1/726F Conversion, having a few problems - UK Vintage Radio Repair and Restoration Discussion Forum

mini670 · 31st May 2010, 12:21 pm

Hi,

Im a new member on this forum, I came across it whilst searching for information on my 1/726F. Ive most the threads in this section of the forum, and come away with some really useful info. I have sucesfully converted a 746, and a 706, but im strugling with my latest addition.

Ive wired the phone up as per a 710 model. The website "www.britishtelephones.com" states that the 726F is `SIMILAR TO 710, BUT WITH PUSH BUTTON D.C. SIGNALLING`.

Im slightly unsure what D.C. signalling is. The phone will recieve calls, but will not ring out. I have a dial tone, when a button is pressed I get a click in the ear piece, but no pulses. Sometimes halfway through dialing a number the phone will seemingly cancel off and revert back to a standard dialing tone.

The extra circuit board above the contact tabs contains a relay, but this does not seem to operate. Im i right in presuming that board is the D.C. signalling part, and the relay should provide the pulses?

Any help would be great.

Cheers
Tom

Ive added a few photos showing the phone and circuits (sorry about the grain on the photos)

julie_m · 31st May 2010, 5:22 pm

DC signalling (also known as Code C) was an extension of earth recall functionality. Depressing keys connected the A and B wires to earth directly or via diodes. Each of the two wires could be in any of 4 states (anode to earth, cathode to earth, short or open) giving 16 possibilities (one of which is the normal state and one of which is just an earth recall state).

It was used on some PABXes, but never on the PSTN -- adding the functionality at the exchange to receive DC signalling would have been too expensive, and long lines might have made the system unreliable. Pushbutton phones for PSTN operation were made to use LD signalling at first, because this was known to be robust; then DTMF support was added as exchanges were upgraded to digital

The only way you will be able to make calls from this phone will be to use a hand-held DTMF keypad.

mini670 · 31st May 2010, 7:17 pm

Thankyou for your reply AJS.
It seems I have an awful lot to learn about how telephone systems worked back in the day.

I have researched some of the acronyms you used in your reply, its made it a lot clearer to me i think.

I am presuming the telephone was intended for internal use only, and maybe went through some seperate equipment to dial out (labels on the phone state; OPERATOR key05, ENQUIRIES key 06, and EMERGENCY key 00)?

looking at http://britishtelephones.com/t726.htm again mentions that the phones were `only used on the following customers premises:Only used on the following customers premises:-`

BP, Harlow.

BP, Brittanica House.

Midland Bank, Lombard St.

ICI, Aberdeen.

ICI, Brockley.

31st May 2010, 12:21 pm	#1
mini670 Retired Dormant Member Join Date: May 2010 Location: Sheffield, UK. Posts: 3	1/726F Conversion, having a few problems Hi, Im a new member on this forum, I came across it whilst searching for information on my 1/726F. Ive most the threads in this section of the forum, and come away with some really useful info. I have sucesfully converted a 746, and a 706, but im strugling with my latest addition. Ive wired the phone up as per a 710 model. The website "www.britishtelephones.com" states that the 726F is `SIMILAR TO 710, BUT WITH PUSH BUTTON D.C. SIGNALLING`. Im slightly unsure what D.C. signalling is. The phone will recieve calls, but will not ring out. I have a dial tone, when a button is pressed I get a click in the ear piece, but no pulses. Sometimes halfway through dialing a number the phone will seemingly cancel off and revert back to a standard dialing tone. The extra circuit board above the contact tabs contains a relay, but this does not seem to operate. Im i right in presuming that board is the D.C. signalling part, and the relay should provide the pulses? Any help would be great. Cheers Tom Ive added a few photos showing the phone and circuits (sorry about the grain on the photos) Attached Thumbnails Last edited by Dave Moll; 31st May 2010 at 3:53 pm. Reason: embedded images converted to thumbnails

31st May 2010, 5:22 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 1/726F Conversion, having a few problems DC signalling (also known as Code C) was an extension of earth recall functionality. Depressing keys connected the A and B wires to earth directly or via diodes. Each of the two wires could be in any of 4 states (anode to earth, cathode to earth, short or open) giving 16 possibilities (one of which is the normal state and one of which is just an earth recall state). It was used on some PABXes, but never on the PSTN -- adding the functionality at the exchange to receive DC signalling would have been too expensive, and long lines might have made the system unreliable. Pushbutton phones for PSTN operation were made to use LD signalling at first, because this was known to be robust; then DTMF support was added as exchanges were upgraded to digital The only way you will be able to make calls from this phone will be to use a hand-held DTMF keypad. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

31st May 2010, 7:17 pm	#3
mini670 Retired Dormant Member Join Date: May 2010 Location: Sheffield, UK. Posts: 3	Re: 1/726F Conversion, having a few problems Thankyou for your reply AJS. It seems I have an awful lot to learn about how telephone systems worked back in the day. I have researched some of the acronyms you used in your reply, its made it a lot clearer to me i think. I am presuming the telephone was intended for internal use only, and maybe went through some seperate equipment to dial out (labels on the phone state; OPERATOR key05, ENQUIRIES key 06, and EMERGENCY key 00)? looking at http://britishtelephones.com/t726.htm again mentions that the phones were `only used on the following customers premises:Only used on the following customers premises:-` BP, Harlow. BP, Brittanica House. Midland Bank, Lombard St. ICI, Aberdeen. ICI, Brockley.