UK GPO 746 line polarity confusion

[Jack Ryan]

Personaly I'm not a fan of the Rotatone. If you must have outgoing tone then the Dialgizmo is better because it's in a box which you plug in between your outside line and your phones. So you can have lots of extensions with lots of phones but just one box.

The internally mounted convertors sometimes attempt to use the dial ON contacts to mute not only the dial pulses, but the DTMF burst that follows it.

I find that the Dialgizmo works but I hear the DTMF burst as the dial returns. The advantage of the externally mounted convertors is, as you say, to allow the use of multiple phones per convertor.

While I am at it, the trouble with using small PABXs like the Panasonic is that you have no control over the numbering scheme, progress tones or ring cadence.

As to the problem at hand, since you can remove the additions and the phone works, there must be a wiring error in the conversion. I do not know if that is in the instructions or in their implementation.

A questions:
1. Once converted, are the dial pulses transmitted to the exchange as well as the DTMF?
2. Are there any (equivalent) circuits of the convertor and converted telephone?

Regards

Jack

[royalbox]

Hello Jack,

Currently it is working fine but I'm not using the rectifier, I just made sure I wired it correctly using my meter to test polarity. Not ideal though if it's ever plugged in somewhere where the wiring is reversed.

With this project, as with rotatone, the dial is completely removed from the phone circuit. The pulse and one of the shunt contacts is connected directly to the converter, therefore there is no muting of either mic or receiver. In fact, if dialling *, # or a quick dial number, you hold the dial at the finger stop for 2 seconds until it beeps so you shouldn't mute the receiver. I don't mind the tone when the dial returns, though it is a bit loud compared with the dial tone. It uses a transistor to amplify the tone coming out of the micro-controller as can be seen in the circuit. Maybe that can be attenuated, perhaps by changing one of the resistor values but I'm not sure, bit of an amateur.

Are there any (equivalent) circuits of the convertor and converted telephone?

Sorry, I don't follow what you mean here.

Thanks for your help. I'm off to bed now, will look in tomorrow.
Barry.

[Jack Ryan]

Does the convertor work if you just connect power and a dial? An earphone, CRO or telephone to detect the tones

If it does, connect it to the phone, across the line with the polarity correct, after the hook switch. Does it work?

If it does, sort out the polarity convertor.

By the way, the zener in the phone serves the same purpose as the one in your converter.

Jack

[Jack Ryan]

Hello Jack,

Currently it is working fine but I'm not using the rectifier, I just made sure I wired it correctly using my meter to test polarity. Not ideal though if it's ever plugged in somewhere where the wiring is reversed.

With this project, as with rotatone, the dial is completely removed from the phone circuit. The pulse and one of the shunt contacts is connected directly to the converter, therefore there is no muting of either mic or receiver. In fact, if dialling *, # or a quick dial number, you hold the dial at the finger stop for 2 seconds until it beeps so you shouldn't mute the receiver. I don't mind the tone when the dial returns, though it is a bit loud compared with the dial tone. It uses a transistor to amplify the tone coming out of the micro-controller as can be seen in the circuit. Maybe that can be attenuated, perhaps by changing one of the resistor values but I'm not sure, bit of an amateur.
Sorry, I don't follow what you mean here.

Thanks for your help. I'm off to bed now, will look in tomorrow.
Barry.

Sorry, I crossed posts.

So the convertor works and the phone works, the bridge rectifier is not right.

A circuit of the telephone with the convertor and bridge rectifier installed would help. The circuit of the convertor you posted answered a few questions.

Jack

[royalbox]

After some testing with 2 phones I've found the source of the problem. It is the transmitter in that particular phone. It only works in one polarity.

I had a read about the transmitters in these phones and see that they have a bridge rectifier in them also. My electronics knowledge is very basic but could it be that one of the diodes is faulty in the transmitter causing it to only work in one polarity? Anyway, I don't think it can be opened without breaking the case so probably best to leave it as it is.

Thanks all for helping me track down the problem anyway.
Barry.

[twocvbloke]

You can pop the top off a 21A case quite easily, they're not glued on (to make it easier to repair the electronics when necessary), I had to do that a few years back to properly (kind of!) shield the first revision of GEC's 21A (which it lacked on one side, introducing a lot of annoying mains hum, and mine came in the wrong colour case):

http://www.classicrotaryphones.com/forum/index.php?topic=7254.msg81372

Reading the link to the 21A article though, the mention of a problem (in Bold) with early A.P. Besson sounds familiar:

There were problems with early production units made by A.P. Besson, these resulted in terminal posts coming loose from the circuit board and transmission cut-off on line reversal.

So it may be that you have an early transmitter with that fault, though I haven't any idea how to resolve it, as my A.P. Besson 21A  is "somewhere" and is a later version without the polarity issue...

[twocvbloke]

Given it's the transmitter at fault due to an inherent flaw, might just be easier to get another, such as this one made by Rathdown Industries (seller also has an A.P. Besson one too for a quid less, but an unknown revision):

http://www.ebay.co.uk/itm/391672715527

[royalbox]

Thanks for the info. Mine looks like the Rathdown Industries one with the all black top and red case.

I may as well have a look inside. How did you pop it open, I'm unable to do it with my fingers alone so far. Don't want to break it, it works in one polarity.
Barry.

[twocvbloke]

I popped mine open by hand, they're quite tight though, so do take some force to open, but they do open, kind of like a Pringles tube, but a lot tighter, though that said I haven't tried an RI one (I don't have one, either the Red & Black or later Blue & Black), I don't know if those ones are glued or not...

[royalbox]

Okay, done it. My guess is that it will be one of the four diodes but I'll probably be really suprised and find they're all okay!
Thanks again.

[royalbox]

One of the diodes (top left in picture above) reads 0.023V in both directions (tested out of circuit also).
It is marked BZY 97C 8V2. Found a datasheet, says it is a zener diode, BZY97C3.3-BZY97C240 (3.3-240V), 1.5 Watts. So I assume this is 8.2V, please correct me if I'm wrong as I'm an amateur at this kind of thing. I don't have one like it, initial eBay search shows they are too expensive to be worth it.

I assume (again) that a substitute 8.2V zenor of 1.5W or higher should work. Please tell me if I'm wrong!
Thanks all,

Barry.

[twocvbloke]

Yes, you can substitute that one with another zener diode of similar value, heck, you could probably use a 1n4001 diode in there (but replace both BZY diodes at the same time!) as those are up-to 50v and can handle up to an amp...

[Jack Ryan]

I didn't think of that sort of problem. I don't normally collect telephones that are so new as to use an electronic transmitter - Id better start.

Yes, you can substitute that one with another zener diode of similar value, heck, you could probably use a 1n4001 diode in there (but replace both BZY diodes at the same time!) as those are up-to 50v and can handle up to an amp...

I'm not familiar with the transmitter but I would assume that the diodes are part of a bridge rectifier which perform the same function as the one you are adding - polarity correction. Wired appropriately, you don't need to fix your transmitter.

Why would there be zener diodes in a bridge rectifier and why use zeners if they can be replaced with normal rectifiers?

All very strange.

Jack

[twocvbloke]

I just realised I'd mixed zener up with schottky, been messing about with zeners on a microphone I got from china in 2015 so that kind if got stuck in my head!!

[royalbox]

Thanks chaps. 2 of those diodes are BZY97C, one is a 4001 and the other is not visible but I expect it is also a 4001.

The rotatone documentation says add a 5.1V zenor if there is no electronic transmitter so there is enough voltage for the converter. Perhaps that's what the zenors in the transmitter do, make sure there is enough voltage to run it. I don't know.

There doesn't seem to be any modern zenors on ebay that are 1.5W but lots that say 1.3W. It's tempting but all those listings when you read the spec or datasheet say 1W. I suppose I'll have to go with the 5W even though they are a bit bigger physically. Will replace both I think.

Barry.