Field telephones Type F Mk.II

[G3PIJpeter]

I have recently acquired a pair of these telephone sets complete with their wooden carrying / housing boxes. Much soap and water followed by Paste, Polishing No. 5 has been applied to remove the grime of ages and restore the original bakelite lustre. I have also demolished a 6-volt lantern battery to provide two dry cells for each of the sets (2 x 1.5 = 3 volts).

The handsets are 'Press when talking'
The handset cable is hard-wired to the body of the set and not via a four-pin plug (as the Mk. I ?)

Results so far when wired together with a short twin flex:
1. Buzzers buzz on pressing the side button but nothing is heard from the opposite set.
2. Bells tinkle on Set A when handle cranked on Set B, and vice-versa.
3. A very slight click is heard in the corresponding earpiece when the cradle button is pressed up-and-down.
4. Nothing is heard in the earpiece when tapping or blowing into the mouthpiece (both sets).
5. Apart from the bell signals, nothing seems to be passing from one set to the other.

Is there a 'prime suspect' for causing this behaviour or should I start laboriously cleaning all the switch contacts and replacing the capacitors?

All thoughts gratefully received - after 40 years of shouting from house to shed, I am hoping that my wife will appreciate the provision of a dedicated telephone line.

Peter

[G6Tanuki]

If they've been stored in times-past in a damp environment the carbon-granules in the microphones could have clumped together.

If you're lucky, putting them in a warm, essentialluy-zero-humidity environment [think Silica Gel desiccants in a closed jar] for a few weeks and giving the capsules a regular thwack-against-your-thigh might get them back to life.

But also remember that if any fungal spores or bacteria sneezed out by some squaddie in times-past ever managed to get in there, a decade of a nice secure and damp environment could have converted the carbon granules to mush...

[G3PIJpeter]

Both of the TX capsules make a 'swishing' noise when shaken next to my ear. It sounds like sand inside a small tin can. I hope this indicates that the carbon granules are still nicely separate from each other and have not clumped.

Although the carrying boxes are labelled Telephones Type F Mk. II, the two sets I have do not contain a bridging coil and only have two capacitors in rectangular cans. The circuit diagram for the Mk. II shows four capacitors and a bridging coil. My handsets are both 'Press while speaking' - is this a way of not having to fit the coil and omitting two of the four capacitors?

Peter

[G3PIJpeter]

A couple of photos to show (1) the lack of bridging coil and only two capacitors and (2) the 'Press while speaking' handset. Note that the strip adjacent to the bells does not contain manufacturer mark or other embossed information.

[Station X]

Originally Posted by G3PIJpeter

Both of the TX capsules make a 'swishing' noise when shaken next to my ear. It sounds like sand inside a small tin can. I hope this indicates that the carbon granules are still nicely separate from each other and have not clumped.

Peter

You can test a carbon microphone by measuring its resistance out of circuit, preferably with an analogue meter. If you then blow into the mic the resistance should change indicated by a flickering meter needle.

[dagskarlsen]

Congratulation with 2 nice phones! Your description of the how it reacts makes me believe that it is pretty good hope for it. For me the most easy way to test the transmitter capsule is to take it out, connect it in series with a receiver capsule and a battery. (a small 9V batteri si OK for a test) You should hear what you say in the tranmitter. Only a click when connecting may indicate problems. If moisture in the capsule (that should hve been "watertight" may take long time to dry out) After theat radial shaking may loosen up the carbon, but I had to put in newer transmitter capsule in mine) If you use a transmitter capsule from a CB telephone , you may need to rise the voltage, that may not be OK for the buzzer. Take a look at this: https://fieldphones.org/episode61/ and this: https://fieldphones.org/episode17/

[G3PIJpeter]

Originally Posted by Station X

You can test a carbon microphone by measuring its resistance out of circuit, preferably with an analogue meter. If you then blow into the mic the resistance should change indicated by a flickering meter needle.

Done with the Avo 8 Mk. 6. Resistance of both is around 500 - 1000 ohms, which decreases on tapping. Nothing much when blowing. Both capsules are now sitting on the rack above the AGA-style range cooker - currently used for hot water / CH only and not steamy cooking.

[G3PIJpeter]

Originally Posted by dagskarlsen

... the most easy way to test the transmitter capsule is to take it out, connect it in series with a receiver capsule and a battery. (a small 9V batteri si OK for a test) You should hear what you say in the tranmitter.
Take a look at this: https://fieldphones.org/episode61/ and this: https://fieldphones.org/episode17/

I wired each TX capsule in series with a pair of army DLR headphones and a variable DC PSU. One capsule worked OK down to 3 volts; the other was rather distorted with lower volume at a given voltage compared to the other.

I have already accessed the two links - very useful - the internal views include four block capacitors (I have two) and an extra inductance. Does this difference explain the PTT handsets that I have with my MK. II sets? Are they an undocumented Mk. IIa?!

Peter

[Dave Moll]

I suspect the PTT handsets to be non-original, given that Tele "F" has a cradle switch. In my experience the handset for this set is normally unswitched. I am also suspicious of them being hard-wired rather than with the usual four-pin plug. This doesn't, however, explain the communication problem, especially as the buzzer isn't heard in the other set either.

As a click is heard in the earpiece when the cradle switch is operated, it seems the earpiece is OK, so I can't immediately think what to check next.

[G3PIJpeter]

While the TX capsules are spending the next fortnight drying out, I shall clean all the switch contacts and make sure that they are making / breaking as they should. Deoxit on a piece of paper pulled from between closed contacts usually works (not emergy paper - or a file!).

Although complete with their transit cases, the telephones were fairly grubby inside and out. It is likely that they have had a hard life, with a decade or so of irate brigadiers shouting into them amidst a cloud of tobacco smoke.

Peter

[RadarAl]

I had 2 type 'F's and one had the same handset, the one that I have left doesn't.
I have attached a photo of the instruction plate in case you haven't got one.

[G3PIJpeter]

Originally Posted by RadarAl

I have attached a photo of the instruction plate in case you haven't got one.

I have an instruction plate screwed into the lid of one of the wooden carrying cases. As is usual in my case, I hadn't read it. On reading your thumbnail, the final sentence leapt to my attention ("connect the receiver ... across L1 and L2 ...). I wired each set in turn to a pair of DLR headphones (R = 80 ohms) - both resulted in buzzer and voice in the headphones. Oddly, I did not have to press the "Press while speaking" lever on the handset to hear my voice - both switches seem to not to be a functioning part of the system.

Wiring the two sets together gave nothing in each earpiece, both of which were subsequently measured at around 60 ohms each (so they're not O/C). The circuit diagram shows the receive pathway to be through C3 and also the secondary of the buzzer transformer. However, it is unlikely that both are U/S.

The presence of two and not four capacitors and the absense of a bridging coil indicate that these two sets are not standard Mk. IIs. Any thoughts?

Peter

[dagskarlsen]

It looks like it has been modified by some reason. Probably not to much to making it working again. The 2 missing capacitors may easily be replaced by new, the bridging coil could be replaced by a relay or something if you need to use it on a CB-line. The rest seems to be there, maybe except a wire or two.
The Press while speaking switch might have been bypassed inside the handset, or given a new function. It may be a job to measure everything to find out how it is wired today.

[G3PIJpeter]

Originally Posted by dagskarlsen

It looks like it has been modified by some reason. ... the bridging coil could be replaced by a relay or something if you need to use it on a CB-line. The rest seems to be there, maybe except a wire or two.

The inside looks original - there's no sign of molestation. All the wiring seems original and all solder joints carried out at the same time (judging by the uniform layer of age-related grime).

I want to use this pair of telephones on a single twisted pair type of line. Not sure what a CB line is - via a switchboard? Next job will be to trace the wires and components associated with the earpieces.

Peter

[cmjones01]

CB = Central Battery, where power for the phone comes down the line from an exchange, rather than phones connected directly to each other relying on their internal batteries.

I can't make it out clearly from the photo, but what condition is the internal wiring in? I've got a couple of Telephone Sets 'F' (Mk I and Mk II) and in the later one the wiring was insulated with rubber which had gone crumbly. This stopped all sorts of things working. I rewired it with silicone rubber insulated wire some years ago but never finished debugging it. The earlier phone seems to have fabric insulation which has survived much better.

Chris

[G3PIJpeter]

Originally Posted by cmjones01

CB = Central Battery, where power for the phone comes down the line from an exchange, rather than phones connected directly to each other relying on their internal batteries.

I can't make it out clearly from the photo, but what condition is the internal wiring in?

Ahh - CB = Central Battery - that's sorted that one out. I have been using 2 x 1.5 V (= 3 V total) dry cells in each set.

The insulation in both sets is rubber but it's in good condition with no signs of rot. Whatever the problem is, it is causing the same symptoms in each set. Each transmits sound down the wire to L1/L2, as heard in a pair of separate DLR headphones but nothing is heard in each earpiece when connected directly by a twisted pair. Bells tinkle as they should. Buzzers buzz, but nothing heard on opposite set. All switch contacts cleaned with DeOxit.

I shall take photos of specific internals if required.

Peter

[dagskarlsen]

Good ideas, it may have been modified to be used on CB lines only without the risk of a generator that may ruin the modern CB equipment. Still a little strange, but I could have done such things my-selves as a teenager, before I knew better

[G3PIJpeter]

Looking at posts Nos. 5 & 6 at https://www.vintage-radio.net/forum/...ad.php?t=45104 I understand that testing a mic capsule with a DVM on the ohms range does not pass sufficient current for the reading to be meaningful - ditto an analogue AVO.

As advised in that thread, I have measured the current through the two capsules (YA1077-MK.IV) from my handsets at 3 volts and at 9 volts, with the following results (converting to ohms via R = E/I).

Capsule 1
3V = 60 ohms when shaken; 100 ohms when tapped
9V = 150 and 300 ohms.

Capsule 2
3V = 30,000 ohms
9V = 3000 ohms

The thread notes that "... as the resistance changes with the granule movement. The book ('The Telephone Handbook', Poole) says '... between 30 and 80 Ohms...' ..."

It seems to me that capsule 2 is totally duff; capsule 1 is on the high side. When banging the mouthpiece on one handset, I can just about hear a noise in the earpiece of the other. Interestingly, a set of DLR headphones wired to either set's L1/L2 reproduces speech - clear in the case of capsule 1 but weak and distorted in the case of capsule 2.

Looks like I need new mic inserts (?)

Peter

[G3PIJpeter]

Update . . . . The mic capsules have spent the past four months sitting on top of the hot water cylinder in the airing cupboard, with occasional outings for a vigorous shaking. Now fitted back in the field telephones, they both give clear undistorted speech into a pair of DLR headphones connected to the terminals.

However, nothing is heard when the field telephones are connected to each other by twin flex. I have previously cleaned all contacts on the various switches. Would it be possible to cut my losses and rewire the internals so that I have a pair of battery-driven handsets with magneto / bells for calling? The buzzers buzz, but nothing is heard from the opposite handsets. Is it possible that both earpieces are defunct?

Any and all comments gratefully received.

Peter

[dagskarlsen]

The diagram from here: https://www.vintage-radio.net/forum/...ad.php?t=45104
modified for measuring enclosed.

This is a pretty hi-teck phone regarding the age.


If you do not know witch phone that causes the trouble, just try to put on, and off a 9v battery to the line terminals, that should make loud clicks in the receiver of the working phone.

If you unplug the induction coil and measure with the handset off hook. A-B should be close to 0 ohms. (Less than 10)
B-C should not be measurable. C-D should be around 400 ohms.
E-F should be close to 0 ohms.

Measuring the contact points of the unplugged induction coil D-E should be measurable and probably less than 100 ohms.

If not OK look at the contact sets at the generator, and cradle-switch.
The simplest test might be to move the wire from the A terminal to the capacitor terminal B or C and try to speak to the other phone.