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Old 20th Oct 2016, 9:31 am   #1
boombox
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Default BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rings

I had understood that the BT Inspiration supported pulse dialing (so is there maybe a setting to enable this?) and also when the phone is connected to an extension (though I've only tried the system extension port), when the line rings the phone only sort of faintly rattles, it doesn't ring.

The weird thing is we have other, modern, push button imitation rotary phones connected (with real bells inside) which DO ring.

When the telephone is connected directly to the BT line socket it rings.

Bit odd?

Thanks
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Old 20th Oct 2016, 12:38 pm   #2
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

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Originally Posted by boombox View Post
The weird thing is we have other, modern, push button imitation rotary phones connected (with real bells inside) which DO ring.
When the telephone is connected directly to the BT line socket it rings.
What's the ringing voltage and frequency of a telephone port on your BT Inspiration exchange?

I ask because we once had an 'INDeX' exchange and it ran at 36V d.c. (instead of 50) and the ringing voltage was only 36V RMS too, being a digitally generated 25Hz 'built-up' sinusoid. It worked fine on bells with a 4 kilohm d.c. resistance (REN 1), but I suspect it might not have worked so well on bells having a 1 kilohm d.c. resistance (REN 4).

I've found that a 'traditional' 59A bell will ring on as little as 12mA (30-ish volt supply) but may be a bit flaky.
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Old 21st Oct 2016, 2:41 am   #3
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

The Inspiration system does support dialling BUT it is very fussy on dial speed and mark/space ratio. A good way to check this is to get a modern push button phone than can be switched to pulse dial. As the pulses are made electronically the timing is usually spot on the money and the Inspiration will accept it no trouble.

I'm pretty sure the ringing voltage is 50v if it uses similar circuit design to earlier PABX's such as the Revelation and others. As Russell says you may need to check the phone has either a 4k ohm ringer or the REN is dropped to 1 using a 3.3k resistor in series with the bell coil.

I planned to replace the electronic PABX's in my mini museum with a Inspiration but didn't due to the fussiness of the system, that said it will work fine with older phones is the dialling pulses are spot on and i've had them ringing old phones with no trouble.

All the best,
Jason
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Old 21st Oct 2016, 6:48 am   #4
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

It could be enough to mechanically tune the ringer to get a ring.

You may monitor the dial speed by just connecting the phone to the microphone input on the pc. (No other things connected) How it looks when its analyzed may vary from phone to phone, but usually not to far from whats described here: http://tinyurl.com/z2fm56n

dsk
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Old 25th Oct 2016, 9:33 am   #5
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

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Originally Posted by jjwayle View Post
A good way to check this is to get a modern push button phone than can be switched to pulse dial. As the pulses are made electronically the timing is usually spot on the money and the Inspiration will accept it no trouble.
Bizarrely the phone which won't dial out IS an electronic push button phone - an old Tandy phone.

Quote:
Originally Posted by dagskarlsen View Post
It could be enough to mechanically tune the ringer to get a ring
Sorry - I don't follow. I can understand using a resistor to adjust the electrics to get a ring but don't (yet) understand how I could mechanically tune the ringer.

Cheers guys
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Old 25th Oct 2016, 11:09 am   #6
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

Hi, I can't see any reason why your 746 shouldn't ring when connected to an extension port of you Inspiration especially as it rings OK when connected directly to an exchange line.

I suspect that you have already tried to following - Are you using a master socket on the extension port for your 746 and have you tried more than one extension port in case the one you are using is faulty?

I seem to recall having a similar problem once with a Revelation PBX (which is similar to the Inspiration albeit older) and a full reset of the software solved the issue, a full reset can be a pain if you have already programmed your PBX to your requirements but might just do the trick.

There are some documents for the Inspiration PBX at this site

Regards

Andrew
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Old 1st Dec 2016, 10:11 pm   #7
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

The old type telephone (746) had an arrangement of straps to connect the telephone leads to the internal cct. The speech and bell ccts need to be connected internally by these straps.

The standard telephone line is two wires (A+B) with the ringing current supplied (30vAC) on one leg while the other has batt feed (50v) thus giving 80v (30v+50v). The master socket has a capacitor from one of the lines to take off the 30v AC to a third wire (bell cct), the two lines plus the bell cct are then fed to any extension sockets in the building (the bell wire plus the capacitor is what can screw up the broadband signals and give slow internet)

Modern telephones only need two wires as they detect the frequency and polarity of the ringing to then signal an incoming call, indeed the VoIP telephone exchanges send the ringing down both lines at at the same tine (balanced ringing)


for full details look at BTNR 315 its on the web
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Old 3rd Dec 2016, 11:11 pm   #8
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

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Originally Posted by qazxsw123 View Post

The standard telephone line is two wires (A+B) with the ringing current supplied (30vAC) on one leg while the other has batt feed (50v) thus giving 80v (30v+50v). The master socket has a capacitor from one of the lines to take off the 30v AC to a third wire (bell cct), the two lines plus the bell cct are then fed to any extension sockets in the building (the bell wire plus the capacitor is what can screw up the broadband signals and give slow internet)

Modern telephones only need two wires as they detect the frequency and polarity of the ringing to then signal an incoming call, indeed the VoIP telephone exchanges send the ringing down both lines at at the same tine (balanced ringing)
That's not quite as I understand it to work but I digress:

To 'tune' the mechanical bells for non standard ringing current; 1) first check that the bell circuit of the phone is as original and that it isn't a 1,000 Ohm bell set which has had a 3K resistor added in series - if it has, remove the resistor and re-fit the link. Later 8746s had 4,000 Ohm coils. 2) slightly loosen the screws which hold the bell gongs in place to an amount which will enable you to slightly rotate the bells with some friction.
The mounting holes for the bells are not perfectly central and rotating them increases or decreases the 'travel' of the clapper. As the clapper and it's armature has a mechanical resonance of its own, getting the bell gongs positioned so the rebound from them assists the resonance will enable the bell set to work on weak & non std ringer frequencies. When you've found the optimum re-tighten the screws. Remember, originally these bells sets were intended for 16.667 Hz (what my mate calls 17 cycles nominal) ringing current.
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Old 3rd Dec 2016, 11:56 pm   #9
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

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Originally Posted by Nicklyons2 View Post
Remember, originally these bells sets were intended for 16.667 Hz (what my mate calls 17 cycles nominal) ringing current.
I understand the 'bells, 59A' in the 700 series were originally designed for 25Hz. The older 'bells 59A' were designed for one-third mains freq (16 2/3 Hz), as found in older 300 series instruments. In practice there isn't much difference between a hand-generator supply (about 17Hz) and the exchange ringing supply (25Hz).

There's an instruction to adapt the 59A bell for 50Hz but its ringing quality is only moderate, IMO.
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Old 4th Dec 2016, 3:13 pm   #10
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

Quote:
Originally Posted by Nicklyons2 View Post
Quote:
Originally Posted by qazxsw123 View Post

The standard telephone line is two wires (A+B) with the ringing current supplied (30vAC) on one leg while the other has batt feed (50v) thus giving 80v (30v+50v). The master socket has a capacitor from one of the lines to take off the 30v AC to a third wire (bell cct), the two lines plus the bell cct are then fed to any extension sockets in the building (the bell wire plus the capacitor is what can screw up the broadband signals and give slow internet)

Modern telephones only need two wires as they detect the frequency and polarity of the ringing to then signal an incoming call, indeed the VoIP telephone exchanges send the ringing down both lines at at the same tine (balanced ringing)
That's not quite as I understand it to work but I digress:

<SNIP>.
Not as I understand it either - having been 'dabbling' with systems since the days of Strowger and first generation electronic systems (was senior commissioning engineer at Birmingham Rectory the first TXE4) and on digital and now VoIP systems. The place to look for what is sent down a normal single PSTN line is the BT SIN - and the relevant one - SIN351 dated October 2016 says -

6.1. Call Arrival Indication
Where appropriate, the BT network interface will provide call arrival indication to any off-line terminal as shown in annex A: Call Arrival Indication (Ringing).
The voltage of the call arrival indication at the NTP will be between 100 V and 40 V a.c. r.m.s as measured between the 'A' wire and the 'B' wire. A Ringer Equivalence Number (REN - as defined in ETS 300 001, Chapter 3 Section 3.1 (GB) [4]) of 4 will be supported.
Note. There is currently a relatively small scale deployment of a version of the line interface design that applies the a.c. call arrival indication voltage across the 'A' wire and 'B' wire, rather than across the 'B' wire and earth. The wider deployment of such line interfaces in the future will be considered by BT as the need or opportunity arises to replace or update existing equipment.
Call Arrival Indication may be presented with or without a d.c. voltage bias (within the limits specified in section 3.1 Off-line d.c. Condition) on the 'A' wire, the 'B' wire, or both. When Call Arrival Indication is presented with a d.c. voltage, this d.c. voltage may be present during the whole cadence or may be confined to the silent periods. The polarity of this d.c. voltage may be reversed.

Note that the 'A' line is normally at earth potential when 'off line'.

Ian
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Old 5th Dec 2016, 12:43 pm   #11
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

I once had an old GPO phone that would not ring because the hammer was jammed against the bells. I found that loosening the bell mounting screws and moving the bells apart slightly cured the problem.
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Old 5th Dec 2016, 1:38 pm   #12
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Default Re: BT Inspiration: GPO 746 telephone can't dial out and only vibrates instead of rin

Quote:
Originally Posted by D.Finney View Post
'I found that loosening the bell mounting screws and moving the bells apart slightly cured the problem.'
Bob has it here, including 50Hz adjustment, in his GPO telephone notes: http://www.britishtelephones.com/adjacbel.htm
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