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Old 3rd Oct 2020, 4:02 pm   #1
s900t8v
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Default Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

Hi guys

I just picked up a nice what I think is a N2500 here in Australia, it has an oak cabinet, it has a 4 magnet generator/magneto.

its for my mum shes always wanted one to go in her circa 1913 house and this fits the era by the research i've done since I grabbed it today.

I am lead to believe these units should ring when wound when speaker is off the hook and obviously also when recieving when they are working properly. This doesnt do anything so I decided to do a few tests

Well I'm only getting 5v ac output from the generator which seems fairly low, I measured the windings resistance and im only getting 150ohms and I can see some corrosion on the armature winding. I removed the magneto and cleaned the armature bars as I thought some corrosion may have been causing it to bind, turns out it was dirt in the housing.

What resistance should the windings be and what should the voltage output be?

thanks a lot

I am wondering whether I should get it rewound, I'm really only keen for it to ring the bell when wound as a bit of a nostalgic novelty for my mum
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Old 3rd Oct 2020, 6:55 pm   #2
Pellseinydd
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

Looking at the circuit diagram for the Tele 135 MW (PMG Drawing C1464 Sht 2), when the magneto is turned contacts on the end of the magneto short out the bell such that just the magneto is straight across the line. The same contact set shorted out the magneto such that just the ell was across the line when the receiver was on its rest. It was usual to turn the handle to call and also 'to ring off' with the earpiece in situ rather than being picked up. Otherwise the AC generated also went 'half' into the telephone and the rest down the line to the exchange.

Are you measuring the voltage with receiver on the rest?

You could try by disconnecting the black red wire that runs from line terminal 'B' to the switch contact on the end of the generator. Other alternatively stop the contacts operating on the end of the generator by insulating with pieces of card/paper between both the make and break sides of the 'change-over' .

Then connect Line Terminal A to Line Terminal B - then when you wind the generator, the bell should ring. It may even ring without the short if it is done with the receiver off.

Ian
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Old 3rd Oct 2020, 7:06 pm   #3
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

Forgot to mention that the 4 bar magnet should be 1000 ohms

But due to its age, may not be the original generator? Any signs of rewiring/soldering?
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Old 4th Oct 2020, 12:54 am   #4
s900t8v
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Default

Thanks a lot for your messages. I’ll upload some photos. The magneto looks original. I had it disconnected from the rest of the circuit and the most I can get is about 5VAC the armature windings are corroded and I guess given I only have 50-150ohms. I’ve tried shorting the line and isolating the armature contact process to get it to ring and have tried on and off the hook. . There is plenty of resistance in the magnets so I don’t think that’s the issue and the armature bars are definitely not contacting the housing and shorting that way. I think it needs a rewind.

Any idea what voltage it needs or should have in normal operation? Surely 50-100VAC ?

The earpiece speaker is not wired in, does it need to be for the bell to work? I didn’t think so that’s why I didn’t bother with that.

I’ll call around and see if anyone can give me an idea what a rewind would cost. Mum Is pretty keen for the novelty aspect.

Also what’s the purpose of the induction coil?
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Old 4th Oct 2020, 10:50 am   #5
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

Just had another thought, are you sure when you say 1000 ohms you are not referring to the ringer, the ringer has 2 500 ohm coils.

The last ericcson guide I looked at had a spec for 3 and 5 magnet units and all the units (4 or 5 were listed) had a 500 ohm spec.

I suppose if I take the armature to a rewinder they should just be able to match the number of turns and the gauge and I should end up in the right ballpark I would imagine .

Can these things kill you if you contact them? 110 is probably not enough right?

I live in the world of 240v in australia where any shock can kill you
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Old 4th Oct 2020, 9:29 pm   #6
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

Wiring diagram: https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAD
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Old 4th Oct 2020, 9:38 pm   #7
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

About all magneto telephones should give at least 50V when cranked at a normal speed (2-3 windings pr second) Usually a lot more than 50V maybe even close to 100V

Some telephones are made with a switch engaged when cranking to disconnect or short out the ringer. shorting the line on the phone and disconnecting this function of disconnecting the ringer should result in a clear ringing.

dsk
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Old 6th Oct 2020, 7:29 am   #8
s900t8v
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Default Re: Ericcson N2500 Aus model 35MW/135MW I think - magneto fault question

I'm almost tempted to try and rewind the armature myself.

Has anyone had any experience with rewinding these AC generator armatures?
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