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SP1240/1353 Disassembly, Ferro, Adhesive

 
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kevd
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Joined: 07 Dec 2019
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PostPosted: Mon Dec 16, 2019 11:02 pm    Post subject: SP1240/1353 Disassembly, Ferro, Adhesive Reply with quote

I recently dissassembled my SP1240 from my 105/3 that was stored for years after it stopped working consistently. KEF had sent me replacements years ago and this one sat around. Now that my surrounds need replacing I decided to replace the fluid as well and go through both speakers as a total refresh. Before working on the actual working tweeters, I took the old one out of the box and practiced on that one. It appears there are 3 different layers glued together and cannot be separated without breaking the bond. It appears the best layer to separate these at is between the magnet and PCB, or first interface. The ferro was like hard molasses and after a complete clean and an impedance check I re-positioned it on the magnet. Using my signal generator and sweeping from 500hz-10khz, it worked again like a charm! I want to reassemble with adhesive that will give me enough setup so I can align with the generator and yet not so strong in adhesion that I cannot remove the voice coil at a later time.

-Can anyone tell me if the disassembly I noted above makes sense?
-What did you use to separate the pieces?
-What adhesive was used originally, if known, and what adhesive is good for this purpose?
-Does the SP1353 come apart differently in anyone's experience? I have two new ones in the box for over twenty years.

I'm using APGO77n from Ferrotec to replace the original APG512 as recommended based on high Gauss/low viscosity, FYI

Thanks,
Kevin






Last edited by kevd on Thu Jan 09, 2020 7:46 pm; edited 1 time in total
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Thu Dec 19, 2019 9:27 am    Post subject: Reply with quote

Your disassembly was correct. A Stanley knife blade was the usual tool of choice. The tweeter was originally built up on the pcb and then stuck down to the magnet top plate using a thin double sided adhesive tape. The production tape was made by 3M, I think, and was supplied as die cut rings on a continuous reel. BITD if you were careful with the blade, you could re-use the original tape. Probably not possible after 40+ years!

Re SP1353, remind me, which unit was that?
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kevd
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PostPosted: Sat Dec 21, 2019 4:20 pm    Post subject: Reply with quote

Thanks SG,
The 1353 is the next gen tweeter, vintage 1996. It appears to be made slightly different and the two I have are brand new. I want to check the fluid but don't want to experiment taking them apart unless I have to. Yeah, when I separated the PCB from the magnet, it just snapped apart as if it were dried glue. Would you say that the tape that was used was in the .001-.003" range of thickness? Just looked at 3M website, tough to find a strong enough tape less than .005". Part of me wants to find an adhesive tape that is close to original, and the more practical side of me says find an adhesive that will suffice. Also, can you tell me if the laminated layers that sandwich the diaphragm used tape as well? Mine partially separated when I was prying layers apart before I realized that wasn't the way to go.
Kevin
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speakerguru
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PostPosted: Sat Dec 21, 2019 6:36 pm    Post subject: Reply with quote

Sorry, my memory is not that good, but I seem to recall that cigarette paper is about 0.001" thick. The tape was not that thin, so, 5 thou is probably OK. The original was yellow but I don't suppose 3M have stuck to that (pun intended) Laughing .

re diaphragm to ring adhesive; again, not sure. I have a dim memory of there being pre-cut rings of hot melt adhesive with a paper backing, but I may have made that up just now.
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kevd
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PostPosted: Sat Dec 21, 2019 6:59 pm    Post subject: Reply with quote

Thanks again, I just spent some time studying the construction. I'm wondering if it was an adhesive transfer since there are no signs of a membrane left behind. I may go with an adhesive like the MI-3035 that I use for the speaker grill/cloth repair. It sticks to these materials well and gives me some time to set the voice coil. Super glue would be great but too fast setting. If you have any suggestions let me know.
Kevin
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kevd
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Joined: 07 Dec 2019
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PostPosted: Tue Dec 24, 2019 5:13 am    Post subject: Reply with quote

Just finished rebuilding 3 SP1240's tonight. One of them actually had a cracked PCB before I took it apart. I managed to glue the piece back and continue until finished. I ended up using Loctite "GO2" It gives you work time to center voice coil, dries thin and clear. I used sparingly and applied with a toothpick. Some observation below after doing all three.

-All interfaces had become weak and needed to be cleaned and glued
-The center "hard foam" was ready to fall off on all of them. I scuffed the bottom of them with some 2000 sandpaper before gluing. Used CRC nitrile adhesive.
-Used alcohol wipes in channel to clean out old grunge
-I filled fluid to about 3/4 full around groove
-I ended up separating the tweeter between the two PCB's instead of at the magnet. This was the weaker interface and it also allowed me more access to the voice coil for cleaning.

The sound output improvement is very noticeable, see below before and after for one of them.
600hz - 0dB/56dB
1khz - 54dB/76dB
10khz - 82dB/90dB

I took the measurements using my signal generator with preset voltage and dB meter 12" away for all measurements. Not a shabby improvement at all!

Thanks
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