N717CG questions from a new member

[jhutzler]

Hi Folks,

Just joined and wanted to say hello. I'm an electronics teacher in a high school here on Vancouver Island that owns an old heritage home and I recently bought a wooden phone from Quebec and hooked it into our Shaw Cable digital phone service.  The phone works... sort of. It only works when one of our wireless phones answers the call first. Then you can use the old phone to talk to and hear the caller. You cannot complete the call without hanging up the wireless phone. Answering a call with the old phone means you can continue to hear ringing on the line then a voice says "please hang up and try your call again".

The old phone guy says I should use an old phone equalizer to limit current draw on my line but a previous post about an n717ah suggests a slightly different wiring schematic.

Old phone guy says use a 2.2 mF cap and a 250 ohm 5 W resistor in parallel and hooked into the line in series with the old phone while DSK and G-Man in this post http://www.classicrotaryphones.com/forum/index.php?topic=8543.15 suggested 2.2 uF from line 1 into Grd then out.

Could use some advice on this one.

Thanks,  Jeff

[unbeldi]

The N717CG is a local battery telephone, which means that the transmitter is powered by a battery connected locally into the transmitter circuit. You cannot normally operate such a phone on a common battery telephone line unless it is properly converted.  I don't know if anyone has looked into doing that for these.

Were you told it should work? Has the previous owner worked on it?

If you have two phones off-hook on the same line, then of course both must hang up to free the line.

I think you first need to determine the state of the phone, examine the wiring and record it as a diagram.

With respect to the capacitors, you have to watch out for units.  2.2 mF  is 2200 µF. mF is millifarad, whereas µF is microfarad. A 2200 µF capacitor is unreasonable for telephony, so this was just sloppyness and µF was meant, but it is important when you order the part.  When people write 2.2 uF, they mean µF, because of the similar shape of the character and they don't know how to type a mu.  Before the metric system was adopted, the common symbol for microfarad was M.F. as is printed on most old telephony capacitors, but this usage should be avoided today.  Today, MF is megafarad, one million farad.

The correct size of components would depend on just what the actual impedance is of the ringer in your set.

[unbeldi]

From your picture it appears you have a 104A induction coil and a B-type ringer.  Hopefully it is a high-impedance ringer, B1 equivalent with long clapper, which should only require a 0.5 µF capacitor in series, bridged across the two line wires.

[jhutzler]

Thanks Unbeldi... that was fast!

I agree with what you said about units and getting them right. The attached pics should clear up any misunderstandings. As for the old phone... I've powered the transmitter with an old cell phone charger and it works great!  http://www.islandregister.com/phones/telephone1.html

Its just that the phone isn't being recognized by my voip-like service as being off-hook and answering the call.

The confusion around mF and uF isn't about me misunderstanding capacitance its just that I don't know what advice is better. The circuit from DSK or the one from Colin.

[dsk]

Hi and welcome.

I do not have the diagram for your phone, but its important to know:
To high ringer current may cause problems, and this circuit needs a non-polarized capacitor to block for DC. The right capacitor may have voltage of at least 200, and a capacity to let enough current pass to run the ringer motor, but still not to much. It is no realy right value here, but something between .47 and 2.2 uF will be right.  (I have measured some ringer loads in this thread: http://www.classicrotaryphones.com/forum/index.php?topic=219.msg91056#msg91056    )

BUT!
When you go off hook it is a DC current who signals that, and the ideal current are near 25 mA.
but it will usually work from 18 to 50 mA.  Nothing here are polarity sensitive, but the receiver element will usually work best if it is isolated from the dc. (I would not kare to much abaout this in the first hand) The last thing are, backfeeding of current may not be OK, so the hand cranked generator should not be used!


dsk

[unbeldi]

If you have a B-type ringer there really should not be much reason to add extra components.  The B1 has 4600 Ω DC resistance the combination with a 0.5 µF capacitor provides a REN value of about 1.3.

I would carefully check the circuit to make sure the switches are all wired correctly.  On a common battery line, there should really be another capacitor to protect the receiver element from DC.

[unbeldi]

Here is the wiring diagram for a N1717CG telephone, which I think is the same as a N717CG.

Your set should indeed have a B1N ringer with a DC resistance of 4600 Ω. With a 0.5 µF capacitor it should ring just fine, like a 302.I don't see a suitable capacitor, so you have to add one.   There is a spare capacitor on the C terminal shorted, but that is too small and normally used for loop compensation, if needed.[/s]

Correction:  That is a 195A condenser package of two capacitors, one of them is a 0.5-0.63 µF condenser, unused, with lead colors slate and yellow. You can use that for ringing.


The N717AP on the other hand has a low impedance ringer.

The other item that is different is that yours has a N1P key which complicates the wiring quite a bit.

[jhutzler]

Hey Guys...

Took a close look at the ringer .... B1N   2-28-52 and the coil looks like 104A.

The DC current issue sounds like it may be the thing. I am ignorant about what the Shaw cable Digital phone service is looking for in order to detect an off hook signal and thats been part of my problem. I will try the cap across the lines first and let you know how it goes.

Another point... the button below the handset (used to signal the operator when you needed to place a call after you crank up some AC from the magneto that I will disconnect) ... when I press this button for a moment I get dial tone then silence. I should have said from the start that this old phone works when a call is connected but there is no dial tone. Could the N1P switch be this button? .... and could the grounding of one of the lines when I push it be giving me dial tone for a second or two?

Cheers... Jeff

[AE_Collector]

I do not have the diagram for your phone, but its important to know:

dsk

Hi Jeff:

Also important to know s that dsk is in Norway! Isn't the internet great!

For modern PSTN telephone networks we would have up to 80ma or more of current flow on short loops and at the other end of the scale we wanted at least 26ma of current flow but would settle for as low as 23ma if nothing else could be done. This was the BCTel requirements at least.

Where are you on "the island"?

Terry in Coquitlam.

[jhutzler]

Well Hey there Terry,

I'm in Nanaimo. 80 mA... thats a lot of current. 

Do you know why the button (N1P?) makes a brief dial tone for a second then nada... the handset is dead as usual unless someone answers a call on the wireless phone first... then the old phone works great?

Cheers,

Jeff

[AE_Collector]

This type of phone is a Magneto Phone just like the big old wooden wall phones that pre dated it. This little one is a much more modern design from the 40's and 50's. It is a smaller version of the big wooden magneto phones though it has a "modern" handset rather than a seperate transmitter and receiver and it has a smaller Magneto than the big phones limiting it to locations reasonably close to the exchange. This one would have its batteries in a separate box maybe down in the basement instead of in the bottom box of the big old phones.

I am not certain but suspect that in this case the key is used in conjunction with signalling (turning the handle on the magneto) only the other phones on your line without contacting the operator OR signalling the operator to answer in order to make a call to anyone else who is not on your line. It would do this by grounding one side of the line while pushing the button as you turn the handle. Im not certain if you pushed the button to signal the operator or to NOT signal the operator.

I have never attempted to get a Magneto phone to work properly on a modern phone line. In many ways it would be easier to get a similar phone that is a "common battery" model rather than a "local battery" model. Northern Electric made these little maple phones as either local or common battery and also with a "one piece" handset or a two piece (separate transmitter or receiver) and common battery models were available with or without dials.

But, I'm sure you like a challenge! They just werent really made to be used on the modern phone network.

Terry

[unbeldi]

Here is a circuit diagram that I just drew up to understand how this phone works.

In many ways it's like a standard anti-sidetone local battery set, such as the WE 400 series.
The key is for signaling the exchange to request operator attention, when pushed it closes the loop completely, while the hand generator is grounded as long as it is pushed.  So, the hand generator is only intended to be used when the user wants to call another party on the line, while the operator is signaled by shorting the line.  This means it is actually intended for common battery signaling, but without a dial.  For this reason there is already a 1 µF capacitor in the circuit that protects the receiver.  At the same time this capacitor performs the function to block the DC from the ringer.

The only thing I don't like here, is that the hand generator is permanently connected across the line when the button is not pressed.   This may interfere with modern operation.  I would disconnect the generator. There is no use for it anyways on a modern line.
Correction:  I think the 41BN hand generator has a built-in switch which connects it to the line only when in use.

Since you already have a power source for the transmitter, the set should work just fine when the generator is disconnected.

The circuit is very similar to the Western Electric 400 K subscriber set, which I wrote about in this post:
http://www.classicrotaryphones.com/forum/index.php?topic=12301.msg129832#msg129832

The phone is even more similar to a Western Electric 307 desktop telephone, which I talked about here:
http://www.classicrotaryphones.com/forum/index.php?topic=10281.msg109274#msg109274
The 307 doesn't have a magneto of course, but it does operate on common battery signaling lines and has a dial.

The audio circuits work identically, essentially, in all these sets. They are based on the same induction coil.  The anti-sidetone balancing network is designed slightly differently in each, but that's not critical. There are many ways to do that.  The 400K does not work on common battery local loops; it has no DC protection for the receiver, while the 307 does.

So, looking at this diagram now, I have to say that it should function to at least answer an incoming call.

[unbeldi]

This type of phone is a Magneto Phone just like the big old wooden wall phones that pre dated it. This little one is a much more modern design from the 40's and 50's. It is a smaller version of the big wooden magneto phones though it has a "modern" handset rather than a seperate transmitter and receiver and it has a smaller Magneto than the big phones limiting it to locations reasonably close to the exchange. This one would have its batteries in a separate box maybe down in the basement instead of in the bottom box of the big old phones.

I am not certain but suspect that in this case the key is used in conjunction with signalling (turning the handle on the magneto) only the other phones on your line without contacting the operator OR signalling the operator to answer in order to make a call to anyone else who is not on your line. It would do this by grounding one side of the line while pushing the button as you turn the handle. Im not certain if you pushed the button to signal the operator or to NOT signal the operator.

I have never attempted to get a Magneto phone towork properly on a modern phone line. In many ways it would be easier to get a similar phone that is a "common battery" model rather than a "local battery" model. Northern Electric made these little maple phones as either local or common battery and also with a "one piece" handset or a two piece (separate transmitter or receiver) and common battery models were available with or without dials.

But, I'm sure you like a challenge! They just werent really made to be used on the modern phone network.

Terry

In general, I would agree with you Terry, about making magneto sets work on common battery systems. It's not worth the effort in general and the benefit is extremely dubious, since the old magneto phones would require grand modifications and one can more easily use other phones.  But I knew from some NE subsets with 104A induction coils that it is not so hard for the more modern ones as this.  That's why I took the time to prepare the circuit diagram. It just may be useful in the future.
 

[AE_Collector]

But I knew from some NE subsets with 104A induction coils that it is not so hard for the more modern ones as this.  That's why I took the time to prepare the circuit diagram. It just may be useful in the future.

Yes, good job with the wiring plan. It makes sense that some of these last Magneto phones made would be very flexible in their capability to solve "one off" problems.

This one then would work into a Common Battery Exchange, possibly in an exchange that was about to be converted to Common Battery. It would allow the Telco to get the Exchange conversion done while putting off having to solve situations on a couple if long lines out of town where Magneto was still the best way to keep the service working.

Terry

[dsk]

After a good night sleep, I'll ask you to do a test.  1 disconnect the ringer, 2 strap c1 and c2. 3 try if you get a dial tone when going off hook.  If yes, try to call in, and answer when you hear the other phones are ringing.
4 Tell us how it turned out.

dsk