Field Telephone Type F Mk II - Help needed
 

Hello,

This is my first post on the forum and I would be grateful if anybody could help me out.

I am a scout leader in Sheffield and on clearing out our stores I came across a pair of old field telephones in excellent condition in the original wooden boxes that are marked Type F, Mk II. I presume these date from WWII and would love to get them working again so the kids could use them on camps and have some fun with them.

I would also like to give them some history and information about the phones such as what they were used for and when they would have been used, could anyone help me out with this?

I wired the sets together and connected them with some 9V batteries which was all I had to hand, but I gather they should use 3V, would a 4.5V flat battery do the job?

I managed to use the handsets so they are working fine and the quality seems to be pretty good. However, when turning the generator handle the bells on either handset do not ring, which I presume they should? Nor could I get the buzzer to operate but after reading other posts on this forum I gather there should be two black knobs inside, which are not present on either handset, there is just a smooth black buzzer piece that slides in and out of the slot on the right. Any ideas how I could get the bell and buzzer working?

I don't have the handsets with me at the moment but I will try and get some pictures up if needed.

Thanks
Simon

Re: Field Telephone Type F Mk II - Help needed
 

Hi Simon,

I have a pair of field telephones model "F" and, whilst there are Forum members with a lot more knowledge than me, I managed to get mine working quite quickly.

Have you tried the trick of placing your fingers across the terminals and winding the handle? You should feel a tingle, which indicates that the magneto is generating. If OK, look at the bells. Small mechanical adjustments can make a huge difference to the ring. The trembler arm tends to get bent, so try removing the lid from the telephone and then see if the trembler is trying to work. Careful adjustment may be all that's required.

I also help with the Scouts' JOTA every year, and my field telephones will be in use this October!

Re: Field Telephone Type F Mk II - Help needed
 

Some type 'F's were fitted with induction-coil buzzers ('Buzzer, T Mk:1'); others weren't. Buzzers are interchangable with the ordinary coil (both mounted on a withdrawable frame), so you might come across some, somewhere.

Agree with Phil about testing the magneto - should produce about 90V AC RMS! Fingers are good for testing.

Type 'F's were not used in the 'front line', being more robust versions of a desk telephone with a gravity-operated cradle switch. They could be connected either directly to each other, or to a field-exchange. I would guess they would work about five miles apart with good batteries.

9V should be OK, and you're unlikely to fry the mics on the current produced by same. I use 4.5V batteries on the 'house' telephone system (2 x type 'F's and an Ericsson desk magneto, one leg to earth and the shed link via my long-wire aerial) without any problems.

One other thing: the original rubber-covered wires can go brittle and/or 'gooey', so you might have a duff connection in there. Are the magnetos stiff to turn (indicating a S/C)? When you turn the magneto handle, it should pull a pair of contacts together at the back of the magneto-generator. Are these contacts clean?

Attached is a cct diagram, in case you don't have one. Note that the buzzer version is shown. Yours will be different. Have a look at my 'Flickr' pics for comparison.

HTH

Re: Field Telephone Type F Mk II - Help needed
 

The battery voltage should be 3V, but it may be ok with 4.5. If the current reaches to high values the granulated carbon my be damged, and you will need new transmitter capsules.

The ringers and inductor may need some cleaning and oiling. The cradle switch and the switch on the inductor may have some corrosion. A dash of WD40 could be what it needs.

dsk

Re: Field Telephone Type F Mk II - Help needed
 

I've never had problems with 9V (I've probably been very lucky here...), but I would keep the voltage down to 4.5V if you can get away with it. My Soviet TAI-43s were designed to run on a single 1.5V cell, but benefit from 4.5V.

The energising current of a British field-telephone mic (similar to, but not the same, as an Inset No:13 that sits with typically 6V across it in a GPO 300 series, say) is over the range 30 -200mA. I've only come across one mic insert with a resistance as low as 38 Ohms - in a field telephone!

Below are the results of the R. W. Barnes 'straw poll' mic insert tests:

Test-telephone: Siemens 366 'Neophone' wall-telephone.

Note that a direct multimeter resistance check is inaccurate! Current is needed to obtain a meaningful result.

• Ericsson round-back N7735A
  22mA at 5.3V = 241 Ohms.
• Red-front BPO Inset No:13
  20mA at 3.4V = 170 Ohms.
• Black round-back MOD field telephone No:9 (like yours, probably)
  32mA at 1.1V = 34 Ohms.
• Red front Ericsson modern N7752
  18mA at 3.4V = 189 Ohms.
• 'New' BPO type, as fitted to older 706s, stud on spring disc
  20mA at 3V = 150 Ohms.

Re: Field Telephone Type F Mk II - Help needed
 

Interesting when 9V applied directly across the mic! Currents and derived resistances below in italic.

• Ericsson round-back N7735A
  22mA at 5.3V = 241 Ohms.
  200mA at 9V = 45 Ohms.
• Red-front BPO Inset No:13
  20mA at 3.4V = 170 Ohms.
  100mA at 9V = 90 Ohms.
• Black round-back MOD field telephone No:9 (like yours, probably)
  32mA at 1.1V = 34 Ohms.
  210mA at 9V = 42 Ohms.
• Red front Ericsson modern N7752
  18mA at 3.4V = 189 Ohms.
  60mA at 9V = 150 Ohms.
• 'New' BPO type, as fitted to older 706s, stud on spring disc
  20mA at 3V = 150 Ohms.
  70mA at 9V = 129 Ohms.

Depends on position, I suppose, as the resistance changes with the granule movement. The book ('The Telephone Handbook', Poole) says '... between 30 and 80 Ohms...'

Re: Field Telephone Type F Mk II - Help needed
 

Here are a couple of induction coils from telephone sets "F". The one on the right is the buzzer version, the one on the left is an induction coil without a buzzer.

Incidentally, if you replace the carbon transmitter insert with a modern electronic one, my experience is that you need at least 4.5V to operate it. I use a "flat" battery pack - seems fine, but my Fs only get occasional use.

Re: Field Telephone Type F Mk II - Help needed
 

Thanks guys for all your info.

I have taken some pics and it would appear that one is actually a Plessey from 1942 (as printed on the magneto) and the other is a T.M.C which seems to be a bit newer.

Still struggling to identify why the ringer on the T.M.C is not working. I have an idea from using the multimeter, that it could be capacitor 2 (pictured) that could be at fault or the actual magneto bell, as both the magnetos are giving good voltage readings. I think I may buy a capacitor and see if does the trick.

Dave, thanks for the pics of the buzzer, it would appear that both these sets have the induction coils, so no buzzer for me then! I have also gone for the 4.5v flat batteries and these seem to do the trick.

It would also appear that one of the mics in the Plessey handset is not working very well, it is marked YI/YA 7873 No 9 . In the T.M.C there is a red mic marked YA2308-MK.V, any idea where I might get another one of either of these to replace the duff one?

Re: Field Telephone Type F Mk II - Help needed
 

I haven't a No:9 spare (the type obviously dreamed up by an alien spaceship designer), but I can let you have a couple of 'newer' type 13s, as fitted to the earlier 706 telephones (130-150 Ohm as opposed to 34 - 42 Ohm).

Send me a PM with your address, and I'll get them to you. If they are no better than what you have, bin them.

Re: Field Telephone Type F Mk II - Help needed
 

For what it's worth, I have acquired a couple of original Tele F wooden carrying boxes, and coincidentally I fitted the phones into the boxes earlier this evening! All I need now is some proper webbing carrying straps...

If all the rest of the circuit checks out for continuity, I agree that C2 could be open-circuit. You'll probably obtain a suitable replacement from Maplins; 2.2uF will be fine. They sell these for audio applications.

Regarding the carbon mic inserts, someone once advised me that these can absorb atmospheric moisture through the little pressure-equalisation hole behind the diaphragm, and gentle warmth from a radiator or airing cupboard for a few days might dry it out and evaporate the moisture. It didn't help in my case, and I managed to find another insert that was better.