wish to communicate.

If you wished to be able to select which extension to communicate with, you would also need something to emulate the Strowger selectors (unless you already have a PBX of some sort).
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Old 2nd Oct 2006, 10:29 pm   #4
AndiiT
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Default Re: BT type 746 intercom

Hi,
Some time ago I designed a simple unit using a couple of relays and a mains transformer salvaged from an old video machine (a transformer from a video was used simply because it had a 40 (ish) volt winding which was high enough to ring the bell in the phone - albeit at the wrong frequency) this design was published in a copy of "Television" magazine some years ago.

If all you require is a point - to - point intercom using two 746 (or similar) phones let me know and I will draw up the details again and post them in this forum

Regards
Andrew
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Old 5th Oct 2006, 10:30 pm   #5
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Default Re: BT type 746 intercom

As promised, please feel free to comment - also ensure to read the disclaimer at the end of the second page of the description

Apologies in advance for the scrawly line drawing - CAD hasn't reached this part of the frozen North

Regards
Andrew
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