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F type Field Telephones (British)
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Author:  Martin [ Tue May 10, 2016 11:46 am ]
Post subject:  F type Field Telephones (British)


I have a pair of F type field telephones which work albeit very quietly.

An expert kindly diagnosed and demonstrated that the units have the wrong No9 microphones - they currently have the blue ones rather than the black domed ones which work infinitely better (my ear is still ringing after proving it with his microphones).

Does anyone know where I could get a pair of the microphones from?


Author:  McVickers [ Tue May 10, 2016 14:48 pm ]
Post subject:  Re: F type Field Telephones (British)

Martin wrote:
Does anyone know where I could get a pair of the microphones from?

You're very best bet would be to attend the National Vintage Comms' Fair (a once annum event) which happens to be this very Sunday. If you do go, I'll send you my mobile number and we can meet up and help you search out the bits you need - I can also bring along some items to help trace what exactly may be causing your quietness.

Martin wrote:
An expert kindly diagnosed and demonstrated that the units have the wrong No9 microphones...

...hmmm, was he/she an expert on British military field phones, or just telephones in general*? The "Transmitter Insert No. 9" microphone is certainly the wrong mic' for the phone, but there also is not a 'right' version of the No. 9 mic' either. What mark are your Type Fs? If metal cased, it's a MkI which aren't at all common; if Bakelite cased it's a MkII which are very common. The handsets of Type F ("Telephones, Hand, No.1" as used with the MkI and MkII, "Telephones, Hand, No.3" as used with the MkI*) were that same pattern as the GPO standard "Handset No. 164" (much of the specialist parts and internals were standard Ericsson designed GPO pattern civilian telephone components), and these (like most other British field phones of the late '30s/'40s/'50s) used "Transmitter Inset No. 10" (Blue painted, Ericsson part unknown to me, but the army called it "Capsule, Microphone, Mk.IV") originally, or the more improved "Transmitter Inset No. 13" (Red painted, Ericsson part number N7752) when it became available. A later, further improved, version (Silver painted, Ericsson part number N7753A) was produced in 1949, but I don't know if any of these primarily civilian telephone inserts made their way 'officially' into military field telephones.

If your handset uses a "Transmitter Inset No. 9", it might either be the wrong handset, or someone's bodged it to fit - but you say their "blue", and so I think what you've actually got is the correct handset with the correct No. 10 microphone inserts fitted, albeit the original poorer performing version.

You speak of a 'black domed' mic'? Any more info' on what it is you're looking for?

As a further note, the Type F isn't strictly a "field" phone, in that it isn't designed to be used in the middle of the heat of war - it's a "remote office" phone. Meant to be placed on a desk, or at very least a basic table, at an area HQ or similar, they are noticeably not as 'loud' as the other contemporary 'phones (e.g. Type D MkV/V*, Type L, etc...) as they were expected to be used in a quiet office surrounding. Though I have seen a photo of Type Fs being used in the North African campaign "out-in-the-open" by a battery of RA, but again, they weren't exactly on the front lines of the fighting.

*or, if I may be as bold to suggest, even an "expert"?

Author:  Martin [ Tue May 10, 2016 20:11 pm ]
Post subject:  Re: F type Field Telephones (British)

Thanks for responding

Unfortunately I'm not free so cant pop along.

I can send you a few pics so you can see, its definitely a mk2 set which i use with my 8yr old son when I'm at events as he enjoys using them. I can send some pics so you can see.

Author:  McVickers [ Tue May 10, 2016 21:25 pm ]
Post subject:  Re: F type Field Telephones (British)

Well, if there's any missing bits, or bits you'd like to replace, I have some limited spares I could look through (though have far too many whole Type Fs, that I might be persuaded to break down one of the worse ones in the "to restore" pile) or I could look out for any bits for you at the Comms' Fair?

Yes, please feel free to send me any photos. I'll PM you my email address.

A great tip to help get old carbon microphone inserts working better is to give them a sharp knock - this honestly isn't an old wives tale type remedy, like thumping a flickering TV - it works because the carbon granules inside the capsule settle and become compacted, and the percussive strike helps loosen them back up.

Also, don't be tempted to put a higher voltage battery source in it to "get it to work better". Always use a 3V battery (two D-cells in a two-cell holder works great). It's only needed to provide a potential difference for the carbon granules to "disrupt" and cause a variant in. Unfortunately "more power" doesn't mean "more workyness", but can instead glaze the surface of the carbon granules and burn-out telephone coil windings.
I can go into the electronics theory if anyone is interested (basically, it's primarily due to the mic audio being on the non-line side of the 'telephone coil' [functioning as a basic transformer for the speech audio path], and as DC doesn't travel across a transformer, only the AC waveform [not true AC naturally, but the varying of the DC potential is seen as AC by the transformer] of the audio is seen on the line-side of the telephone coil; so the increase in DC potential is practically* irrelevant.

*I say practically, as there will naturally be a slight increase to the AC waveform - divition of potential, yada yadda....)

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