Over the years there have been reports in tech literature of subscribers adding various odd phones they have acquired as extensions on an existing PMG/Telecom service. One of the tasks assigned to staff at an auto exchange was to measure the capacitance on a subs line to see if it more than expected for the number of phones connected. If just one phone was connected the value should have been in the range of 1.5 to 2.2μF but if a subscriber had plugged in an unauthorised second phone then the value would be doubled. The PMG/Telecom were always chasing revenue for extension phones and turn up at the door of any scriber they detected as having more phones than they were paying for. The other problem was that a 'resourceful' subscriber would adapt a GPO plug and socket to connect the extra phone which was very dangerous for all concerned. A recent item on eBay provided proof that this did happen. I wonder if the item came from a 'deceased estate'.

The phone is British Tele 702 but they were used in Australia on PAX systems.

It was possible, using the jumpers in the 1st phone on the line and the extra circuits on the standard wall plug and socket, to wire the extension bells through the "master" phone. In this way, other phones on the extension would not tinkle when you were dialing, and the exchange would only see one phone.

Most Telecom techs worthy of the name knew how to do this.

Yeah, that capacitor bypass trick was pretty well known.

Photos uploaded.

My apologies for lateness. I have just returned from a camping trip.

Wow!
Plug that baby into 240 volt and I bet the bell would give more than a tinkle!!!!!
Fred.

Seen this many times when working for Telecom . I believe its natural selection in action, if you remove the signs lol the problem will eventually not exist.
I once heard of a house in America that had all his extensions wired into GPOs. To make things worse there were no polarised GPOs.

Photos uploaded.

Did you mean to post the same photo twice?

Thanks Brad for up-loading the photos but you have doubled up on the first photo. The second photo gave a better view of the way the plug had been wired to the connection block. However I think that the readers have grasped how dangerous the configuration was.

In fact I purchased that phone because I wanted the internal parts to restore another phone. It may be that someone plugged it into a GPO to "see if it still works" like the great 'unknowing' are prone to do with old radios; I will report the condition of the phone when I get my hands on it.

As Ian and Carl mentioned, if someone had access to the techs installation handbook they would have known how to wire an extension phone and set the internal jumpers so that the capacitor in the main phone was in parallel but the two bell motors were in series. That way the unapproved extension phone was fairly well undetectable from the exchange. However most punters just connected the extension in parallel with the main phone and wondered why the bells tinkled when dialling.

All fixed.

The old phone has arrived so I have been able to examine it. The case was badly scratched but fortunately the internal components (that I wanted) were in good condition. On checking the line wiring I found that it had been connected between Neutral and Earth in the three-pin plug which might have provided some protection had the phone been plugged into a real GPO. However if the GPO had Active and Neutral transposed then the result would have been fireworks and lots of 'magic smoke'.

As I had expected the phone was a version of the Tele 706 intended to be used on a business PAX. Although the phone was marked 'CSA' on the back of the case it had been made by Ericsson Telephones Limited (ETL) in the UK in the 1960s. The style of point-to-point internal wiring and the 'N' number on the circuit diagram was indicative of ETL as the OEM. The diagram (when posted by Brad) shows the simplified RC bridge circuit used on 24V PAX systems where lines were less than 1Km. The standard Tele 706 as used by the BPO had an induction coil (type of transformer designed to pass DC) and operated on 48-50V with a regulator to maintain the line current at about 30mA.

In fact the simplified circuit can be made up using standard 1W carbon resistors (three 330R in parallel for R2), a 1μF greencap for C2 and 2μF 250VW greencap for C1. I have used this circuit to restore/convert other old phones to working condition several times in the past.

British Telecom 706 Circuit Diagram

Had a neighbour back in the '60s who rigged up an extension speaker for his radiogram using 240VAC plug and socket and a length of two core flex. Took a fair bit of diplomatic explaining to convince him that this was not a good idea.

When I was aged 5 or 6, my father borrowed a neighbour's electric mower while our Atco was having its blades sharpened. The neighbour had attached a female 3-pin Bakerlite socket to the wooden handle of the mower and used a suicide cable to connect to an extension cord. I vividly recall my father asking me why it was a very bad idea.

Document uploaded to Post 10.