Car Intercom

[PaulR]

A friend has a pre war Humber limousine complete with a telephone-type intercom so that the occupants could speak to the chauffeur. This has never worked during the period he has had the car and I wondered whether we could get it to work. It consists of a microphone in the back with a press button switch and a bakelite trumpet by the chauffeur's ear rather like those the operator spoke into in old telephone exchanges. There doesn't seem to be any sort of separate battery so I assume that it works off the car battery.

Will this be a simple series circuit and is it likely that it worked off the car supply or should I be looking for a battery somewhere? The wiring is buried in the leather and walnut trim of the car and I don't want to set about it without some prior knowledge. If it is a simple as that I assume I would just be looking for continuity of the wiring, switch, coils etc.

Thanks

Paul

[Peter.N.]

Interesting! Due to the age it has to be simple, probably just a carbon mic driving a diaphragm type ear piece. If you can get either end apart you should be able to check it for voltage, I expect it runs from the car battery.

Peter

[richrussell]

I would suspect like many home intercoms of the same era it's simply a series circuit of carbon microphone, battery and diaphragm earpiece too.

It could well be run off the car battery, but I wouldn't be surprised if there's a small 6V battery tucked away somewhere. The car battery supply may be too noisy depending on the charging system.

[Dave Moll]

If it is run off its own battery, the non-functionality may even be as simple as a dead battery.

[PaulR]

Would the 12V car battery be too high a voltage in fact?

[KeithsTV]

If it was a factory fit I would assume it would have been designed for use on 12v. A separate battery would have to be replaced at regular intervals although I would think that would have been the chauffeur's job.

Keith

[kalee20]

I'd like to think something as classy as a Humber limo would have been designed to use the car battery. Having a separate battery smacks of technical inelegance...

[PaulR]

Thanks all, I will have a dig around it. The owner tells me he has found a fuse inside the car that is live but nothing stops working when he removes it, so that might be a starting point.

Paul

[ColinB]

Surely a meter reading from the terminals of the accessory nearest the battery to the chassis would indicate if 12v is present or not? This might confirm that the system is 12v, but a lack of 12v won't confirm it isn't, if you see what I mean....

[60 oldjohn]

There are not many of these Humbers about now and I believe there is a collector of these cars in the Bridlington East Yorkshire area. I think they called him Mr Marshal, he has featured on local television. He and his helpers may be able to help if you can track him down. John.

[PaulR]

Thanks all

I will check voltages etc when I next see the car.

It is a 1934 Humber Pullman Limousine and I believe it is one of fewer than 10 remaining in the world. I imagine that the setup of the intercom will be as standard as such things were in its day and it seems it will be a simple setup. Fortunately the owner is a mechanic who specialises in classic cars and he has done a lot of work on it since he bought it, but he doesn't do electronic stuff.

Here is a photograph of it from just before he bought it.

[rherber1]

Hi PaulR,

I have come across just such a beast as you describe in your original post. Like yours, the one I was asked to take a look at did not work.

My analysis of the circuit showed it was a simple series circuit consisting of the vehicle battery, carbon microphone via push-button PTT switch and a telephone type receiver mounted behind a Bakelite horn. The carbon microphone was attached by a 2 conductor cord to a spring loaded cord retractor mechanism which was designed so that it maintained a two wire circuit wen the cord was pulled out or retracted.

I don't know whether vehicle battery voltage during the production of 1930's UK vehicles was 6V or 12 V but I measured the resistance of the receiver coils and they totalled 1.3 Ohms. This seems to indicate that the device operation was based on a reasonably heavy current in order to produce sufficient diaphragm deflection to be heard over vehicle and road noise - something I am very dubious about ever working as intended. The receiver could not be repaired because the U shaped pole piece on which the coils were mounted also served as the polarising magnet and there was absolutely no evidence of any magnetism remaining. It was not possible to re-magnetise the pole piece. This put paid to the whole restoration project.

Did you have any success with yours?

Ross Herbert

[TonyDuell]

If there is a substantial DC current flowing through the receiver coils, couldn't that provide the 'bias' magnetisation? The pole pieces may well have not been a permanent magnet.

Would the 12V car battery

Could be a 6V battery.

[AC/HL]

You still need a magnetic field for the receiver coil to work against, but it should be connected so as to enhance the field rather than working against it, as with headphones. The battery voltage wouldn't matter too much with the sort currents needed for speech.

[TonyDuell]

I would have thought that the resulting magnetic field (and thus the effect on the receiver diaphragm) was much the same in these 2 cases :

1) A permanent magnet (the magnetised pole pieces of a normal receiver) with a coil carrying AC (with a mean level of 0)

2) Unmagnetised pole pieces with a coil carrying the sum of the AC current and a DC current (the latter providing, in effect, the same field as the permanent magnet).

So if you arrange things so the carbon microphone varies in resistance as normal but never gets too high a resistance, you may not need a permanent magnet in the receiver. (I am assuming the normal series circuit of battery, microphone and receiver).

[Herald1360]

The last post from Paul on this was 6 years ago.

Still, a follow up could be interesting.

In a simple series circuit I would expect the current to be limited by the carbon mike- there were some heavy duty types around.

A better result might be had with a suitable matching transformer. If the dc field in the core from high magnetising current in the secondary were countered by an opposing field from the mike current in the primary it should work OK.

Weren't there some interesting early loudhailer type systems that did something similar?

[Station X]

Yes. I think some Pye loud hailers were nothing more than a carbon mike, battery and speaker in series.

[G6Tanuki]

Yes - more sophisticated ones used the carbon-mic as the base-feed resistor to something like an OC28 transistor. An amp of DC through the speaker-coil, 10 Watts of audio!

A suitably arranged capacitor and inductor would cause the thing to oscillate - a little press-button let you use it as a "Squawker" siren.

[PaulR]

I have just picked this thread up whilst defying swmbo and coming on here whilst on holiday!! I never did any more about the car telephone set as my friend lost interest in it a bit having bought two more '30s limousines and a Standard Flying 12 in the meantime. Tell that to partners who object to the occasional radiogram . It is 12v but I have never looked into it further than that.

I might resurrect the idea of getting it working if we can get the car down my drive.

Paul