Field telephones

[keithinuk]

This has probably been covered in the past, so forgive me for my ignorance, I have a common type F MK2 field telephone that I have had for some time and as you do I had to get another one to try them out, they do ring each other, but that's as far as I got to be honest.
The other one I have also had for some time is still a type F, but a MK1/1 and pretty much mint inside.
It is date stamped 1939, but made of aluminium, I wouldn't have thought aluminium was all that cheap in 1939, so what I'm asking is, is this set originally from that date.
Thanks.

[ThePillenwerfer]

The early ones were cased in aluminium but when the war started this was needed for more important things, like aircraft, so the cases were changed to Bakelite.

- Joe

[Dave Moll]

First, my experience is that the MK I (with metal case), although not rare, is much less common than the MK II (bakelite) ones.

As to your problem, you don't mention what batteries you are using to power them. They would originally have had two "battery dry 1½V" cells (as shown below) in series, but they can be run off a pair of "D" (or "C") cells as the modern equivalent, and in a holder these fit quite well into the battery compartment.

If you are powering your sets successfully, your problem may lie within the carbon granule transmitter insets. Please note that if you replace the transmitter inset with the electret-type transmitter 21A, the voltage will need to be increased to at least 4½V for it to work. It is also worth checking that the receiver insets are not open-circuit. This can either be done with a resistance meter or by momentarily connecting a low voltage battery (a 1½V cell will do) across the receiver - which will make a crackling sound if it is working.

Another useful check, if one or both sets have a buzzer unit installed*, is to press the buzzer button adjacent to the handset plug and listen whether (a) the buzzer in the sending set is operating and (b) whether the buzzing is also audible from the receiving set - best not to hold the receiver too close to your ear as it's quite loud to be audible from a set with the receiver on-hook.

*This can be checked by opening up and seeing whether the induction coil removable unit (at the opposite end from the battery compartment) has a pair of thumbscrew knows on the top. The MK I should usually have a buzzer, but the MK II may not.

[keithinuk]

Would I be able to get away with, a 2v battery, as I did pick up two new C type 2v rechargeable batteries that I no longer have use for.
Both units look pretty much the same inside.

[J T Goldfinch]

New boy here with what I hope is useful information:

The photo shows the coil insert of the original buzzer variety, which has the two adjusting knobs on top. NB: If these had both been screwed down by someone trying to get the buzzer to work, they will make a shorted turn on the coil which seriously affects it properties as a telephone coil. They should be wound out so as not to contact while getting the speech part to work.

For the buzzer to work these are gently screwed in, assuming that the windings are intact, but if this is not successful it may/will be necessary to dismantle the coil enough to clean and flatten the moving contacts and round the ends of the screw-in contacts with a fine diamond hone or similar. This process is the same needed to get a Fullerphone chopper reliably operational. I have had recent experience in this with expert advice from someone in the services who knew.

John.

[dagskarlsen]

The battery should be approx 3V, not more than 4.5
This powers the transmitter, and buzzer if installed.

The buzzer is not needed when you use these telephones, but may be nice to get working when the others works.

I have one probably made when it was difficult to get mateials for production of buzzers, the unit has only an induction coil (transformer) and no buzzer function.

dsk