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Old 16th Jul 2017, 6:11 pm   #1
crackle
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Default Telephone Set H Mk III

We bought one of these today at a boot sale for 6, my son is involved in WW2 re-enactment.
It all seems to work, we tested it using a stereo earpiece connected to the L1 & L2 terminals, and used one earpiece as a mic and the other in the ear.

Can anyone please confirm what the resistance of the original mouth and ear pieces should be. I measured each as about 27 ohms, which seems very low and would not help efficiency when used with a long run of cable.

Up to what length of cable were these used with.


Thanks
Mike
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Old 17th Jul 2017, 7:00 am   #2
Dave Moll
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Default Re: Telephone Set H Mk III

I'll get back to you later when I've had a chance to measure mine. I'll also see how it compares with my Mk II.

I don't know about cable lengths. but being sound-powered (i.e. unamplified), I would imagine it will be fairly limited.

By the way, you did well to obtain this at a boot sal for 6. The only ones I've ever seen for sale are the two that I have acquired.
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Old 17th Jul 2017, 8:20 am   #3
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Default Re: Telephone Set H Mk III

Hi Dave
I was not sure whether to to put this on the Military comms. or the Telephone thread.

That would be great, I suspect the transmitter and receiver elements are original, but just wondered if they had shorted turns, which again is unlikely as they both measure about the same.

thanks
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Old 17th Jul 2017, 8:32 am   #4
Dave Moll
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Default Re: Telephone Set H Mk III

Definitely telephones and telecommunications. The military section is radiotelecommunications - and I don't generally check it!

Anyway back to the original question, transmitter and receiver on my Mk III each measure 27Ω, so it looks like yours are good. On the Mk II, however, transmitter reads 100Ω and receiver 68Ω - and it's noticably a different handset from the Mk III.
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Old 17th Jul 2017, 8:37 am   #5
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Default Re: Telephone Set H Mk III

Hi Dave
Thanks very much for confirming that, it all seems to work, and you get a nice little tinkle on the bell when the magneto handle is turned and there is another headset connected to L1 & L2.

We have not really worked out what difference the handset switch makes, it seems to make little difference if you operate it whilst listening or talking.

One of the other members of his group has one of these, http://3.bp.blogspot.com/_k07pirzBU3...00/DR8RL39.JPG so they are almost there to connecting up to another group during one of their re-enactments.

thanks
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Old 17th Jul 2017, 1:34 pm   #6
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Default Re: Telephone Set H Mk III

Quote:
Originally Posted by crackle View Post
We have not really worked out what difference the handset switch makes, it seems to make little difference if you operate it whilst listening or talking.
Me neither! Looking at the "paster" diagram, all it seems to do is short out a 0.8μF capacitor that, along with a 2μF one is in series between the handset and the line - effectively reducing from 2.8μF to 2.0. The Mk II doesn't have this switch - or at least mine doesn't.

Quote:
One of the other members of his group has one of these, http://3.bp.blogspot.com/_k07pirzBU3...00/DR8RL39.JPG so they are almost there to connecting up to another group during one of their re-enactments.
So, you have one telephone and a reel of cable. You just need something to attach on the other end.

For interest, I attach below images of my two telephone sets "H".
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Old 17th Jul 2017, 6:01 pm   #7
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Default Re: Telephone Set H Mk III

I had two of those handsets complete with the 4 pin plugs when I was a youngster.
I bought them from Globe Radio in Manchester.

I made a pair of light-beam telephones with them.
The transmitter was carbon, the receiver was balanced armature, I remember the resistance was much less than the similar inserts in a pair of headphones, but I still think they were around the 1000 ohm mark, so I was surprised at the readings taken above.

The PTT switch was either N/O and in series with the mic or N/C and across the mic I can't remember which.
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Old 17th Jul 2017, 6:50 pm   #8
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Default Re: Telephone Set H Mk III

The handset of the Mk II may look externally similar to that of other field telephones with four-pin plugs, but the insets are very different - particularly the transmitter inset, which is similar to a receiver inset and generates a current rather than the varying resistance of a carbon-granule inset.

The handset switch of the Mk III is normally open and shorts out (as I noted earlier) a 0.8μF capacitor.

While talking about appearances, the Mk II looks externally similar to a telephone set "F" Mk I, but the insides have a lot of empty space with very little other than the magneto. The Mk III likewise externally resembles a telephone set "J", but with a similarly sparsely-populated interior.
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Old 17th Jul 2017, 7:26 pm   #9
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Default Re: Telephone Set H Mk III

I have had a go at cleaning up the circuit diagram, and typed up the instructions.

It seems like the .5uF capacitor is there to limit the pulses from the magneto from damaging the sensitive ear and mouth pieces, but it is shorted out during speaking so as not to reduce the signal.
Maybe the operation of the switch would be more noticeable when a long wire is in use.

Mike
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