MW6502 Repair Questions

kevviek

I picked up some Monitor 5's this week, supposedly working but when I got there both drivers had issues. I had driven an hour and a half so against better judgement I took them at a reduced price. One had magnet shift and one has a torn cone, interested in thoughts on if they can be revived. Photos are attached, I'm not sure if the voice coil is still ok or how to verify?
These ones have the SL2000

pitdogg2

No photos, now the good thing is mw6502 is the most common driver available. Get on ebay and get your buy on.

kevviek

I'm in Australia, so to get replacement/copy drivers is $70US each plus $65US for shipping. Not really worth it, the cabinets are in poor shape too.

Gardenstater

Looks like the driver was overdriven and the voice coil bottomed out; judging by how the end of the former is mashed. You would check it with a meter. I think the voice coil is out of center in the frame piece now and I don't think your chances of repair are good. I'm dealing with that situation on my original MW6500s that have less clearance than yours but mine do not have damaged cones, just crooked voice coils.

kevviek

To clarify, one had a shifted magnet but the cone is good on that one, and although the voice coil looks a bit damaged, it reads 3.8 ohms which I think is in spec.
The other cone has a minor tear, but still plays ok. I’m wondering what th best way to reinforce the cone might be.

Gardenstater

Posting the photos was good but you didn't identify which photos were for which driver; the one with the tear or the one with the slipped magnet.

Slipped magnets can often be fixed. What caused the cone distortion in that one photo would be the question. Hard to believe there was no damage to the voice coil or spider.

First step with both is to check the ohms with a meter on both and compare to what it should be. 3.52 Ohms.

xschop

I just repaired a 6516 shifted magnet and crimped cone just like that from shipping anomoly. After re-setting magnet, I used the black window paint. After 24hr cure time I tested in the 8Tls. Good to go, and can't see repair within 3 ft. distance or so.

As long as they ohm out, coils are good. Polk has DCR all over the place on these drivers. 3.8 is good, 3.5 is better, but not a biggie unless you're OCD.

Gardenstater

I am OCD but the 3.8 wasn't given when I posted lol. Many meters can easily be off by as little as 0.3 ohms. What does the one with the wrinkled cone read? Importance should be mainly placed upon them being reasonably close to each other. What does your meter read when both probes are placed together? Probably not 0.

That one with the wrinkled cone had a majorly traumatic event lol.

I'd try to re-assemble and align that one that is slipped but you must be aware of the methods since you already have it apart. You're on the right track. The misalignment of the voice coil in the frame piece that I *think* I see in your first photo is concerning. Were you pushing on the cone when you took that photo? Check it with shims even if it isn't rubbing to see where it may be too close for comfort.

kevviek

ok, so it may be better to start again to remove confusion. Both read 3.3-3.4 ohms, the figure above was from memory but I just checked again.

The first driver with the damaged cone looks to have been replaced since new, it is not locked but does scrape the voice coil if pushed in by hand. But I guess that is to be expected given the state of the cone. I was wondering if that’s possible to repair somehow but I guess not.

The connection to the spider looks ok, but interestingly the leads are connected there rather than on the cone itself.

kevviek

The second driver was locked and I have pulled the magnet off. The voice coil looks a bit corroded and damaged and the attachment to the spider doesn’t look perfect, but it moves freely within the first section of the magnet assembly, which is still attached to the frame.

kevviek

xschop wrote: »

I just repaired a 6516 shifted magnet and crimped cone just like that from shipping anomoly. After re-setting magnet, I used the black window paint. After 24hr cure time I tested in the 8Tls. Good to go, and can't see repair within 3 ft. distance or so.

As long as they ohm out, coils are good. Polk has DCR all over the place on these drivers. 3.8 is good, 3.5 is better, but not a biggie unless you're OCD.

Hmmm...I thought the one with the damaged cone would be toast but I'd like to give it a try. Would this approach work?
- remove the voice coil cover
- insert shims to align the coil within the magnet assembly
- apply something (?) to the cone to add rigidity
- allow to dry before removing the shims and replacing the dust cover

The question I have is how to do the cone repair. Is black window paint a special paint, would it add strength, would I paint both sides? Or should I add some sort of reinforcement (worried about adding too much mass as well)

xschop

Mine was creased pretty good, not torn. Can't tell by your pics if yours is torn. I got lucky and only had to massage front and back of cone till crease was fairly flat, then applied the paint. Made By Me brand, black color of course.

For better insurance, do shim the coil like you suggested and glue up.

Gardenstater

I'm a little concerned with this image. Is this the relaxed position or are you pushing on the coil? Xmax should only be 3.18mm which means you should only see about 1/8" of the coil at rest. That gives the driver 1/8" of movement in the outward (push) and the inward (pull) directions. It looks like this one got overextended in the pull direction and the voice coil former (aluminum) got mashed on the pole piece. I don't think the white is corrosion but it is an indication that the coil has been rubbing and the insulative coating on the wires has gotten scuffed, causing the white appearance.

kevviek

Yea I was pushing to see what to condition of the voice coil was. it looks more like corrosion than rubbing, the metal part has the white flaky stuff as well

kevviek

xschop wrote: »

Mine was creased pretty good, not torn. Can't tell by your pics if yours is torn. I got lucky and only had to massage front and back of cone till crease was fairly flat, then applied the paint. Made By Me brand, black color of course.

For better insurance, do shim the coil like you suggested and glue up.

It’s not torn yet, but is pretty weak. I might try a coffee filter and some shellac on the back

Gardenstater

kevviek wrote: »

Yea I was pushing to see what to condition of the voice coil was. it looks more like corrosion than rubbing, the metal part has the white flaky stuff as well

Copper corrosion is green in color and the coating would have to protect the wires from corrosion. It is scuffing of the coating. If you sand your car between clear coat applications it will appear like that.

kevviek

Ok, I have reinforced the cones, have a jig and ready to glue the magnets. I have Gorialla Glie and Loctite Red, I gather Loctite is the one to use?

Gardenstater

I have not yet done this although lots of others have like @xschop and @lawdogg. All I can say is that the original adhesive that Polk used was very thin. I measured 0.003" thick when I checked the chips, and it was also magnetic. Whatever they used did not last obviously and I believe a lot of the problem is the difficulty of getting a good long lasting and strong bond to zinc chromated steel.

I'm sure whatever adhesive you use, you want a low viscosity adhesive especially since the only clamping pressure is going to be from the magnetic attraction, whereas Polk was able to use clamps because they did it in the non magnetized state. You want it thin to not reduce the BL of the magnet motor too much.

Be careful with the Loctite Red that you don't use so much that it gets into the voice coil gap but I'm sure you already know that. Gorilla makes lots of adhesives and you don't say which one.

kevviek

I've used the Loctite to bind the magnet to the pole piece. I did a test fit onto the speaker with the jig and had it lined up so that the voice call moved freely (no glue yet) but found that the plastic pipe is not strong enough to be able to make fine adjustments with the screws, it just distorts. Will have to think of something else.

Gardenstater

kevviek wrote: »

I've used the Loctite to bind the magnet to the pole piece. I did a test fit onto the speaker with the jig and had it lined up so that the voice call moved freely (no glue yet) but found that the plastic pipe is not strong enough to be able to make fine adjustments with the screws, it just distorts. Will have to think of something else.

That's a lot of magnetic force to overcome. There was a member here who made one out of PVC but he put helicoil type inserts in for the adjustment screws. He bought a kit that includes the special tap for something like $25.

You might try that so that the screw threads don't strip out, unless you are saying that the pipe is bending and that is the problem? If so can you find another pipe that will fit over that one and you can use PVC adhesive to glue them together?

Edit to add - Check out PVC Hub Repair Couplings

xschop

If you can wait, check in here....

https://forum.polkaudio.com/discussion/195928/forum-member-mw-magnet-repair-tool-set#latest

kevviek

I’m in Aus so no chance of me getting the the real jig. I had it a go anyway, a bit of a sad outcome, I mashed the voice coil when connecting the two pieces together. So I decided to do a bit of exploration and pulled the whole thing apart. At least the magnet is lined up now ;-)

Interestingly I found there almost zero chance f getting it aligned without a jig, as the pole is attracted to one side of the magnet, trying to adjust it by hand it just moves from one side to the other.

Gardenstater

Ahh bummer mate! I think anyone doing this should cut the dust cap off and do xschop's phase plug mod at the same time as all this work because, not only does it produce a significant improvement in sound quality, but it also should enable some properly sized shims to be used in a circular shoe horn arrangement to guide the voice coil in without damage.

Thanks for donating your driver to science. It is interesting to see the interior view.

How is the spider attached to the stamped basket? Is it simply butt glued against a flat area of steel or is there a steel flange that we can't see in the photo?

What is the depth of the recessed part of the basket from the spider plateau area to the rivets plateau area?

kevviek

The spider is butt glued, there is no flange. I'm not 100% clear on what the other measurement is that you are looking for.

Gardenstater

kevviek wrote: »

The spider is butt glued, there is no flange. I'm not 100% clear on what the other measurement is that you are looking for.

Thanks for that! Pretty much what I thought and it should be possible to separate the spider from the basket with the right Xacto knife.

No worries on the other measurement because I realize it can be measured from the outside just fine. My curiosity is related to installation of a sound absorption donut under the spider, above the magnet assy..