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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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16th May 2016, 10:52 pm | #1 |
Retired Dormant Member
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GPO 746 second microswitch and button
Hello
I have a GPO 746 phone that I am converting All seems fine, but I am curious about a second micro-switch and button, in addition to the normal microswitch on the hang-up mechanism: The second microswitch is wired to the on-hook mechanism on the far side from the "normal" spring-loaded switch The push button on the top (type 5A) is wired in parallel with this second switch Both are wired into T11 (brown) T12 (blue) and T13 (grey) on the PCB As I understood it though, these terminals are spare... So does that mean these two switches do not actually have any function as things stand? |
17th May 2016, 8:16 am | #2 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,820
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Re: GPO 746 second microswitch and button
Hello and welcome.
Have a look underneath. Does it actually say "746" underneath, or is it one of the many other 700s-series variants that were produced? Nick. |
17th May 2016, 4:13 pm | #3 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
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Re: GPO 746 second microswitch and button
Are you talking about a microswitch which is operated when you place the handset on the rest? There were 1, 2, and 3 pole versions (basically stacked V3-size microswitches with the appropriate actuator) that were used for various purposes. For example, if you have the telephone wired for 'Plan 4' (plug-in wiring using the old, 4-contact jack plugs, Plug 420), the bells of all the telephones plugged in end up in series and each is fitted with a microswitch operated by the handset rest to open the bell circuit if any telephone is off-hook to prevent tinkle.
I am pretty sure that a telephone wired for Plan 105 or Plan 107 external (extension telephones to a planset) need both one extra button and one extra contact on the handset rest. I can't think of a use for having said microswitch and the button wired in parallel/. My guess is that somebody has converted the telephone to 'normal' wiring and just parked the leads there to prevent them shorting to something else. |
17th May 2016, 6:10 pm | #4 |
Retired Dormant Member
Join Date: May 2016
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Re: GPO 746 second microswitch and button
Thanks all
The unit is stamped on the base with "746 GEN 74/1A C" There is the standard sprung microswitch on one side of the cradle rest, but on the other there is a second microswitch - type 19A This second switch is wired in parallel with a switch of type 5A, which can be actuated by pressing the single button above the dial Both of these switches are wired back to T11,T12,T13 - but there are no further connections to these terminals, and they are marked as spare on the diagrams I have found, which makes me think this button and extra switch may have been there "just in case" someone found a use for it... |
17th May 2016, 8:42 pm | #5 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
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Re: GPO 746 second microswitch and button
I think it's unlikely they were fitted 'just in case'. As I understand it, if you rented a telephone for a use that needed said switches (and remember back then you rented the telephone, you didn't buy it), the GPO engineer would get the appropriate bits (telephone, Switch 19A, Switch 5A, the appropriate button, etc and assemble them.
Does the telephone currently have a new-style plug (Plug 431A)? If so, it's been converted at some point (a telephone originally supplied with that plug is an 8746). My guess it that it was obtained surplus, it was converted to the new type of plug and the extra switches, not needed for this, were just left in place, not connected to anything. |
17th May 2016, 8:46 pm | #6 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
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Re: GPO 746 second microswitch and button
Hi, the additional gravity switch and auxiliary button were most likely legacy items which were fitted for 'C' wire signalling which was used with many PMBX switchboards. As others have mentioned they have no purpose when the telephone is connected to a standard exchange line.
Regards Andrew |
17th May 2016, 10:00 pm | #7 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: GPO 746 second microswitch and button
Another idea is for an PL1A extension to a shared service where one switch was in lock mode to short out bell and other was momentary mode to call exchange, and both buttons now taken out of use. Might depend on any writing on the switch surface ( e.g. "PUSH")
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19th May 2016, 11:22 pm | #8 |
Retired Dormant Member
Join Date: May 2016
Location: London, UK.
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Re: GPO 746 second microswitch and button
Thanks all - for the moment I have let them doing nothing, and the rest of the phone is working fine.
It still has a nice old jack plug / socket, instead of the modern BT one - so I wired up a little converter box between the two plug types. Now my only issue is that I use VOIP - and I have two VOIP adapters - an SPA3000 and a PAP2 Both are almost identical (same manufacturer and firmware) but the phone rings on the former, but not the latter. My modern phone rings on either, perhaps something do do with power draw... though both units claim to support multiple REN |
20th May 2016, 10:09 pm | #9 |
Heptode
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 707
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Re: GPO 746 second microswitch and button
Are the ringing settings the same? There are various settings which can be altered such as voltage, frequency and either sinusoidal/square wave. Could be that one is set up for one set of settings and the other for a different one. I must admit that in ten years of using various Linksys units set up for British tones & cadences and UK ring settings with CNet, our replica of the old GPO phone network, I've never had a problem with a Linksys ATA and the small ring RJ45 to BT adapter (which 'ring' capacitor) when using phones converted to modern plug & socket.
Be interesting to see if they are the same - only a thought. Ian Jolly CNet 0352 2979 |
21st May 2016, 12:23 pm | #10 |
Pentode
Join Date: May 2015
Location: Hove, East Sussex.
Posts: 147
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Re: GPO 746 second microswitch and button
It could have been for 'Secretarial-Awareness'
When the boss was one the phone, a second uswitch would, via the switchooks, illuminate a lamp on his secretaries phone, so she knew he was on the phone. The push button would serve the same purpose. (in a meeting, etc.) There would be an N-diagram for this somewhere. These were a popular in large bank and insurance branches, and were a total pain to do. You would have to mod the bosses 746, add a lamp to the secs 746, change both cords to 6-core, change the BTs, and then prove a pair between both phones. In practice, you'd start with the bosses phone, mod a new 746, connect it directly to the bosses 746, to prove it worked, and then swap the secs phone. Nightmare ! And then is the boss changed his secretary, you'd have to start all over again ! In your case, your 746 would have been the bosses phone. |
21st May 2016, 9:18 pm | #11 |
Retired Dormant Member
Join Date: May 2016
Location: London, UK.
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Re: GPO 746 second microswitch and button
Hi,
Wrt the ATA, I wonder if the ring capacitor could be the issue. Both units have the same settings, and both can ring my "modern", self powered phone. However, the SPA3000 can ring the old phone as well. If I pick up the old phone when it should be ringing (but isn't), the call works fine. Perhaps the SPA3000 has an internal ringing capacitor, and the PAP2 does not. I don't think the RJ11 to BT converter I have contains one - so that might explain it. Easy enough to test... Apparently this unit came from a local council building, so that could make sense - though the cord was only 4 core, terminated in a nice brass jack connector (which I have retained and use, converting it to a modern BT plug with an inline converter) |
22nd May 2016, 6:51 pm | #12 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
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Re: GPO 746 second microswitch and button
Hi, the line cord terminated with a jack plug (jack 620?) and the additional gravity switch suggest that the phone was wired as one of a pair (or more) of portable phones wired to Plan 4 diagram N4506 with two telephones (fig2.) or N4526 which was for party line/shared service working; the addition auxiliary button certainly suggests the latter.
Regards Andrew |
22nd May 2016, 10:29 pm | #13 | |
Heptode
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 707
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Re: GPO 746 second microswitch and button
Quote:
Ian CNet 0352 2979 MR ETD 053-6278 |
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