Telephone wiring
- Thread starter russ8585
- Start date
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I had identified on a number of occasions that some simply did not work on 37.5V systems due to the lack of line current. The loop resistance was low enough that there may only be a volt or two across the tele. It was enough of a problem that the company built test adapters, to insert between the socket and the tele with 2 4mm sockets for a meter and also a test button which placed a resister across the line in lieu of the tele socket, very crude affairs built into a line jack.
If a customer complained their phone didn't work we would try our test phone first, if that was OK we'd plug in the adapter and meter and check there was at least a minimum voltage with the test resistor. If their phone took the voltage down below the test resistor it was declared unsuitable.
The joke was: I/we used to hate going into the switch room due to the smell of 50 or more women wearing different combinations of shampoo/deodorant /perfume etc. To this day I think it's what triggered my first migrain.
I feel like that may be true sometimes.
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With modern systems where line current is controlled by the line card in the exchange. With a high resistance loop the large voltage drop along the line to the Off Hook phone could result in there being insufficient voltage for the phone to work.
Back to the old days when the exchange could supply 96 milli-amps and the phone regulated the current it took when Off Hook
Copied from http://www.britishtelephones.com/t706dismantle.htm
Contrast of Constructions Methods
It was stated [Ref 1] that printed circuit techniques were quite novel in 1959 and some manufacturers preferred to stick with the traditional hand-wired construction. After taking the wired version apart and re-assembling it, I am surprised at this. The sheer simplicity of assembly of the printed circuit board must surely have made it quicker and cheaper to assemble.
Transmission Circuit
The circuit diagram starts out from the incoming line in a similar way to earlier telephones. Magneto bell isolated by large value capacitor, C1A, which also forms part of the receiver circuit. Three winding hybrid transformer with balance components C1B and R2. Dial off-normal contacts, one which shorts the transmitter for improved pulsing and the other mutes the receiver to avoid clicks during dialling.
The innovation is on the left of the circuit diagram with the automatic sensitivity regulator, the Regulator No 1. A detailed explanation is given in the POEEJ article [Ref 2]. Here is a simplistic description. The line current is passed through the resistor lamp (shown as a positive temperature coefficient resistor). This is the black painted glass bulb that looks like a lamp. As line current increases it provides an increasing potential difference across the back-to-back diode bridges - the red tubular item with nine leads. These diodes in turn partially shunt the receiver to reduce the sensitivity. At the time of design the public network was entirely Strowger based and current on short lines could reach 95 mA. Recent experiments [Ref 4] have shown that the regulator only provides a shunting effect at currents above 40 mA. On modern electronic exchanges where the line current is limited, it would have no effect.
Early regulators had clear glass lamps. Of course they glow a dull red as the line current passes through them. The story goes that nervous customers telephoning in the dark saw the phone glowing and dunked it in a bucket of water thinking it was on fire. Whether this is true or not, all later lamps were painted black.
The regulator can be reversed and as you can see from the circuit track layout, in that position it simply shorts out terminals B, C and D to maintain continuity. I don't know when it would be necessary to use the telephone in unregulated mode - maybe for local battery working.
That's a good article Bernard, one or two points I either don't agree with are simply wrong but nothing to get involved in discussion over.
From your reply I assume my line current comments have been unclear.
Some of the early phones had a low internal DC loop resistance, such as the 300 type and the same it true of some of the early electronic phones, when these were used on long lines (high resistance) the volt drop in low resistance phone resulted in insufficient current to operate the phone properly. (Low voltage across the phone)
We automatically turned the regulators over on the lower voltage systems.
The only issue I was aware of with the 700 type tele were the original bells had 1000Ω bells (REN4) and they struggled to move properly on high resistance lines, they could possibly be adjusted so something could be heard.
Yes but these phones were made at a time when extensions bells were wired in series, officially 5 maximum, later versions of the phone had 4KΩ bells (REN1) to use with PSTN system.
If fact phones with 1KΩ bells were around before the term REN was invented.
No, as explained, back in those days the bells were wired in series - so 2 bells makes 2k, 3 bells make 3k and so on. If you had plug in phones, it was a multipole jack (I think I have one or two still lying around the junk box) which included a switch to short out the bell contacts when the plug was removed. The phone needed 4 wires - the A&B connections for the phone and 2 more for the bell, with teh latter 2 being shorted in the socket when the plug is removed.
This all changed in the (IIRC) 80s when they introduced the flat socket we are used to now - to make it easy for users to use their own phones - and I guess they decided they needed to make it easier by having everything with parallel connections.
As a bit of trivia, I vaguely recall the plug/socket combination came from Vero, and was available with different keying (shape of the non-contact side of the plus) and also left and right handed (retaining clip on left or right of plug). Some phones used the opposite handing plug for the handset.
Great. So now, if you haven’t done so already, you need to get a contract with BT for telephone service.
If you already have, then report it as a fault.
If you already have, then report it as a fault.
Or one of the myriad other telephone providers.
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