Bell Telephone Manufacturing Cie model no 2724

[Matilo Telephones]

A while ago I got this one: an Antwerp phone, with older style handset.
I have been curious about that handset, so I was very excited to get one. I have the French version of it, I read about the US version, but never had my hands on a Belgian version of it.

At first I thought that the handset was a replacement and not original to this phone, as this model usually comes with a different model handset.
I found a pic in a BTMC catalogue of this phone and handset combination (in the TCI-library).

Unfortunately somebody has had a go with this phone. Capacitor has been changed, cords. Also the bolts with which the base plate are fastened do not seem original to me. I have other Antwerp phones with different bolts.

Sadly the innards of the handset have been changed too. That grey cap in the transmitter opening is from a Dutch T65, so presumably a countryman of mine has done this.

No diagram inside.

Nevertheless I am very happy with it. The rest of the phone is origanal and in good condition.

[unbeldi]

This phone is labeled 2724-B.
Does this phone not have an earthing button? I am not sure I can make it out on the inside view. I think it should be right above the dial.

Does this version already have an anti-sidetone induction coil?  I know the later versions did, but I am curious just when BTMC put them into phones, this being an earlier one with the old handset.
Unfortunately it may hard to determine just looking at the coil, because the three windings would be in series and still only have four terminals coming out.

[unbeldi]

No diagram inside.

Actually I do believe that your phone should have the anti-sidetone circuit already. It is Doug's phone that should still have a Booster circuit.

The first image should be the correct diagram.
This is from the 1936 BTMC catalog.  It shows two choices for the dial.

The circuit principle is exactly like the Western Electric anti-sidetone circuit, only the switches are slightly rearranged. For comparison, look at the principle of the WECo circuit.  If you leave out the switches in the BTMC diagram, you should get the identical layout.  I omitted the ringer though in the WECo circuit.  It would connect between L1 and C to share the capacitor, while WECo installed a separate capacitor for the ringer.
Finally, if you want to unfold the induction coil, you get the last diagram which is more similar to the BTMC diagram.

[Matilo Telephones]

Thanks Karl,

it does not have an earthing button.

Should I look for certain markings on that coil?

In the catalogue in the TCI library this model with the older handset is shown, but the one with the newer handset too.

On the base plate the year 1936 is scratched in, by some previous owner. Perhaps because that date was on the capacitor. But I will never know for sure, as I have not been able to find any date on it (yet).

[unbeldi]

Thanks Karl,

it does not have an earthing button.

Should I look for certain markings on that coil?

In the catalogue in the TCI library this model with the older handset is shown, but the one with the newer handset too.

On the base plate the year 1936 is scratched in, by some previous owner. Perhaps because that date was on the capacitor. But I will never know for sure, as I have not been able to find any date on it (yet).

The few coils that I have seen, didn't have anything written or stamped on them, IIRC, and since the coil is usually standing up, very little can be seen in pictures.  Is there anything on yours?

Without an ohmmeter, it's probably best to note the number of terminals on the coil.  It seems some have only four terminals, despite having three windings, but other diagrams I have seen show six contacts, for example, primary = 1-2, secondary = 3-4, and tertiary (across the receiver) = 5-6 or simply 2-4, where 5 & 2, and 6 & 4 are the same.

[Matilo Telephones]

Well, it does have 4 terminals. But the markings are clear enough to read.

[unbeldi]

Well, it does have 4 terminals. But the markings are clear enough to read.

Indeed, and those show that it is a three-winding induction coil.
It also shows the DC resistance values, which are in line with expectations.

[Matilo Telephones]

Thanks. Because it has 3 lines of values and characteristics, it has 3 windings, right?

This phone has me puzzeled. I thought maybe the handset was a later replacement. But the catalogue in the TCI library shows it was made original like this, with the older style handset, belonging to the previous model.

When you mentioned the 2 different side tone suppresion circuits, I thought perhaps the older handset went with the older circuit. But that is not the case.

[unbeldi]

I think finding phones with different, especially older, handsets is not so unusual, especially in administrations that sourced telephones from multiple manufacturers as was the case in most of the European countries.  For examples, I have seen many Ericsson handsets on Heemaf phones, for example, and I am certain you have too. Even in the US, we sometimes find E1 handsets on 302 telephones, especially around the war years when telephone companies struggled to piece together equipment that had been suspended from manufacture.

The 1936 BTMC catalog still shows both handsets as available in the piece parts section, the older style, No. 2288/2289, and the newer one, 2290, mentioning that the newer style is the preferred model on the 2700 and 2800 series telephones.

I don't think the style of the handset would have much to do with the type of circuit used.  Perhaps the type of transmitter capsule is more significant in various circuit arrangements. For example, local battery telephones often use special transmitters.

One aspect that I find unusual though is that it seems your handset has a plastic capsule type receiver.  Is that white looking element original?

[unbeldi]

Thanks. Because it has 3 lines of values and characteristics, it has 3 windings, right?

:-)     Actually it is the other way around.    Because it has 3 windings, they listed 3 lines of values.

The three windings are straight in sequence and could possibly be created by single action of winding, only bringing the wire out to a terminal at the two points of the process, creating essentially two additional taps on a single winding.  However, often the tertiary winding needs to have a balancing impedance attached, which was achieved by simply using higher resistance wire for that winding and winding it non-inductively. So I suspect it was added separately. From what I can read, the specification is: Winding    Terminals    Resistance (Ω)    Function    Primary 1, 2 37 transmitter circuit Secondary 4, 3 40 receiver circuit Tertiary 4, 2 60 anti-sidetone compensation The Western Electric 101A coil had DC resistances of 22, 19, and 75 ohms, which produced AC impedances at a frequency of 1000 Hz of ca. 2000, 900, and 300 ohms.  As the tertiary winding as only 300 Ω impedance, despite having a 60 Ω DC resistance, i.e. about 3x the value of the primary or secondary, it follows that most of the winding is actually non-inductively wound. I suspect that BTMC did something similar with their coil.

[unbeldi]

It appears this 2724 has the identical induction coil as my 1947 BTMC set. I am not sure just how many terminals are brought out on mine—I think four—but the designations and DC resistance values match mine.

The circuit is identical with the exception of an added capacitor across the dial pulse contacts to prevent arcing.  Mine has the 7019A dial. 

[Matilo Telephones]

Do you have a picture of that 1947 BTMC Set? Is it an Antwerp phone? Of is it that later model, that looks like the WE 302?

[unbeldi]

Do you have a picture of that 1947 BTMC Set? Is it an Antwerp phone? Of is it that later model, that looks like the WE 302?

It is a rather different looking beast:  http://www.classicrotaryphones.com/forum/index.php?topic=8786.0
... undoubtedly a result of WWII destruction and shortages.  I was surprised that something like this was still made so late.

Yes, it was made in Antwerp.

I would like to find a more correct handset for it, as I think that AE handset was placed on it for lack of one.  I think it should have the newer version of your handset, I would think.

[Matilo Telephones]

Aha, that one.

I know the Germans used old stock to make mixed up models after the war.

Perhaps these were made because there was old stock around.

Anyway, I usually often them with the 2288a handset here. So an E4 handset may be a good place holder.

They also appear with the handset for the Antwerp phone. Not sure if they were delivered like that by the factory.