Rotary dial telephones and broadband

Other pages (this site): Demystifying Telephone Wiring Broadband filters
External links: Sam Hallas’ Telephone Projects History of British Telephones
(These are useful sites I used to verify information I have provided here.)

There was an old fashioned rotary dial phone in this house when I moved in.  That was in pre-BT days when the GPO had the monopoly on the British phone system and, in addition to paying line rental, you had to rent the phone itself as well!  When deregulation came, I told BT (as it had become by then) I didn’t want to continue to rent this obsolete pile of junk (as I considered it to be at the time) - and went out to buy a shiny new push-button thingy.  All fine, except the electronic attempt at a ringer it produced drove me nuts...

BT never asked for the old phone back (I’d almost certainly paid for it many times over), so I trotted it out just to have a bell again.  Then, several years later, I found they were becoming ‘collectable’ (though not worth megabucks, even today - there are loads of them on eBay, and you can buy cheap Chinese copies from the likes of Amazon).

What I have is a type 746 - the GPO’s last offering that had a proper bell - and precursor to that awful Trimphone, which was as light as a feather (so it waltzed round the table as you dialled it) and a dreadful warble that some birds could mimic (was that the phone - or another pesky magpie?)

Yes, it’s clunky - but actually extremely well-made and definitely over-engineered.  And, above all, it’s very simple and reliable.  Though in these days of DTMF push-buttons, ancient dial phones baffle some of today’s kids - as this hilarious video shows!

Tele 746

746 wiring

As an engineer, I was curious to see what went on inside.  Moreover, by now I had a problem with my broadband occasionally dropping out, which I found didn’t happen anything like as often if I disconnected the old girl.  There are several circuit diagrams on the ’net, but they’re all in ‘1950s wiring diagram’ style (like this ‘official’ circuit that BT inherited from the GPO), which aren’t at all easy to follow.  So I took it apart by unscrewing the single screw at the back (thoughtful design - it is retained once loose) and taking off the cover.  A single screw in the centre of the PCB (by the left orange wire in the photo) released it, so I could trace out its circuit.

Although it looks an utter ratsnest inside, it’s not that difficult to find out what’s going on.  The grommets holding the handset (left) and line (right) cables have been removed from the central retainer at the back for clarity.

The circuit diagram I traced out is below (click on it to see it in more detail):-

746 circuit

*1 Broadband filter added to stop the bell pranging the Internet.  Standard links T8<->T9 & T17<->T18 removed.
Standard connections: Dial orange moved T8 to T9; Bell return moved T17 to T18; Line 5 moved T19 to T16.
The bell inductance is very high (8.4 Henrys for a 4kΩ bell and 2.6H for a 1kΩ bell).
Thus, if a ring pulse ends with current flowing in the bell, there will be a significant ‘inductive kick’ that could upset broadband.
This rather crude filter attenuates any spike so formed.
*2 T2A, T19A and T19B are on silkscreen but are not fitted.  LK1 and LK2 are bypassed with PC track.
*3 A 3.3kΩ resistor replaces link T4<->T5 to reduce the REN to 1 if used with the earlier 1kΩ bell.
(REN means Ringer Equivalence Number, which determines the load put on the ring generator at the exchange.
A REN of 1 is equivalent to 4kΩ: most exchanges can drive 4 such loads.)
*4 Shown with handset ON HOOK.  GS1B makes before GS1A changes over when the handset is lifted.
*5 Marked as ‘Regulator’ in old GPO diagrams.
MR1 (the green thing that looks like a 9-legged caterpillar in the photo above) is laid out as a diode stack:
Diode stack
The ‘Regulator’, which also incorporates R3, R4 and RU1, is a separate item in the earlier Tele 706.
MR1 and MR2 are selenium disc rectifiers - NOT silicon.
(The purpose of MR2 is to prevent ‘Acoustic Shock’ - ie. not having your ear blasted if the switch intended to short out the earpiece while dialling failed to do so.)
*6 C1/2 and link T6<->T7 were not fitted to latest Tele 746s (Tele 8746), when the bell capacitor was fitted as standard in the master LJ box.  Intermediate 746s (as above) had a 4kΩ bell and one 900nF capacitor for C1/C2.
The bell impedance in early Tele 746s (and 706s) was 2* 500Ω/1.3H.  These units had two 900nF or a single 1.8μF bell capacitor.

The Tele 746’s predecessor - the Tele 706

Tele 706 This is broadly similar - the main differences are:

Future-proofing a Tele 746 (or any other rotary phone)

All rotary dial phones use pulse (or loop disconnect) dialling.  This works by the dial effectively ‘hanging up’ and reconnecting repeatedly, at a pre-determined rate, for a number of times determined by the number dialled (ie. once for a ‘1’, twice for a ‘2’ etc. up to nine times for a ‘9’ and ten times for a ‘0’).  These pulses activate uniselectors (or later, crossbar switches), which are electromechanical switches in the exchange.

[An amusing anecdote about the inventor of the first automatic exchange, Almon Brown Strowger, is that the wife of a rival firm of undertakers, who worked as a telephone operater, was diverting calls away from his business to that of her husband.  Strowger sought to cut out the ‘middle person’ with his invention, in 1889, of an automatic exchange.  He patented it in 1891 - the same year that he attained a patent for the rotary dial.]

When the first electronic exchanges (System X) were introduced, dialling using DTMF (Dual-tone multi-frequency) tones replaced pulse dialling and electromechanics in the exchange.  This enabled push-button dialling and additional keys - the ‘#’ and ‘*’.

BT continued to support pulse dialling with its electronic exchanges, and still does so at the time of writing, but this is not guaranteed forever.  So, at some point in the future, rotary phones might not be able to dial out.  From my point of view, this isn’t a big deal - they’re a pain to use - and it’s all too easy to mis-dial.  Let alone no speed-dialling or last number redial...  Plus, all this repeated disconnecting the line is a recipe for upsetting broadband.  So I keep my 746 only for its bell and to receive calls, which it will continue to do - ensured by an REN of 1, courtesy its 4kΩ bell.

There are at least two manufacturers dedicated to ‘future-proofing’ a rotary phone - by converting its pulses to tones.  This is done in one of two ways: either by inserting a box between the phone and the line (Old Phone Works - [USA]), or by fitting a gizmo inside the phone (Rotatone) - the supplied box is wired between the dial and the rest of the phone, which requires a bit of work with a soldering iron.  Neater, but a purist would say the phone was no longer ‘original’.  Extra features some of these include are: last number redial; ability to dial a ‘#’ and ‘*’; and a limited number of speed dials.  These are usually invoked by pulling the dial round as usual and waiting a second or so before releasing it.

Steve Glennie-Smith   25th August 2020

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