GPO Tele 312F Bell tinkling and low receive volume.

[dtmfman]

Hi,

I have two issues/queries as folllows:-

1. Everytime I dial from my 312F other modern digital phones in the house ring in unison with me dialling.
What could cause this? Is it something to do with the 312F telephone?

2. The receiver on the 312F telephone handset is quite low in received volume. I have tried three receiver swaps including the handset cord and all received levels are to me quite low.

Is there a history on these phones which details what I am hearing?

Is there a newer modern receiver I can replace on this handset and would it need modding to work?
If so what type of receiver and what do I need to do to mod it?

Thanks in advance for any info.

Cheers

[OscarFoxtrot]

1. Your 312F might be mis-wired and not suppressing bell tinkle.

Is it wired as here:

http://www.britishtelephones.com/pstconv1.htm#312

2. Your modern phones might only have a 2-wire connection to the line and will tinkle if an extension dials out in loop-disconnect. You probably cannot easily rewire the modern phones.

3. You might have the phones plugged into different ADSL microfilters which break the pin 3 bell wire and recreate it at each filter. That will prevent the 312 suppressing bell tinkle.

[dtmfman]

Hi,
thanks for the info.

I converted the phone as per your link originally and I got the same symptoms of muffled receive.

I also converted the phone as follows:


1) Remove the base plate of the telephone (using the 4 screws in the corners)
2) Remove all existing wire straps
3) Insert strap between T1 and T2
4) Insert strap between T8 and T9 (as seen on the diagram below)
5) Connect the white wire of the line cord to terminal T1
6) Connect the red wire of the line cord to terminal T9
7) Connect the blue (bell) wire of the line cord to terminal T11
8) Connect the spare green wire of the line cord to terminal T3
9) Insert a 3K resistor between terminals T11 and T12.


Which is stated for a 332 but the quality of receive /ring/dial is exactly the same.

Although I believe the above conversion is a lot cleaner.

Is there any reason why I cannot use the above conversion? i.e what are the differences/impacts on the 312 between the two conversions please?

I have also just tried plugging my phone into the master jack and no other modern phone bell tinkles when I dial from my 312F.

Narrowing it down the only modern phone which tinkles is the one originally plugged into the master jack when the 312f phone is plugged into the slave-so I believe you are right with the 2wire loop-disconnect info.

With reference to adsl filters which ones should I use?

I assume they should also be standard throughout the house?

I'm also just wondering if the slightly lower receive tone is a design limitation of the 312f when I am comparing it to a more recent 332l?

Is it possible this is just the way 312f phones are?

My 312f is a 1953 model.

[mikelect]

I had this bell tinkling problem many years ago when I was dabbling with phones the answer was to connect a thermistor (donated by another phone) into the GPO phone but for the life of me I can't remember what the connections were it was told to me by a friendly GPO engineer but he has long since dissapeared, the answer must be out there somewhere should I remember I'll get back to you .....................

[OscarFoxtrot]

Originally Posted by dtmfman

With reference to adsl filters which ones should I use?

Ideally one filter at the master socket, with your router plugged in there, and all phones downstream of the filter.

[julie_m]

Unfortunately, it's what they do

A telephone bell rings when energised with AC. The line normally carries DC. A capacitor is used to send only the AC component to the bell. Any sudden change in line voltage -- such as when loop-disconnect dialling is in progress -- looks like temporary AC, and thus can cause tinkling.

In the early days of multiple extensions, all bells were wired in series, with one ringing capacitor in the "master" phone; and lifting any handset interrupted the series chain, thus preventing tinkling.

When the need to have pluggable phones came along, all bells were wired in parallel with one ringing capacitor in the master socket. This time, tinkle suppression was done by having the bells shorted out while any handset was lifted.

Then came two innovations which finally rendered tinkle-suppression measures unnecessary: electronic "ringers" (which could be equipped with a delay to discriminate between stray pulses and sustained bursts of ringing current) replaced bells, and DTMF replaced loop-disconnect dialling.

The advent of ADSL required every phone to have a filter between it and the line, and each filter contains its own ringing capacitor, downstream of the filter, to regenerate the ringing signal. (Otherwise, shunting the ringing signal would also shunt out the ADSL carrier.) By this time, it was assumed that everyone would be using modern phones with electronic ringers (which won't be set off by LD dialling on an extension), and DTMF signalling (which won't make extensions tinkle even if they are connected via ADSL filters).

Hence your tinkling .....

A standard plug-in ADSL filter only supports an REN of 1 (i.e., the ringing capacitor only lets enough current through for 1 standard ringer). But you can get faceplate filters which replace the removable faceplate of an NTE5 (modern master socket) and have a ringing capacitor which lets enough current through for up to 4 REN (which is as much as the exchange line will support). You will need to have the ADSL router/modem near the master socket, obviously, and extend the cables to the switch which serves your computers (unless you're all wireless).

This should improve the reliability of your Internet connection, though, as ADSL + phone is much less robust than ethernet.

[dtmfman]

Yeah,
I believe you are right about the bell tinkle.
Thanks for your info-very much appreciated!

My only real issue is the more muffled receive volume I am getting.
I'm not sure if this is a design issue or what.
I have terminated a different handset and no change so this is why I am wondering if I am chasing/analysing a fault which is not there.
Th transmit is fine.

I have also swapped out the Receive module and replaced with known good ones and still the same quality.

Is there anything I should verify on the main telephone board?

Any known issues/info on diagnosis would be much appreciated.

Cheers,

[russell_w_b]

Originally Posted by ajs_derby

You will need to have the ADSL router/modem near the master socket, obviously, and extend the cables to the switch which serves your computers (unless you're all wireless).

At work, we have NTE5 faceplate sockets on a couple of master LJUs, and we routed the ADSL output via a pair in a CAT5 for a distance of about (as the cable flies) fifteen metres. No difference in speed or performance whatsoever.

Of course, if you do this at home, you'll have a girt big CAT5 running around the house to wherever your router is! Next to the master LJU is probably more convenient.

[dtmfman]

I don't think my wife would appreciate the cable running throughout he house .

I have just resolved my low receive tone and muffled voice issue.

The no12 dial contacts were the issue.

Specifically the switching springs needed retensioning as they were slightly to obtain the following:

Therefore when the dial is normal, the minimum contact separation should be 10 mils. When the dial is off-normal, there must be a space of approximately 20 mils between the buffer of the first level spring and the face of the second lever spring.

The two lever springs (springs 2 and 4) should exert a pressure of 29 ± 4 gm. (re-adjust), 29 ± 7 gm. (test), on the contact springs (springs 1 and 3), measured with the gauge detail opposite the contacts and they weren't which needed the retensioning.

So before when the dial was normal there was very very slight contact between the spring buffers.
This allowed dial tone through but did diminish its receive level by at least half.

So all well that ends well.

I do have one remaining question why do we have to convert the 312 telephones quite significantly different to the 332s in respect to converting their line cords?

Both conversions work fine on the 312 (as far as I can test)

[russell_w_b]

Originally Posted by dtmfman

The receiver on the 312F telephone handset is quite low in received volume. I have tried three receiver swaps including the handset cord and all received levels are to me quite low.

Further to your receiver sound level...

Don't expect the same receive level on a bakelite handset as on an instrument with a balanced armature receiver fitted (706, 746, 8746, etc...)!

If your 312 is fitted with the older 1L receiver, it will be about 10-15db down on the newer 2P receiver. What sort of receiver has your telephone? Have a look here to see.

[dtmfman]

Hi,
fixed the receive level issue as detailed above.
Originally the receive level was very low but is now at expected nominal levels.
The handset has a No. 2p model installed which is the latest for this model (I believe!).

Thanks for your help an info.

The only question I have remaining is what the difference/impacts are if you convert a 312 exactly the same way as a 332 instead of following the recommended way?

I think I will raise another question on the forum for this specifically as I am deviating form my original issues.

Cheers,

[russell_w_b]

Originally Posted by dtmfman

The only question I have remaining is what the difference/impacts are if you convert a 312 exactly the same way as a 332 instead of following the recommended way?

Briefly, then...

They are two different instruments! A 312 is for what was known as 'shared line working' where an earth return was used as a common wire in the ringing circuit - 'A' wire & earth for calling one sub on the shared line; 'B' wire and earth for calling the other sub: shared pair for speech. it was possible to lift the receiver and listen to the other party's conversation, but that's how these things worked.

Two subs could be connected to one copper pair. A bit like the DACS philosophy today, but no eavesdropping on DACS!

The thermistor and diode in the 312 were functional to shared service operation, and to wire a 312 to use on today's POTS lines, you can forget them. Note that the bell in a 312 is actually cut out of circuit by one of the cradle switches, unlike a 332.

Try wiring your 312 as in the attached diagram and see what happens.

[migette]

The bell tinkle accurs when dialing is taking place,due to surges caused by the make and break of the dial. This can be overcome by two ways, some telephones have a bias spring attached to the bell hammer and provides a small pressue which prevents spurious tinkles without stopping normal ringing. US phones use this method and their bells are in parellel unlike here where one cap is used and the bells are connected in series during ringing. By wiring a GPO thermister in series with the offending bell this will stop this, I gave an explanation of the workings of thermistors in the Rotary Phones Forums. Hope this helps I would agree about the volume of the 332 being quieter due to the older style, rocking armature better but will not fit in the solid handset Peter Excuse spelling I cannot get the spell check to work

[russell_w_b]

Traditionally in the UK, the bell was shunted during dialling by one set of dial-off normal contacts and a resistor (30R) non-inductively wound on the ASTIC. The dial pulses were quenched by the same resistor in series with a further dial-off normal contact and one set of the cradle switch contacts, making a series CR circuit across the dial pulse contacts to include the 2u bell capacitor. This prevented bell-tinkle. As all bells were connected in series (up to six), then all bells were quenched.

On the modern UK three-wire system, one series capacitor is mounted in the master LJU and all the bells are connected in parallel, in seires with it. The blue (typically) third 'anti-tinkle' wire and bell connection to the master LJU will form part of the quenching circuit and bell-shunt (in other instruments on the three-wire system), with the capacitor in the master LJU taking the place of the original bell capacitor in the telephone instrument, which is no longer used.

It is important when connecting, say, a 300 series instrument, to ensure there is no break between the blue (typically) third wire to the standard BT plug-and-lead and the resistive winding on the ASTIC, and subsequently the DON contacts.

[russell_w_b]

Incidentally, it is possible to 'shoehorn' a balanced armature receiver into an older type 1L earpiece - I have done it several times! Not possible on a 2P, as the receiver element is of different construction, and the threaded sections of the receiver are different, so won't match the respective earpieces.

For authenticity, it is bad to stick a 2P earpiece on a 1L receiver, and vice-versa; tch!

See here for how 1L to B/A is done.

[migette]

Thanks for info re fitting xmtters into solid h/s I should of realised this as I bought replacement receiver for one of my projects which cap was welded due to what looks like the receiver being left somewhere damp. Nice to know this can be done. Peter