Telephone Set L51 "sound-powered" telephones

[Dave Moll]

I have just acquired a pair of "sound-powered" (batteryless) L51 telephones.

Understanding the principle of a "sound-powered" handset (i.e. that it relies on the small current generated by a magnetic transmitter - in fact I have elsewhere a pair of handsets wired to work on this principle), I appreciate that the sound level to be expected in the receiver is going to be quite small, but I am unable to detect anything getting through in either direction. I can almost convince myself that a signal is getting from the "transmitter" to the "receiver" (which appear to be two identical elements connected in parallel) of a single handset, rather than simply acoustic conduction - but absolutely nothing audible at the other end.

I have established that all four transmitter/receiver elements are working using a multimeter to obtain a crackle from both "receiver" and "transmitter" by probing across the common connector - in fact the crackle is also audible in the remote handset. I also obtain (in each direction) the appropriate hooting sound in the remote handset when winding the magneto handle.

Does anyone know what sort of functionality I ought to be expecting from these?

[mikelect]

I have in the past connected two magnetic earpieces together with 100 mtrs of bell type wire and got a definite signal from them not as loud as a standard telephone signal but definitely loud enough to be coherant HTH

[russell_w_b]

Have you measured the resistances of the transmitter and receiver balanced-armature elements, and are you sure you're using one type to transmit and another type to receive?

Lots of B/A modules look identical, but some are designed as receivers, and some microphones, although they will do either, to a greater or lesser degree.

I used to have a couple of sets of sound-powered breast-mic and earphone sets (ex MoD) and they worked quite a distance with clear sound.

[Dave Moll]

Originally Posted by russell_w_b

Have you measured the resistances of the transmitter and receiver balanced-armature elements...?

Although I find I don't currently have a meter capable of giving an accurate resistance measurement (haven't used meters recently for resistance measurement - except continuity checking - and seem to have a rash of flat batteries), I do get a consistent measurement (by disconnecting from the system) of each element, whether transmitter or receiver. Also, I note that the markings on "transmitters" and "receivers" are identical (A.P. 18221 - along with 3 and 1958, which I take to be date of manufacture).

[russell_w_b]

Are the B/A units all the same colour and do they have the same number of holes in the front?

[G8VAT Wolfman]

You might try a crystal earpiece to detect the presence of a signal on the wires. seems like you need a third item to help point the finger.

[Dave Moll]

Originally Posted by russell_w_b

Are the B/A units all the same colour and do they have the same number of holes in the front?

I don't entirely understand the question, but here are pictures of the balanced armature units - front and rear. The "transmitters" are at the top of the images.

[russell_w_b]

Ah... They're older balanced armature units than the ones in my TMC head / mic sound-powered set! Mine were the same type (Besson manufacture, as I think yours might be) as the receiver element of a 706 or 746. The mics were coloured red and the receivers were green, and the mics had more holes in them. I've come across other variants, but that's by the by...

Have you a 'scope? Or an assistant and sufficient cable to eliminate the acoustic connection? What's their provenance? If some previous owner of the telephones had inadvertently stuck a battery in there, then the d.c. may have rendered the B/A units insensitive.

[Station X]

I had a pair of SPTs years ago. I found that the only way to test them was by trying to talk to someone some distance away outside accoustic range. The lack of sidetone or "blow" makes it impossible to detect your own voice.

[terrybull]

I remember a construction article, I think in the Meccano magazine, to make a set of these sound power telephones using 4 'standard' telephone earpieces with two joined by a meccano strip to make the handle. I never made them as I couldn't source earpieces.

[Peter.N.]

I was given a couple of balanced armature units and a long coil of wire as a present by my uncle when I was a kid back in the '40s, they worked very well out of shouting distance but as expected the signal was not very strong. As mentioned, if you have a scope you should be able to easily see the signals from them or even on a sensitive DVM on the lowest AC range.

Peter

[russell_w_b]

Sound-powered telephones have been a Naval (and Board-of-Trade) requirement on vessels since the SS Morro Castle disaster, when conventional telephone arrangements were damaged and lives were lost. The ones used on modern naval vessels haven't changed much to look at, but they will have improved electrically.

I can't imagine emergency sound-powered telephone commands in a noisy, possibly hostile, environment being issued quietly!

[Dave Moll]

Originally Posted by russell_w_b

What's their provenance? If some previous owner of the telephones had inadvertently stuck a battery in there, then the d.c. may have rendered the B/A units insensitive.

Their provenance is the local auction house - so goodness only knows to what abuse they have been subjected over the past fifty-odd years.

As I mentioned in the initial post, I have a pair of handsets wired up as a demonstration of unamplified communication, so I'll try wiring up the transmitter elements from those (which do result in a signal I can detect for myself) in place of the inserts in the L51s. I'll report back when I have done so.

[Dave Moll]

Originally Posted by Dave Moll

I have a pair of handsets wired up as a demonstration of unamplified communication, so I'll try wiring up the transmitter elements from those.

Having susbstituted one of these (see picture below), I can definitely detect sound in the local earpiece (though not as loud as the remote in the above setup) - transmitter is on a short length of wire and not in physical contact with handset, so I can discount acoustic conduction. It still doesn't seem to be strong enough to reach the remote handset though.

I think it is simply a case of elements that have become feeble with old age.

[Peter.N.]

Possibly the magnets have lost power, I wonder if its possible to remagnetise them?

Peter

[russell_w_b]

Have you tried connecting them all to a low-level signal source, perhaps an oscillator or the 'phone output from a radio (via a series 1K resistor with all the elements in parallel, say)? You could compare the relative levels and so determine if any unit was worse than others.

If they're poor at transmitting, then they'll also be poor at receiving. You may be let down by a single duff element!

[Dave Moll]

That sounds a good idea. If I connect the sig gen output to the line terminals, that should ensure that both handsets are getting the same signal.

Originally Posted by russell_w_b

Have you measured the resistances of the transmitter and receiver balanced-armature elements?

My DMM (now back in the land of the living) gives readings of 80Ω and 82Ω for tx and rx respectively on one handset, and 81Ω for each on the other.

[Dave Moll]

Well, that was certainly a revealing exercise! At a level of 10mV (according to the settings on my sig gen), I get a respectable, and fairly identical, output from the tx and rx of one handset, and a quite audible output even at 1mV (the sg's lowest setting). From the other handset at 10mV I get a much lower (but still audible) output from the tx, but just about nothing from the rx. Just to make sure that signal wasn't getting lost elsewhere in the (somewhat minimal anyway) circuit, I double-checked by injecting the signal direct into the insensitive handset - with the same result.

Testing the latter two elements on their own, increasing the output to 100mV gives a similar output from the rx as 10mV gives from the tx.

It looks, therefore, as though I have one pair of good elements, one middling and one duff one.