Hello,
Can anyone tell me the correct polarity of the white and red wires connected to a UK 746 telephone please? I'll go into more detail of why I'm asking later.
Many thanks,
Barry.
I'm not entirely sure what you're asking exactly here. I can tell you the red one should go to T8 in the phone and the white one to T18 although I don't think it really matters too much under normal circumstances.
If you're asking which way the round they go in relation to the DC speech circuit then I have no idea other than a guess that the red one carries +v.
Assuming you're using plug & socket then it also depends which way round the line connections have been wired in there.
If you need me to have a look inside a 746 just let me know.
Andy.
Hello Andy,
Here's what I've found here: The white (T18) is negative and the red (T8) is positive. If I swap those 2 wires then when you speak or blow into the transmitter, you no longer hear yourself in the receiver.
All the phones I've checked including modern ones send some of the transmitter to the receiver, otherwise it feels like the phone is not connected.
There is a reason for all this, honest! I just don't want to get over-complicated explaining why straight away or it'll get confusing.
I'll keep searching online to see if I can find any more on this.
Thanks for your help,
Barry.
On rotary phones made in North America, polarity doesn't matter. On GPO phones, I have no idea. For all I know, one polarity could cause a problem with bell tinkle whereas the other would not. It could be a matter of using the polarity that works best for you.
Quote from: royalbox on January 09, 2017, 04:32:08 PM
Hello Andy,
Here's what I've found here: The white (T18) is negative and the red (T8) is positive. If I swap those 2 wires then when you speak or blow into the transmitter, you no longer hear yourself in the receiver.
All the phones I've checked including modern ones send some of the transmitter to the receiver, otherwise it feels like the phone is not connected.
There is a reason for all this, honest! I just don't want to get over-complicated explaining why straight away or it'll get confusing.
I'll keep searching online to see if I can find any more on this.
Thanks for your help,
Barry.
Does the phone have a diode (=rectifier) installed ?
I recall that this was often practiced.
Quote from: royalbox on January 09, 2017, 04:32:08 PM
I just don't want to get over-complicated explaining why straight away or it'll get confusing.
Too late.
Generally some background is needed or everyone who reads the question will have a different understanding of it.
T18 is the A line.
T8 is the B line
A = Ground = 0 volts
B = Battery = -50 volts nominal
Reversing those leads will have no effect on any telephone I can think of except:
- Older DTMF signalling telephones. If the supply is reversed, the dial won't work.
- DC signalling telephones. If the supply is reversed, the dial won't work.
- Phones that use a reversal such as payphones
A standard (as supplied) 746 is not polarity sensitive. If yours is, you have a rectifier in circuit that shouldn't be there.
If there is a good reason that your 746 is polarity sensitive, I'll think of it after I press "Post".
Regards
Jack
Quote from: unbeldi on January 09, 2017, 06:11:37 PM
Does the phone have a diode (=rectifier) installed ?
I recall that this was often practiced.
What was it for Unbeldi?
Jack
Quote from: unbeldi on January 09, 2017, 06:11:37 PMI recall that this was often practiced.
I believe you're thinking of the Rectifier 205A, which wasn't related to the line polarity... :)
The Rectifier No. 205A was placed across the receiver connections inside the phone to reduce the loud pop produced when pressing the hookswitch for a follow on call to reduce the "Acoustic shock" issue that can occur (and acoustic shock is not a nice thing, having done it to myself a few times!) with the user still holding the handset to their ear. It's just two parallel diodes wired in opposite polarity to each other, and can be made using cheaply available Schottky diodes (such as the 1n400x types)... :)
Quote from: twocvbloke on January 09, 2017, 06:40:46 PM
I believe you're thinking of the Rectifier 205A, which wasn't related to the line polarity... :)
The Rectifier No. 205A was placed across the receiver connections inside the phone to reduce the loud pop produced when pressing the hookswitch for a follow on call to reduce the "Acoustic shock" issue that can occur (and acoustic shock is not a nice thing, having done it to myself a few times!) with the user still holding the handset to their ear. It's just two parallel diodes wired in opposite polarity to each other, and can be made using cheaply available Schottky diodes (such as the 1n400x types)... :)
But that doesn't affect the polarity of the phone.
Jack
Quote from: Jack Ryan on January 09, 2017, 06:42:41 PM
But that doesn't affect the polarity of the phone.
Jack
Hence why I said that it didn't affect line polarity, in the first line....... ;D
There is also the regulator in there, which is a series of diodes wired in a full wave rectifier manner, but, again, this doesn't affect the polarity (in essence it actually makes it unaffected!!)... :)
Quote from: twocvbloke on January 09, 2017, 07:23:12 PM
Hence why I said that it didn't affect line polarity, in the first line....... ;D
There is also the regulator in there, which is a series of diodes wired in a full wave rectifier manner, but, again, this doesn't affect the polarity (in essence it actually makes it unaffected!!)... :)
Sorry, I thought you said "unrelated" which leaves open the possibility of unwanted side effects. Point taken
Perhaps the diode(s) that Unbeldi was thinking of were in the 312 (and equivalent 700) which *is* polarity sensitive. That won't work properly on a normal phone line though and the diodes didn't affect side tone.
Regards
Jack
Thanks for the replies everyone.
Here's the full details I was trying hard not to post.
I've build a pulse to DTMF converter from an open source project (https://bitbucket.org/310weber/rotary_dial/overview). The device is very similar to rotatone (http://www.rotatone.co.uk/).
The wiring is the same and so I followed the advice on the rotatone site (http://www.rotatone.co.uk/fitting-a-rotatone-to-a-gpo-706-746-8746/) for fitting to a 746 and added a bridge rectifier polarity guard as they describe. The converter worked but I noticed I wasn't hearing myself in the receiver like all the phones I have tried including the converted one before converting.
After a lot of testing I found that it was due to the polarity guard. It sent positive to T18 and ground to T8. I re-wired the phone to normal and I could hear my voice in the receiver once more. I then swapped the T18 and T8 wires so the polarity would be the same as if the rectifier was installed, and again I could no longer hear my voice in the receiver. Hence why I wanted to know if the polarity was important (on the phone, not the converter) as I had read somewhere that it wasn't.
It seems strange that either rotatone got it wrong, or if the polarity doesn't matter, strange that I'm getting this problem when reversing it. Hence why I'm asking.
Quote from: Jack Ryan on January 09, 2017, 06:21:49 PM
T18 is the A line.
T8 is the B line
A = Ground = 0 volts
B = Battery = -50 volts nominal
With the phone wired correctly (white to T18, red to T8) and with the negative lead of my multi-meter on T18 and positive on T8, I read just under 49VDC. (not minus 49).
With the polarity guard fitted as per the rotatone guide, I get -48VDC (minus).
So adding the rectifier as per the rotatone guide reverses the polarity of my phone and has the effect of not sending any signal from the transmitter to the receiver.
That's what happens here anyway.
Thanks all,
Barry.
Well, there is a wiring problem somewhere but I'm not very good with the tab A slot B type instructions that the WEB site gives - I need a circuit diagram.
There is nothing in the phone that is sensitive to polarity so inserting the polarity guard and other associated wiring has just broken the phone.
As a matter of curiosity, why isn't the bridge rectifier and the zener part of the pulse to tone convertor? It would simplify installation.
Sorry, not very helpful I know.
Jack
Hi Barry,
I just did a bit of testing here. I got a phone (basically a 706) with red to T8, white to T18 and blue to T6. This is a normal conversion for plug & socket working. I blew down the handset and could definitely hear myself in the receiver.
Next I swapped the red and white line cord terminals around and repeated the test. I could still hear myself as expected.
I'd say take all your modified wiring off and just test the phone like this for a start. It's not beyond the realms of possibility that you might have a fault in the original phone. It could be a crack on the circuit board or even some metallic debris of some kind (often the nut off the back of the base securing screw on a 746) that's lodged itself between the board and the base and is shorting something it shouldn't be.
Another thing that might be worth doing is chacking how your sockets are wired. When you look at a standard plug you'll see it has teh capacity for 6 contacts but most likely only the middle 4 are used. Of these middle 4 the outer 2 should be the red and white line. As you look at the pins of the plug from above with the gold contacts at the top the left one should be white, next green (generally unused at the phone end), next the blue 'ring' wire and finally the red side of the line. If you trace this back through the SOCKET it's not beyond the realms of possibility that someone might have connected red and white up backwards either on the back of the socket or even done by BT on the back of your master socket where the drop cable comes into the house.
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. However, having said that, for less than the cost of either option you can pick up a cheap PBX (mione was £10) that will convert pulse to tone for dialling out, which means LOTS of phones, and inter-phone dialling! :)
Of course if you're doing the converter for fun then carry on. I just didn't want you to be doing something you thought you needed when there are better ways to connect up old telephones.
Andy.
Thanks for all the replies and help, much appreciated.
Now that I know polarity shouldn't affect it I'll check all the wiring as you suggest Andy and also test with another phone. Tomorrow hopefully.
QuoteAs a matter of curiosity, why isn't the bridge rectifier and the zener part of the pulse to tone convertor? It would simplify installation.
Actually, I used a jump wire where the diode is shown. Rotatone say use the diode if you don't have an electronic transmitter as it will guarantee enough voltage to run the converter. I should have mentioned that. Also, there is a 5.1V zener used to regulate the voltage in the circuit of the converter project.
I agree though, I think it would make more sense to have the rectifier as part of the circuit. I probably will on the next one then.
Here's what rotatone say about it:
QuoteWhen Rotatone was being developed, a decision was made not to incorporate a polarity correction circuit within Rotatone module. This decision was made for a number of reasons, the chief of which is that the polarity protection was best applied to the entire phone, and not just to the Rotatone module.
Doesn't say what other reasons.
Here's something else the rotatone instructions says:
QuoteVoltages up to 35 volts are acceptable, as the module is fitted with its own voltage regulator. Voltages however must not exceed 40 volts, so do not connect the unit on the exchange side of the hook switch where it would be exposed to ringing voltages (it would cause the line to go 'off hook' but this can't be
relied on to protect the circuit). For additional surge protection, a 24v Zener diode can be installed across the line after the switchook as shown in the generic diagrams below.
Do you think it would be a good idea to add a voltage regulator like rotatone has instead of the 5.1V zener? I have a couple of 7805s here already.
Andy, PBX is something I know nothing about although I've heard it mentioned here and there. Will look it up.
I built the project as I like doing things like that and had most of the components in my bits box. Also, I have never flashed a micro-controller before and was interested in trying that. I was quite impressed that it all worked! Also wanted the 7 quick-dial numbers and ability to dial * and #.
I've posted an image of the circuit diagram of the converter in case anyone's interested in looking it over.
Thanks again for all your help,
Barry.
Quote from: andy1702 on January 10, 2017, 10:07:12 AM
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
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.
QuoteAre 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.
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
Quote from: royalbox on January 10, 2017, 06:49:43 PM
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
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 (http://www.britishtelephones.com/trans21.htm) 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.
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:
QuoteThere 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...
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
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.
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...
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.
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.
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 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.
Quote from: twocvbloke on January 11, 2017, 05:28:12 PM
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
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!!
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.
Received the replacement diodes - 1N5344B (8.2V 5W). Paid £1.69 for 5 including postage. All working fine both ways (so far!), so thanks everyone for your help.
Barry.
Sounds like it's all sorted then, makes me wonder why those original diodes conked out, but I guess it's either age or a voltage spike in the line or something, Schottky diodes are usually pretty tough to kill...
I finally found a Transmitter Inset No 21A.
It contains a two transistor amplifier powered from a bridge rectifier so that either polarity works.
Two of the diodes in the bridge are zener diodes; these act as normal diodes in the bridge but the voltage across the amplifier is clamped at, in this case, 16.4 volts (two 8.2 volt zeners). The current when clamped is limited at the maximum loop supply current but would normally be less because of the loop resistance plus the IC resistance.
Jack