Weak Ericofon Receiver

[Spanish_phones]

Hello again,

This time, I just have great news!!!! 

I have just repaired the receiver with a neodymium strong magnet. I sewed a narrow cloth bag, in other to recuperate the magnet if need, and placed it at the back side of the receiver, increasing the volume close to the original one.

I have to say that putting the magnet in place (the exact center of the receiver back) has been much easier with the help of a little camera that I bought.

Ah, thanks for the ericofon.com lick, I already knew that web, its excellent, and they have a lot of wiring diagrams and models information! But the extremely dangerous procedure of disassembling the receiver cap was my last option.


Again, I have to say thanks to all of you, and for your quick answers!

[countryman]

That's a cool repair job!
So the actual problem is a weak permanent magnet in the speaker?

[andy1702]

Has the magnet in the bag remagnetised the speaker magnet, or do you have to leave the new magnet on the back of the original one? The neodinium magnet should remagnetise the speaker I would have thought.

[Spanish_phones]

What I suppose is those original ericsson receivers are not the best quality. What I presume is that the receiver should be light and small, and due to those specs, the final result was "cheap"

I used a neodymium magnet, and I had to leave it permanent at the receiver back. As soon as you start to take the magnet away from the receiver, the volume decreases to the minimun.

I think this technique could save a lot of Ericofons, which unfortunately suffer from this problem more often that we would like to

[Jim Stettler]

I used a neodymium magnet, and I had to leave it permanent at the receiver back. As soon as you start to take the magnet away from the receiver, the volume decreases to a minimun.

This is a very useful observation. Sharing this type of info is the goal of the forum.
Thank You,
Jim

[andy1702]

It's a very interesting discovery. I'm going to check mine now.

[19and41]

I see the housing on the receiver is metal.  Is there any chance that if it is ferrous metal the housing is becoming magnetized and impeding the diphragm's function?  If that were the case, degaussing the housing might also remedy the problem.  A soldering gun can be used to degauss or scramble the magnetic field in an object by passing the side of the tip and it's attachment points over the object a couple or 3 times, while turned on.  I have used this method on magnetized clock movements with good results.

[andy1702]

I see the housing on the receiver is metal.  Is there any chance that if it is ferrous metal the housing is becoming magnetized and impeding the diphragm's function?  If that were the case, degaussing the housing might also remedy the problem.  A soldering gun can be used to degauss or scramble the magnetic field in an object by passing the side of the tip and it's attachment points over the object a couple or 3 times, while turned on.  I have used this method on magnetized clock movements with good results.

The wires look to be soldered on to the speaker, so I wonder if they accidentally degaussed the magnet when they built it? Off the top of my head I can't think of any other phones where the wires are soldered onto the actual speaker.

[19and41]

There must be some sort of insulated passage for the conductors to get through the housing.  Looking at the reddish cast on the housing and reading the performance could be altered with a magnet made think about degaussing it.

[countryman]

depending on the material of the original permanent magnet a slow loss of performance might be considered normal. Taking into account that a new, strong magnet brings back the performance I'd think a loss of magnetic force from ageing is the culprit.
This seems to be a moving iron speaker. (More modern) dynamic speakers don't seem to loose performace in the same way.

[dsk]

  I have probably broken some Cobras when I tried to get them working.

This way of solving it is good news 


I have taken pictures of 2 different receivers.  The black housing has a DC resistance of 120 ohms, and the brown 80 ohms.


dsk

[19and41]

Those appear to be field effect magnets, They ideally shouldn't retain any magnetic force when there is no current applied.  It's looking more like it may be residual magnetism causing the weakness in audio.

[Spanish_phones]

I hope this simple method of putting a strong permanent magnet at the back of the receiver would help to save a lot of Cobras/ericofons.

Correct me if I'm wrong, but I think the receiver housings  are made of plastic.

[countryman]

Those appear to be field effect magnets, They ideally shouldn't retain any magnetic force when there is no current applied.  It's looking more like it may be residual magnetism causing the weakness in audio.

A moving iron speaker contains a permanent magnet. The variable (AC) field of the coils just adds or substracts from the permanent magnetic force, moving the iron diaphragm. The Wikipedia article mentions de-magnetization as a common problem. I read a discussion about re-magnetizing the original magnet by stroking over it with good magnet, obviously with mixed results on old german handsets. So when the new magnet can be added permanently, this would be preferable.

In most telephone circuits/networks the induction coil keeps any DC current away from the receiver element, to prevent premature de-magnetization. On my
french PTT model 1924 I found the opposite: The receiver contains no permanent magnet at all, but is pre-magnetized by a DC current through it. As it's handset is related to American E1 handsets, I wonder which method these use?

Later dynamic receiver systems (moving coil) obviously are not prone to de-magnetization in the same way, maybe because they often use a ferrite magnet.

I hope this simple method of putting a strong permanent magnet at the back of the receiver would help to save a lot of Cobras/ericofons.

It seems you did a great job to help saving a lot of these phones.

[dsk]

I hope this simple method of putting a strong permanent magnet at the back of the receiver would help to save a lot of Cobras/ericofons.

Correct me if I'm wrong, but I think the receiver housings  are made of plastic.

Yes it is plastic! I have tried with an external magnet on the one I pictured, but it did not help on this one 

Still it will be the first I will try before I try to open the glued receiver-cap. 


dsk