150 Candlestick and Bell 1A conversion - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 4th Oct 2015, 6:44 pm

I want to convert a Bell No.1A to work with a 150 candlestick. Initial thought was to replicate the Bellset No.1 induction coil and it was suggested I could use parts from a 700 series phone. On looking at the 700 circuit I am not convinced this will work as the receiver impedances are very different.

So I am now thinking that it might be easier to use an electronic solution that would also deliver some amplification. I found a very basic circuit for a phone here http://circuit-zone.com/?cat=Phone_Circuits. It doesn't look right to me as there is no attempt to cancel out the local mic signal.

Has anyone tried anything like this?

Does an induction coil have any special characteristics? I assume the inductance is low so the ring signal does not damage the receiver and of course it must work with 20mA DC.

TonyDuell · 4th Oct 2015, 6:56 pm

My first question has to be 'why do you want a candlestick telephone in the first place?'. To me, part of the attraction of old telephones is the appearance, but a major part (if not _the_ major part) is the circuitry behind them. If I wanted an electronic candlestick there are many cheaper ways of getting one than starting with a 150.

So if it were mine I would want to keep the induction coil based circuit.

My guess is that a 700-series induction coil will work in that you will be able to transmit and receive speech, but that the sidetone cancellation will be totally wrong. You might do better with the induction coil from a 332 or something like that, if you can find one.

PJL · 5th Oct 2015, 12:15 pm

Tony, I take your point but the wife OK'd the purchase but she wants it working. I don't plan on changing the 150, just the means of connecting it to the PSTN and as I already have this bell box from a previous house, this is the obvious way to go.

Getting the correct induction coil is just not going to happen. Making one is a possibility but I don't fully understand how it works. I have drawn the off-hook arrangement below. Why do they use an open iron core (it needs to carry DC but a small air gap in a conventional transformer works)? I assume most of the receive sound is from the voltage generated across the carbon mic? As well as cancelling the mic, the induction coil must reduce the received sound amplitude?

There is a capacitor missing from the drawing in series with the receiver.

TonyDuell · 5th Oct 2015, 4:33 pm

I suspect the open core was to prevent saturation from the line current (the core was a bundle of soft iron wires, nothing fancy). The 700 series indution coils look more like conventional transformers, I have never pulled one apart to look for an air gap in the core

At the time of the 150 it might not have been an anti-sidetone induction coil, maybe just a matching transformer between the transmitter and line (the latter being much higher impedance). The circuit would look odd, due to (a) needing to pass line current through the carbon microphone and (b) not pass it through the receiver so as not to demagnetise said device. That could well be the circuit you show, the coil in series with the microphone is the primary (low impedance), it carries line current so the carbon microphone works, the actual line circuit is the secondary and receiver in series with the blocking capacitor. The primary circuit current is also what operates the relay at the exchange to indicate off-hook.

PJL · 5th Oct 2015, 10:48 pm

Thanks Tony, if it was just a bundle of wires I can make one. At audio frequencies impedance matching is not going to make any difference so I would definitely think it is sidetone reduction. The resistance of the windings is 17/26 ohms so if I assume they used the same wire diameter then this is the approximate turns ratio. The resistance of the receiver is 75 ohms so if I make the inductance at 250Hz in the same order of magnitude this should give some LF rollloff.

The dial mechanism is now working to specification after a strip & clean. The carbon mic also seems to work but I remember vigorously shaking 300 series handsets as a kid so they were hit & miss then.

What's the collectors view on respraying? The paintwork does have chips and a few rust spots but not enough to detract and the dial has lost quite a bit of paint to reveal the brass. This looks a nice tidy original example, the blueing is in good condition as are the receiver and transmitter housings, and a respray would give it an almost new look but I don't want to do the wrong thing.

Herald1360 · 6th Oct 2015, 12:51 am

Quote:

Originally Posted by PJL

At audio frequencies impedance matching is not going to make any difference .......

I beg to differ there. If the source impedance of the mic is say 30ohms and the receiver impedance is say 600ohm then a 1v signal from the mic will deliver about 0.95v to the receiver or about 1.6mW.

Put a matching transformer (ratio sqrt20:1) in the circuit and you'll now get about 2.25v at the receiver, or about 8.6mW. Well worth the effort!

Taken to much greater extremes, that's why an output transformer is needed to get any reasonable output into a lo-Z speaker from the hi-Z valve anode. The whole story there is a bit more complex when valve non linearity gets in on the act, but the principle's the same.

PJL · 6th Oct 2015, 8:59 am

I think Tony was refering to line impedance matching. However, it is most likely that the transformer turns ratio is determined by the requirement to cancel the sidetone.

This comes back to my earlier question. The receiver is wired in parallel with the mic. so the induction coil is cancelling some of the received signal as well as the sidetone. Correct operation is totally reliant on the resistance of the carbon mic.

4th Oct 2015, 6:44 pm	#1
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	150 Candlestick and Bell 1A conversion I want to convert a Bell No.1A to work with a 150 candlestick. Initial thought was to replicate the Bellset No.1 induction coil and it was suggested I could use parts from a 700 series phone. On looking at the 700 circuit I am not convinced this will work as the receiver impedances are very different. So I am now thinking that it might be easier to use an electronic solution that would also deliver some amplification. I found a very basic circuit for a phone here http://circuit-zone.com/?cat=Phone_Circuits. It doesn't look right to me as there is no attempt to cancel out the local mic signal. Has anyone tried anything like this? Does an induction coil have any special characteristics? I assume the inductance is low so the ring signal does not damage the receiver and of course it must work with 20mA DC.

4th Oct 2015, 6:56 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,220	Re: 150 Candlestick and Bell 1A conversion My first question has to be 'why do you want a candlestick telephone in the first place?'. To me, part of the attraction of old telephones is the appearance, but a major part (if not _the_ major part) is the circuitry behind them. If I wanted an electronic candlestick there are many cheaper ways of getting one than starting with a 150. So if it were mine I would want to keep the induction coil based circuit. My guess is that a 700-series induction coil will work in that you will be able to transmit and receive speech, but that the sidetone cancellation will be totally wrong. You might do better with the induction coil from a 332 or something like that, if you can find one.

5th Oct 2015, 12:15 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: 150 Candlestick and Bell 1A conversion Tony, I take your point but the wife OK'd the purchase but she wants it working. I don't plan on changing the 150, just the means of connecting it to the PSTN and as I already have this bell box from a previous house, this is the obvious way to go. Getting the correct induction coil is just not going to happen. Making one is a possibility but I don't fully understand how it works. I have drawn the off-hook arrangement below. Why do they use an open iron core (it needs to carry DC but a small air gap in a conventional transformer works)? I assume most of the receive sound is from the voltage generated across the carbon mic? As well as cancelling the mic, the induction coil must reduce the received sound amplitude? There is a capacitor missing from the drawing in series with the receiver. Attached Thumbnails

5th Oct 2015, 4:33 pm	#4
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,220	Re: 150 Candlestick and Bell 1A conversion I suspect the open core was to prevent saturation from the line current (the core was a bundle of soft iron wires, nothing fancy). The 700 series indution coils look more like conventional transformers, I have never pulled one apart to look for an air gap in the core At the time of the 150 it might not have been an anti-sidetone induction coil, maybe just a matching transformer between the transmitter and line (the latter being much higher impedance). The circuit would look odd, due to (a) needing to pass line current through the carbon microphone and (b) not pass it through the receiver so as not to demagnetise said device. That could well be the circuit you show, the coil in series with the microphone is the primary (low impedance), it carries line current so the carbon microphone works, the actual line circuit is the secondary and receiver in series with the blocking capacitor. The primary circuit current is also what operates the relay at the exchange to indicate off-hook.

5th Oct 2015, 10:48 pm	#5
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: 150 Candlestick and Bell 1A conversion Thanks Tony, if it was just a bundle of wires I can make one. At audio frequencies impedance matching is not going to make any difference so I would definitely think it is sidetone reduction. The resistance of the windings is 17/26 ohms so if I assume they used the same wire diameter then this is the approximate turns ratio. The resistance of the receiver is 75 ohms so if I make the inductance at 250Hz in the same order of magnitude this should give some LF rollloff. The dial mechanism is now working to specification after a strip & clean. The carbon mic also seems to work but I remember vigorously shaking 300 series handsets as a kid so they were hit & miss then. What's the collectors view on respraying? The paintwork does have chips and a few rust spots but not enough to detract and the dial has lost quite a bit of paint to reveal the brass. This looks a nice tidy original example, the blueing is in good condition as are the receiver and transmitter housings, and a respray would give it an almost new look but I don't want to do the wrong thing.

6th Oct 2015, 8:59 am	#7
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: 150 Candlestick and Bell 1A conversion I think Tony was refering to line impedance matching. However, it is most likely that the transformer turns ratio is determined by the requirement to cancel the sidetone. This comes back to my earlier question. The receiver is wired in parallel with the mic. so the induction coil is cancelling some of the received signal as well as the sidetone. Correct operation is totally reliant on the resistance of the carbon mic.