Induction coils for old telephones - UK Vintage Radio Repair and Restoration Discussion Forum

trevwgb · 12th Apr 2007, 11:36 pm

Hi All
Could I ask the experts another novice question?. For not a lot I aquired a non functioning GPO 150 phone and was pleased to discover that by wiring it up as per the circuit diagram in the base it works. However to test it I had to "borrow" the Bellset 25 from my 162. I realize that what I need is Bellset no 1 but these do not seem easy to aquire at a reasonable price! I do however have Bellset no 1a. As far as I can see this differs from no 1 only in not posessing the Induction coil and the condenser. Since the condenser is shorted out when converting to work on BT, can I turn my No 1a into a no 1 by wiring in an induction coil? If so can I use an induction coil from any series GPO phone or are there differences i.e. are the induction coils specific to individual phones?
Many thanks
Regards
Trev

Darren-UK · 13th Apr 2007, 2:30 pm

Hi Trev,

Go to http://www.britishtelephones.com/ and follow the links to the 150 telephone. Info on the bellsets can also be found from the 150 page, including how to wire these for use on the modern telecoms networks.

Hope that helps

.

AndiiT · 13th Apr 2007, 2:32 pm

Quote:

Originally Posted by trevwgb

Since the condenser is shorted out when converting to work on BT, can I turn my No 1a into a no 1 by wiring in an induction coil? If so can I use an induction coil from any series GPO phone or are there differences i.e. are the induction coils specific to individual phones?
Many thanks
Regards
Trev

Hi,
The condenser may be shorted out when wiring your phone to work on the "new" plug and socket system, but in fact is still present in the form of the ringing capacitor (or condenser) in your line jack (master socket).

Whether coils are specific to individual phones I am not sure, physically they would have to be but electrically there may or may not be a difference between different models of telephone.

Lots more information can be found
here http://www.britishtelephones.com/

here http://www.britishtelephones.com/how700.htm

and maybe here http://www.samhallas.co.uk/repositor...rams/index.htm

Regards
Andrew

plumbweiss · 14th Apr 2007, 7:42 pm

I thought the induction coil (ASTIC) was only a later improvement anyhow and was designed to reduce the side-tone or volume of your own voice.

Dave Moll · 27th Jun 2007, 4:27 pm

Thanks for pointing out that the induction coil is not essential.

I was having difficulty with my candlestick 'phone in that the sound was very faint in the receiver. Having removed the induction coil, reception is much improved - and I don't seem to be deafened by the sound of my own voice!

Incidentally, the coil I was using was the one specified for the associated bellset - though I was not using the bellset itself.

Herald1360 · 27th Jun 2007, 5:48 pm

ISTR the ASTIC (hybrid transformer) may be recent, but didn't earlier phones still have an induction coil (step up transformer) to better match the impedance of the microphone to the line and receiver circuits? Without one the sidetone would be acceptably lower, but the transmission level would be less than optimum.

Chris

plumbweiss · 27th Jun 2007, 9:20 pm

I would say that if the addition of the coil increases side-tone and lowers reception, it may be connected incorrectly? The phasing of the winding is the key!

beery · 29th Jun 2007, 5:59 pm

Hi All,
Induction coils were used in almost all telephones usind carbon microphones going back many years before the ASTIC was invented. The main purpose of the induction coil is to match the earpiece and microphone to the 600ohm impedance of the phone line.
You at least need to limit the current that passes through the microphone, but if you don't use an induction coil the most likely outcome is that the person at the other end will not be able to hear you.

For a no. 150 a no.1 or no.25 bell box will do. Alternatively you can use the induction coil from a linesmans test phone, this is the large brown plastic box with a bakelite handset, dal and leather strap. The coil in the linesmans phone is not the same as the one in the no.1 bell box, but it is close enough to work quite well and looks the part.

Cheers
Andy

easylife · 18th Oct 2007, 8:57 pm

Hi Trev
I was wondering if you have had any success with a replacement induction coil. I've got the same problem, a candlestick 150 that requires a No.1 bell set, but I've only got a No.1A bell. I've seen a picture of a dealer selling a 150 with 1A bell, with what looks like a modern induction coil replacement. Has anybody got any ideas.
Thanks
Dom

trevwgb · 27th Oct 2007, 1:35 pm

Hi Dom
How I got round this problem was that I contacted several of the sellers on ebay who sell telephone spares and someone sold me a number 14 induction coil i.e. as in the no.1 bellset. I then inserted this into the No 1a ( it just fits ) and wired it up as per the no 1 circuit diagram. It is then the same as a no 1 without the condenser, which is not needed when used on a current BT setup. This worked very well with my 150 phone.
I still don't know what would happen if one used a different induction coil and this was the question I origionally asked the experts ( which I am not ).
Some probably think this is "heresy" but I am a bit of a pragmatist when it comes to these things and the 1a can be restored to its origional state at any time.
Regards
Trev

dagskarlsen · 14th Feb 2008, 7:10 pm

Quote:

Originally Posted by trevwgb

Hi Dom
How I got round this problem was that I contacted several of the sellers on ebay who sell telephone spares and someone sold me a number 14 induction coil i.e. as in the no.1 bellset. I then inserted this into the No 1a ( it just fits ) and wired it up as per the no 1 circuit diagram. It is then the same as a no 1 without the condenser, which is not needed when used on a current BT setup. This worked very well with my 150 phone.
I still don't know what would happen if one used a different induction coil and this was the question I origionally asked the experts ( which I am not ).
Some probably think this is "heresy" but I am a bit of a pragmatist when it comes to these things and the 1a can be restored to its origional state at any time.
Regards
Trev

I am no expert, but from experience: On good lines, it's not imortant, but the impedancematching is different, caring about line impedance 600-900 ohms.
(typical 900 on modern urban lines) The receiver may ahve impedances from 30-300 ohms. (typical 120 in EB and Ericsson used in Norway) German brands more likely to be about 40. EB telephones had carbon transmitters of 50 ohms, moas others 200 ohms.

The result of wrong impedance on bad lines may be humming, to strong/weak sidetone (even at different frequecies).

An other imortant task for the coil is to separate AC and DC, but this is easily done by using a simple 1 winding coil and a cap.

e.g.

line A (ring)---transmitter----(cap+rec)in parallel with coil-----line B (tip)

dsk

Majicou · 14th Feb 2008, 10:05 pm

AS mentioned elswhere here, some of the sellers on Ebay may be able to fix you up with what you need. If all else fails there, then Telephonelines in glos are great. I but many a small part from them and have a good relationship with them. I too have been finding Bellset no 1's hard to find as are Bellset 25's in good condition ( any one have one to sell ?)....

12th Apr 2007, 11:36 pm	#1
trevwgb Tetrode Join Date: Feb 2007 Location: Haywards Heath, West Sussex, UK. Posts: 81	Induction coils for old telephones Hi All Could I ask the experts another novice question?. For not a lot I aquired a non functioning GPO 150 phone and was pleased to discover that by wiring it up as per the circuit diagram in the base it works. However to test it I had to "borrow" the Bellset 25 from my 162. I realize that what I need is Bellset no 1 but these do not seem easy to aquire at a reasonable price! I do however have Bellset no 1a. As far as I can see this differs from no 1 only in not posessing the Induction coil and the condenser. Since the condenser is shorted out when converting to work on BT, can I turn my No 1a into a no 1 by wiring in an induction coil? If so can I use an induction coil from any series GPO phone or are there differences i.e. are the induction coils specific to individual phones? Many thanks Regards Trev

13th Apr 2007, 2:30 pm	#2
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Induction coils for old telephones Hi Trev, Go to http://www.britishtelephones.com/ and follow the links to the 150 telephone. Info on the bellsets can also be found from the 150 page, including how to wire these for use on the modern telecoms networks. Hope that helps .

14th Apr 2007, 7:42 pm	#4
plumbweiss Retired Dormant Member Join Date: Nov 2006 Location: Manchester, UK. Posts: 157	Re: Induction coils for old telephones I thought the induction coil (ASTIC) was only a later improvement anyhow and was designed to reduce the side-tone or volume of your own voice.

27th Jun 2007, 4:27 pm	#5
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	Re: Induction coils for old telephones Thanks for pointing out that the induction coil is not essential. I was having difficulty with my candlestick 'phone in that the sound was very faint in the receiver. Having removed the induction coil, reception is much improved - and I don't seem to be deafened by the sound of my own voice! Incidentally, the coil I was using was the one specified for the associated bellset - though I was not using the bellset itself. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

27th Jun 2007, 5:48 pm	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Induction coils for old telephones ISTR the ASTIC (hybrid transformer) may be recent, but didn't earlier phones still have an induction coil (step up transformer) to better match the impedance of the microphone to the line and receiver circuits? Without one the sidetone would be acceptably lower, but the transmission level would be less than optimum. Chris

27th Jun 2007, 9:20 pm	#7
plumbweiss Retired Dormant Member Join Date: Nov 2006 Location: Manchester, UK. Posts: 157	Re: Induction coils for old telephones I would say that if the addition of the coil increases side-tone and lowers reception, it may be connected incorrectly? The phasing of the winding is the key!

29th Jun 2007, 5:59 pm	#8
beery Heptode Join Date: Jun 2006 Location: Ware, Hertfordshire, UK. Posts: 988	Re: Induction coils for old telephones Hi All, Induction coils were used in almost all telephones usind carbon microphones going back many years before the ASTIC was invented. The main purpose of the induction coil is to match the earpiece and microphone to the 600ohm impedance of the phone line. You at least need to limit the current that passes through the microphone, but if you don't use an induction coil the most likely outcome is that the person at the other end will not be able to hear you. For a no. 150 a no.1 or no.25 bell box will do. Alternatively you can use the induction coil from a linesmans test phone, this is the large brown plastic box with a bakelite handset, dal and leather strap. The coil in the linesmans phone is not the same as the one in the no.1 bell box, but it is close enough to work quite well and looks the part. Cheers Andy

18th Oct 2007, 8:57 pm	#9
easylife Retired Dormant Member Join Date: Oct 2007 Location: Stoke-on-Trent Posts: 1	Re: Induction coils for old telephones Hi Trev I was wondering if you have had any success with a replacement induction coil. I've got the same problem, a candlestick 150 that requires a No.1 bell set, but I've only got a No.1A bell. I've seen a picture of a dealer selling a 150 with 1A bell, with what looks like a modern induction coil replacement. Has anybody got any ideas. Thanks Dom

27th Oct 2007, 1:35 pm	#10
trevwgb Tetrode Join Date: Feb 2007 Location: Haywards Heath, West Sussex, UK. Posts: 81	Re: Induction coils for old telephones Hi Dom How I got round this problem was that I contacted several of the sellers on ebay who sell telephone spares and someone sold me a number 14 induction coil i.e. as in the no.1 bellset. I then inserted this into the No 1a ( it just fits ) and wired it up as per the no 1 circuit diagram. It is then the same as a no 1 without the condenser, which is not needed when used on a current BT setup. This worked very well with my 150 phone. I still don't know what would happen if one used a different induction coil and this was the question I origionally asked the experts ( which I am not ). Some probably think this is "heresy" but I am a bit of a pragmatist when it comes to these things and the 1a can be restored to its origional state at any time. Regards Trev

14th Feb 2008, 10:05 pm	#12
Majicou Retired Dormant Member Join Date: Feb 2008 Location: Maidstone Posts: 9	Re: Induction coils for old telephones AS mentioned elswhere here, some of the sellers on Ebay may be able to fix you up with what you need. If all else fails there, then Telephonelines in glos are great. I but many a small part from them and have a good relationship with them. I too have been finding Bellset no 1's hard to find as are Bellset 25's in good condition ( any one have one to sell ?)....