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Old 16th May 2016, 10:52 pm   #1
dbrb222
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Default 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?
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Old 17th May 2016, 8:16 am   #2
Nickthedentist
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Default 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.
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Old 17th May 2016, 4:13 pm   #3
TonyDuell
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Default 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.
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Old 17th May 2016, 6:10 pm   #4
dbrb222
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Default 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...
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Old 17th May 2016, 8:42 pm   #5
TonyDuell
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Default 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.
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Old 17th May 2016, 8:46 pm   #6
AndiiT
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Default 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
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Old 17th May 2016, 10:00 pm   #7
Oldcodger
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Default 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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Old 19th May 2016, 11:22 pm   #8
dbrb222
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Default 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
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Old 20th May 2016, 10:09 pm   #9
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Default 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
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Old 21st May 2016, 12:23 pm   #10
Thyristor
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Default 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.
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Old 21st May 2016, 9:18 pm   #11
dbrb222
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Default 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)
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Old 22nd May 2016, 6:51 pm   #12
AndiiT
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Default 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
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Old 22nd May 2016, 10:29 pm   #13
Pellseinydd
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Default Re: GPO 746 second microswitch and button

Quote:
Originally Posted by dbrb222 View Post
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...
The SPA3000 does not have an 'inbuilt' ringing capacitor. Whilst there are plenty of RJ11 to BT socket adapters, few have inbuilt ring capacitors. Old unmodified telephones with a two wire connection will work direct off ATAs but those modified to modern PSTN three wire system need the ring capacitor. As I mentioned previously, you can alter the ring voltage and whether it is square wave or sinusoidal ringing. Thus one ATA could be set up with ringing setting different to the other. The adapters I use come from eBay see item number 150866655308 - it is cheaper to buy ten at a time - half the price and you never know when you need one! We've had no problems with either the PAP2 or SPA3000 on our GPO replica network working over VoIP.

Ian
CNet 0352 2979
MR ETD 053-6278
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