|
Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
|
Thread Tools |
2nd Oct 2006, 3:49 pm | #1 |
Heptode
Join Date: Oct 2006
Location: Leeds, West Yorkshire, UK.
Posts: 597
|
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 |
2nd Oct 2006, 6:24 pm | #2 |
Pentode
Join Date: Jan 2006
Location: Almeria, Spain
Posts: 221
|
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 |
2nd Oct 2006, 7:03 pm | #3 | |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,129
|
Re: BT type 746 intercom
Quote:
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). |
|
2nd Oct 2006, 10:29 pm | #4 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
|
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 |
5th Oct 2006, 10:30 pm | #5 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
|
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 |