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 Post subject: How to fix disintegrated magnet in high imp spkr? GE P913J
PostPosted: Dec Tue 08, 2020 1:22 am 
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On to number two radio in the box of non-working radios. This is a GE P913J that has a high impedance speaker with reed suspended between two poles. According to https://www.esrv.net/1967_ge_p807g.htm, a guy can glue in replacement magnets. Unfortunately, no real description of how this is done is explained. Anyone ever do this successfully? The radio is in pretty good shape and would hate to let it go to the parts pile... :(

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 08, 2020 4:28 am 
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The referenced article said:
"The magnets that were glued on either side of the reed..."
SO, two magnets originally.
"I decided to super glue on some tiny magnets from a magnetic catch in a cell phone pouch.... These tiny magnets are rather powerful."
From that, I suspect they were 2 little rare-earth magnets, such as neodymium ones. Maybe only 1/4 in dia.

I can see the crumbly remains of a magnet in the picture. Attached is an annotated copy that is how I interpret the repair to have been made. Ideally the old crumbly bits would be removed so the new magnets can sit flat on the iron shoe faces. If not practical to remove, perhaps small magnets can be placed on the shoe to the side of the crumbly parts. Do the best you can. I don't think positioning is super-critical to making it work. You're not looking for the optimal, most-efficient fix.

Note that the facing poles of the two magnets ought to be opposite polarity (want to stick to each other). This implies the magnets ought to be "polarized on the face". (IOW, an imaginary line from N to S on magnet goes thru both of the flat faces. By comparison, those old red & black "bar" magnets from your elementary school science lab have that line go end to end instead of thru the larger flat faces) I think most of the little rare earth magnets you can buy in hobby/craft stores will be face polarized. And probably stronger that the old magnets that were there originally. (Probably making the speaker more sensitive.)


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 08, 2020 4:45 am 
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Hey man, that is a great detailed explanation, much appreciated! I had wondered about the polarity but that part is solved now. The thickness of the magnet is my only question? Too tight a fit would rub the reed but the old magnet looks like it was pretty thick? Very cool, excited to get this old girl speaking again... :D

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 08, 2020 5:28 am 
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I suspect that with the new stronger magnets the spacing between magnets & reed can be much more than it was previously. From the size of the slot the reed extends thru, and the flat(?) speaker diaphragm, I'd guess the reed didn't travel far from it's resting position.
I wonder if the new magnets might stay in place (temporarily stick to the iron shoe) without glue for a quick test before permanently gluing them in place.


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 08, 2020 5:33 pm 
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Ordered some magnets, in a couple different sizes. How would a guy use a test signal to see if the speaker works with this deal?

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 08, 2020 9:11 pm 
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If you have an audio-freq signal generator, try feeding a couple volts peak-to-peak from that to the coil & see if the speaker makes any sound. If the speaker is hi-impedance, it ought to be easy to drive (won't suck a lot of current from the generator). Since the battery was just 3v, I'm guessing the driving signal voltage to the speaker would be in that ballpark or less.

Lacking a signal generator, do you have something like a mil-surplus radio that was designed to drive hi-imp headphones? Then just connect the speaker as if it were a headphone element.

Lacking a radio like that, do you have any spare audio transformers intended to match a speaker to a tube's audio output (e.g., a few ohms secondary to 1000 ohm or more primary)? If so, hook it up backwards to any 4 to 16 ohm radio, etc., output & the repaired speaker to the hi-ohms secondary.

In all those cases, start out feeding the speaker as little signal as possible, just in case it's really sensitive & could be easily over-driven, perhaps damaging the diaphragm.

But until it's driven by it's own radio, it's hard to know if the repair may be more or less sensitive to excitation than originally (i.e., too poor or too good of a fix or just about right).


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Sat 12, 2020 9:56 pm 
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Got the Neodymium magnets today and cripes are they powerful :shock: . Question is, how to glue them? Found on the interweb:

Another reported good success holding cylindrical and ring magnets into holes in an anodized aluminum part using Loctite 609 (acrylic adhesive) and Loctite 638 (urethane methacrylate).

I have some Loctite 609 that I use for bearing mounts. So, should I test it with super glue then use acetone to remove it and purrmanently glue or just go all in with the Loctite?

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Sat 12, 2020 11:27 pm 
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I'm curious if the magnets will stay in place by their attraction to the shoe. Just for testing.


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Sun 13, 2020 8:38 pm 
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hwhall wrote:
I'm curious if the magnets will stay in place by their attraction to the shoe. Just for testing.


Yes, they stay in place nicely and the radio is alive! I didn't realize it was iron so no need to temp bond it in place. In the box of radios, there was another GE with the speaker magnets in purrfect shape. On the OEM speaker, the magnets are very close to the reed but since these new magnets are much more powerful, should be ok? Volume was low on the radio but I haven't gone through it yet, maybe just a weak electrolytic or transistor. Top pic is the new magnets and bottom is the undeteriorated speaker magnets.

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Sun 13, 2020 10:45 pm 
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I do think the old ones were so close because they weren't as powerful as the rare-earth ones we have now. But the magnetic field imparted to the reed is pretty weak, too. Comparing the OEM speaker to the repaired one would be very interesting. I'd _guess_ the repaired would be at least as good as the OEM but I could be wrong. Magnetic field intensity falls off very fast with distance (inverse r-cubed) & the greater spacing in the repair may have more effect than I'd have anticipated.

I would glue the new magnets down, though, if you want to keep that speaker in service.


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 15, 2020 5:37 pm 
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Just an update fur anyone needing to purrform this speaker surgery. Below is the specs on the magnets I used. After replacing the three electrolytics that were very bad, the radio purrforms very well. Thanks to all that assisted, a great learning experience. :D

Neodymium Block Magnet NB11325N-35 - N35¸ 0.25'' L x 0.13'' W x 0.1'' Thk.¸ 1.14 lbs. pull
magnetsource.com

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 15, 2020 7:10 pm 
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Great---glad it works well !

Have a few junk thin-ring "super-magnets" from old printer drive motors which are easily broken. Imagine a couple pieces of one of those would also work---will have to keep this in mind...

John


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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Tue 15, 2020 7:57 pm 
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xrhonda91 wrote:
Great---glad it works well !

Have a few junk thin-ring "super-magnets" from old printer drive motors which are easily broken. Imagine a couple pieces of one of those would also work---will have to keep this in mind...

John


Just thought I would throw this out there, me thinks super glue would be a bad glue to use. Those magnets are so daggum powerful, it ain't easy placing them in the right spot. Super glue would be a fairly instantaneous bond and screw a guy trying to position it. The bearing mount loctite worked great but epoxy would probably work as well. Just need something that doesn't quick contact bond. :wink:

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 Post subject: Re: How to fix disintegrated magnet in high imp spkr? GE P91
PostPosted: Dec Wed 16, 2020 12:06 am 
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"Super glue would be a fairly instantaneous bond"

I wish! Seems like I cannot get any CA glue that will set fast anymore. Have to clamp the work & wait a day like it was Elmer's glue. It's nice to have working time before it sets but sometimes it's just annoying slow.


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