GPO 746 conversion HELP!!???!!
 

Hello folks...

I am slightly confused re converting my old 746 phone, any help would be appreciated (I am pretty clueless with stuff like this)...

I bought a new lead with a couple of components and I have put the supplied resistor between T4 and T5...is this correct? Does it matter which way round it is?

I have another component (diode??), supposedly to prevent acoustic shock...I have no idea what I'm supposed to do with this!! Does it go in the earpiece or the phone terminals as per the resistor????


Cheers guys

Cletus

Re: GPO 746 conversion HELP!!???!!
 

As per the instructions at British Telephones web site, the "rectifier no. 205" should be connected between terminals T1 and T2.

Please note the other instructions there about placement of wires and the addition/removal of straps.

Both the resistor and the rectifier (which is really two diodes in opposite polarity) can be connected either way round.

Re: GPO 746 conversion HELP!!???!!
 

Cheers Dave

I have wired it as the website suggests:

Remove strap T6 - T7
Remove strap T4 - T5
Insert strap T5 - T6
Red wire of line cord to T8
Blue wire of line cord to T6
Insert straps T16 - T17 - T18 - T19
White wire of line cord to T18
Green wire of line cord to T15
Insert a 3.3K resistor between T4 - T5
Insert a Rectifier No. 205 (if needed) between T1 and T2

Unfortunately the phone does not ring, but otherwise functions correctly...

Any ideas anyone???

Re: GPO 746 conversion HELP!!???!!
 

Have you actually linked T17 to T18? From memory, aren't these two terminals on different rows on the terminal block?

Re: GPO 746 conversion HELP!!???!!
 

16/17/18/19 are the final (right-hand) four terminals of the bottom row.

CVD's description of the wiring looks correct, and corresponds with the 746 which I have open in front of me.

It is worth checking that the ringer is working and wired correctly. The two ends of the coil should be connected to T4 and T16 respectively. With the 'phone disconnected from the line, connect a multimeter set to ohms (if you have one) between these two terminals - there should be a reading in the region of either 1K or 4K. If you get the latter reading, remove the 3.3K resistor and replace it with a strap.

Later (or refurbished) 746s were fitted with the higher impedance ringers and do not need the series resistor, which is only added to make a 1K ringer look like 4K to the line. A 4K ringer should still work with the series resistor, but may not pass enough current to ring properly.

Re: GPO 746 conversion HELP!!???!!
 

I'll have to get hold of a multimeter and give that a go...

This phone is starting to wind me up now:angry:

Re: GPO 746 conversion HELP!!???!!
 

Hi,
As has been suggested the bell coils may already be ones which are 4K, to prove this replace the resistor on T4-T5 with a strap for the time being and see if the bells now ring - no harm will be done if the bell coils are of 1K impedance whilst trying this.
Sometimes the impedance of the bell coils can be seen clearly marked on them, these will be either two 500 Ohm coils or two 2K Ohm coils.

I have seen a few "Hard Wired" GPO/BT telephones of this type which have been through their "clean up" process and have had High impedance bell coils fitted before leaving the factory to be put back in to service.

Are you trying the telephone connected direct to the master socket or to an extension socket on the same circuit?
If using an extension socket it may not be wired correctly; many modern phones only use two wires so will ring even if there is a line reversal or no third "Anti-Tinkle"/Ringing wire connected to the extension socket.

Regards
Andrew

Re: GPO 746 conversion HELP!!???!!
 

Cheers for the help fellas but I'm still drawing a blank.

Bell coils are 2x500 Ohm...

Thanks for the tip Andy, but plugging into the master socket has not made any difference....

I've tried to attach a photo, is the black thing between T1 and T2 the rectifier? Anything look odd?

Re: GPO 746 conversion HELP!!???!!
 

Yes; it's the rectifier (more correctly, two diodes back-to-back to clip off excessive voltage spikes giving rise to acoustic shock).

Out of interest, what happens if you reverse the red and white wires? This will prove whether your line-pair is correct WRT your bell-wire. Another thing to do is to plug your telephone in, take your multimeter on the DC Volts range, and measure between a good, electrical earth (connected to the black multimeter lead) and the following terminals (using the red multimeter lead) of your telephone with the handest on its rest:

• Earth and red: -52.3V (nominal -50V)
• Earth and white: -3.6V (nominal -4V)
• Earth and blue: -3.6V (Nominal -4V)

Report back with your findings!

Re: GPO 746 conversion HELP!!???!!
 

Have alook at post#10 in this thread https://www.vintage-radio.net/forum/...ad.php?t=41547 where I suggest a possible cause..it may not help in your case but it's worth a try given that your wiring appears to be correct.

Re: GPO 746 conversion HELP!!???!!
 

Hi,
There is a possibility that the Bell coils themselves are open circuit, have you checked them with a meter as suggested in post #5? I have had a few Bell coils go open circuit in 746/706 type phones.

Another thing to look out for is the link between the bell coils, is it present and soldered on to the coil tags?

Another test you can make is to remove the Blue lead of the line cord and, leaving all other strapping alone, place a strap between T6 - T7; this would then utilize the phones internal ringing capacitor for ringing - don't be tempted to leave the phone wired in this way if strapping like that solves your problem though (if it does solve the problem you have either a faulty line cord, faulty master socket front plate or a polarity reversal as suggested earlier.)

Regards
Andrew

Re: GPO 746 conversion HELP!!???!!
 

Looking at post #6 and all following posts, I think we can assume that CvanD still does not have a multimeter at his disposal.

Until such a time as the situation changes, it doesn't appear much progress can be made. Perhaps, however, he's taken a meter ohm with him this evening :D.

Re: GPO 746 conversion HELP!!???!!
 

When measuring between T4 and T16 I recorded 3.34K with resistor and 66.6 Ohm when strapped....does this mean I need the resistor or not?


Cheers

CVD

P.S. Doesn't ring either way

Re: GPO 746 conversion HELP!!???!!
 

Make sure that you're measuring resistance between terminals 4 and 16 with nothing at all between terminals 4 and 5. What do you get then?

Does your bell-hammer move? Are your magnet pole-pieces fouled by iron filings or dust? Are your wires connected as shown (green not included, but stick it on T15, out of the way). Have you measured the voltages to earth yet, as I suggested earlier?

Re: GPO 746 conversion HELP!!???!!
 

Oh, and another thing...

Make sure your plug and lead checks out as per the attached pic.

Re: GPO 746 conversion HELP!!???!!
 

Hi,
If you still get these measurements with the T4 - T5 strapping removed, as suggested by Russell, this would indicate that the Bell coils are not open circuit but one (or both) of the coils may have developed shorting turns, which is another possibility.

To narrow the fault down further you could try measuring the individual coils direct at their appropriate tags (once again with strap T4 - T5 removed). The readings obtained at each individual coil should be in the region of 500 Ohms.

Either way with readings you mentioned above it may be time to think about obtaining a substitute/replacement set of bell coils.

Regards
Andrew

Re: GPO 746 conversion HELP!!???!!
 

I've learnt a lot from this post, I've spent some time looking at an N-diagram whilst reading everyones advice. Its cleared up a few things for me.

Andrew,

Lets suppose the problem was with CletusVanDamme's master socket (faulty capacitor) or with the line cord and therefore using the phones onboard capacitor resolved the issue. Lets also suppose that all the other phones in his house are relatively modern and therefore even if the master sockets capacitor is bad it doesnt matter as they dont need its capacitor for ringing.

You seem to suggest even if this works its not a long term solution: "...don't be tempted to leave the phone wired in this way". Taking into account my theoretical setup, is there the potential to cause damage if the onboard capacitor is used long-term?

Ambassador

Re: GPO 746 conversion HELP!!???!!
 

Don't see why. You'd effectively be placing a series C - L - R bell-circuit (formed by the original telephone configuration of 1u8 capacitor in series with bell-coils) in parallel with the series circuit formed by the 1u8 capacitor in the master LJU and the bells of any conventionally-wired telephones. Neglecting the 470K resistor in the master LJU, of course.

There may be a bit of spurious 'tinging' caused by increased capacity but I wouldn't think any damage would occur. ADSL filters are effectively little master LJUs and all have their own capacitor and bell supply within. Putting several of those on one's extension sockets seems to work OK (apart from the lack of third-wire continuity back the way!)

If, as you say, the master LJU capacitor is 'bad' (very unlikely, I would have thought) and so the third wire (term. 3 anti-tinkle / bell supply) is not necessary on the instruments you have installed on your system (ie: those which only have two wires and DTMF tone-dialling) there oughtn't be a problem, as all you've done is shift the master LJU capacitor into your telephone.

Re: GPO 746 conversion HELP!!???!!
 

I had this problem on 2 phones . there was a link missing not described in the 'how to' articles. maybe because of the internet filter - This is what I did.

refer to the picture in post 8. The connection with the orange & red wire is 8 (linked with 9)

Make sure the cradle switches are down as if the handset is in place. Ring the phone and short with a metal object between 7 and 6. The phone should start ringing.

If so please report- then one of the more experienced guys may be able to explain why

Re: GPO 746 conversion HELP!!???!!
 

PWH:

The phone's ringing capacitor (C1 and possibly C2 in parallel if fitted) is connected between T7 and T9.

With the modern plug-in system, it becomes redundant and is replaced by one in the master socket or ADSL filter, which is why the strap between T8 and T9 is removed when a phone is "converted", as is the one between T6 and T7.

If you needed to leave this/these link(s) in situ, there is possibly something wrong with the wiring in your house, or the blue wire in the line cord ;)

Nick.