MW6500 - Offset pole piece

gulp35

Hello

I have a pair of Monitor 7Bs (circa 1980) whose drivers had frozen. After taking them apart it seems only the magnet-front plate glue has failed.

When I do the repair should I apply glue/jbweld to the outside of the pole-piece/magnet?

Looking at the pole piece/ magnet assembly on both of them seem to be out of round. Is this normal ? partial failure?

gulp35

Another question, what is the best way to remove the old glue? I've tried isopropyl alcohol, but it didn't seem to do as much as just scraping at it... but that takes a while

Schurkey

Complete instructions are posted on this site. With my forum set-up, the thread is three pages long, with my post 'n' photos on the last page.

http://forum.polkaudio.com/discussion/135716/lets-try-to-fix-a-mw6511-damaged-in-shipping/p1

I scraped old adhesive using a single-edge razor blade style gasket scraper. I think I used aerosol carb cleaner as a final wash.

skrol

If all else fails, I may have a 6500 available. I was going to sell it to some one previously but haven't heard anything back in a while. I'll double check with him first.

gulp35

Thanks skrol, I might need one still, but hoping all goes well, someone else will find better use of it. Wish me luck!

gulp35

Pole piece ended up breaking off while trying to glue together the speaker luckily locktite power grab is water soluble and I was able to save the speaker from being glued and screwed.

PS If these batteries plates are cadmium plated, I think there should be some caution taken when scraping the epoxy off them.

https://en.wikipedia.org/wiki/Cadmium_poisoning

It looks like the biggest threat is from cadmium vapors (unlikely), but cadmium dust is also dangerous. I was not able to find any sort of LD50 for this though, so i can't say if this is potentially deadly or potentially unhealthy.

Schurkey

gulp35 wrote: »

gulp35 wrote: »

Pole piece ended up breaking off while trying to glue together the speaker

I've been wondering about that photo. Figured it was a "set up" posed for easy photography, but not actually the way the speaker was being re-assembled.

The pole piece is the LAST thing you glue onto the rest of the already-assembled speaker. In other words, the magnet goes on first, and the glue should be fully cured.

Loctite Power Grab is NOT the product you should be using.

gulp35

I figured that if that glue had still held, that just gluing around the edges to retain it would be enough since its the suggested glue for that purpose; and that most were using loctite red or jb weld. Will it be able to hold? Will there be an audible difference by using power grab?

Also, I don't think the pole piece HAS to be the last to go, if they had been connected they can be reconnected (and they do, the dry run went great). though i've been wrong before, and I'm open to arguments why.


edit: westmassguy seems to have used it before without issue...

Schurkey

Barring damage to the voice-coil or cone or spider or surround, the only things that need to be reattached are the magnet and the pole piece, and the dust cap if removed.

When considering the typical adhesive bond failure of these drivers, there are two glue bonds, both of which are suspect after all these years. The magnet is glued to the basket, the pole piece is glued to the magnet. The magnet must be removed from both the basket and the pole piece, then re-attached with fresh adhesive in both places to assure long-term success. You discovered the truth of this when your pole-piece fell off the magnet during your re-assembly attempt. (Just a guess: Your pole-piece glue bond had ALREADY failed, the only thing holding it to the magnet...was magnetism. You disturbed it enough that the magnetism couldn't hold it.)

During reassembly, the magnet should be reasonably well-centered, but only the pole piece needs true precision.

Leaving the pole piece until last allows the pole piece to be moved into proper position by locating off the magnet, which is a sturdy and stable mount for the alignment tool once the magnet is firmly re-attached to the rest of the speaker.

Attempting to position the pole piece while it's already attached to the magnet, by moving the magnet around on the speaker basket would be a nightmare. So, yeah, the pole piece has to go on last--except for perhaps the dust cap.

You're right, many folks have had success with Power Grab or epoxy when the original glue bond has not yet broken. Once the glue bond is broken, Power Grab or epoxy is no longer the proper choice.

gulp35

Thank you for responding, but I'm still not clear on why I shouldn't be using Power Grab. If it is strong enough to prevent shift with failed epoxy, then wouldn't it be strong enough to hold the magnet in place (i.e. power grab strength > failed epoxy strength)?

I definitely agree with you that I should have broken apart the pole piece magnet from the start, even if only as "while I'm in there job". My logic at the time was that if the epoxy had not failed, and there was a(n easier) procedure to keep it from failing, I should just reinforce the existing glue.

I've decided to mount the pole piece first since i've designed my jig to adjust plates (front or pole) sitting on a restrained magnet. If I connected the magnet to the front plate first, I would have to make another jig.