Acoustat 2+2: fixing cracked MDF frame- HELP!

[clayturner]

Hello all- my first post here. Skip to ***** if you don't want the story.

I recently got a nice deal on a pair of 2+2s that sound great.
(Relatively young, with MK-121-C Medallion transformers.)

They came with original white/cream covers, soiled and nappy.
I purchased new black covers from The Upholstery Company,
the original provider, who BTW are having difficulty sourcing quality fabric.
(My replacements have seams on each side, not one in the back.)

*****Upon removing the old covers I discovered a crack midway up one side
of each frame, perpendicular to the length.***** The speakers are holding
together OK, with minimal shifting of the broken side on each speaker.

I bought these from a home cleanout guy, and I am pretty sure
he did not know how to handle them- or maybe I did this myself
in transport. I have since dis-assembled them correctly in order
to move them, and that's certainly the only way to do it right.

******
The cracks on my frames have not resulted in a significant shift,
and I think I can easily get each frame back into correct position
in order reinforce it. As I said, I did not notice these breaks until
removing the covers. I plan to remove the frame from the base
and transformer, and use gravity, clamps, and a bit of leverage
(the reverse of the breaking force?) to get it in near perfect position,
then apply my reinforcement technique.

I see two choices-

1- drill holes across the break, then use screws or glue in these holes.
(I don't see a way to get glue into the narrow crack- anyone?) These holes
will need to be drilled at about a 45 degree angle to the surface.

2- add a reinforcing plate to the outside of each support.
considerations: material, dimensions, particularly thickness.
(I'd like these plates not to be obvious under the covers.
If I use aluminum I can bevel the edges and also cover them
with duct tape so they don't snag the covers.

In any case, I have to deal with the frame sides, which are MDF.
I have researched this stuff a bit, but I've never worked with it
(not that I am a woodworker!). I have read that a hole drilled
should be nearly as big as the screw. And I plan to drill any holes
outdoors.

At this point, my favored approach is this-

-use 1/8" x 3" x 12" aluminum plate

-the 12" (or longer?) length would allow for
maybe 6 holes on each side of the crack.

-these holes would be about 1/2" deep- the MDF frame is 1" thick.
(Any necessity for deeper holes? Any reason not to drill 5/8 or 3/4"?)

-also, no reason not to put glue in the screw holes and between
the plate and the frame

-I would put a plate on the un-cracked sides of the speakers
in order to keep them symmetric, and also to strengthen
what seems like a weak point.

Any better material than aluminum plate?

I'd be very grateful for any criticism of my plan, or my thinking on this.
Or better ideas I have overlooked?

Thanks, Clay

[planet10]

When i converted my Model 2 to 1+1 i built a much better frame out of solid + MDF (i'd use plywood these days.

Associated Equipment

dave

[Few]

Welcome, Clay. This might be a case in which a picture is worth a thousand words. I'm not familiar enough with the innards of the Acoustats to be able to visualize what you're up against. Perhaps everyone could offer better advice after seeing a picture or two of the problem area. It does seem like it ought to be fixable, though.

Few

[mavric]

Clay, I am new as well, a pisture is worth a thousand words, post some pics, you might have somethng really nice, just need some pics to unerstand, can be simple, or in my case, can be a challenge, i look forward to both.

[DaveG]

If the frame is holding together naturally in the correct position, can you pry open the crack abit, maybe with a flat blade screw driver, and then squeeze or force or use a putty knife to work white glue into the crack? Let it spring back together and dry, and or wrap it in wax paper and clamp it for a perfect fit.
Unless it is near the very bottom there is not that much load on the MDF.

[silverr1]

I would recommend completely redoing the frames. Your ears will thank you later. The stock frame are pretty flimsy at best. Once you tighten the frames up the image is focused and the bass is greatly improved. I've got a few more mods to add to this album. I added a sand filled box to the base, L brackets, and updated parts in the interface. All small improvement but cheap to do. In the end they all add up.

Picasa Web Albums - Ho - Acoustat

[TheSlimMan]

I add my vote for solid wood frames in place of the MDF ones. Over on the Planar Asylum page of the Audio Asylum board, many people (me included) have gotten great results reframing their Maggies in wood. It seems that MDF negatively affects the sound of planar drivers whereas wood enhances it.

[lvw]

If you have the gear and inclination to make new frames, that's doubtless the best. However, if you are just up for a patch job you should consider putting plates on both sides of the frame (that is, two plates in each cracked area, one each on the front and the back at each location). A plate on one side, while better than nothing, won't really add too much strength.

And, with plates front and back, they need not be very thick. Much thinner than 1/8 would be fine. You would just hold the frame straight while the glue set. With a few screws as well, should never shift.

[planet10]

Originally Posted by TheSlimMan

It seems that MDF negatively affects the sound of planar drivers

I personally think that that can be extended to all speakers. MDF is just not a good material for building loudspeakers.

dave

[clayturner]

Thanks for all the suggestions. I have posted pictures at
Picasa Web Albums - Clay - Acoustat 2+2 ...

Both these breaks are midway on the frames, at a point which
leverage-wise seem likely to take the most strain when mishandled.
(See arrow in third picture.)
(As when possibly these speakers were transported in the back
of a pickup truck- my speculation.)

Add to this the fact that these vertical supports are notched out
just at this midpoint to accept the rear cross braces.
Both the breaks occur at this point, going right to these notches.

I appreciate that completely re-doing the frames is likely
the way to get the best sound from these speakers,
but I'm just not ready for that kind of effort. Plus I'd like to
hear them as close to original for a while. I have not had them very long.

I will try to introduce some glue into the cracks, but my primary
effort will be to use aluminum plate on the sides- the 3.5 inch dimension-
of the supports. The front and back of the support is only one inch-
I don't want to drill and screw here.

But rather than simply correcting the existing breaks, I want to
address what I think is a basic weak point in these frames,
and maybe stiffen them at the same time.

I want to add plates on both sides of the speakers, not just the broken ones,
and I want them to be fairly large. I plan to use 3/16" x 3" plate,
perhaps as long as 36" or 48". I can get four such 36" plates
of extruded 6061 aluminum for about $50 or $60 plus shipping.
That's pretty strong stuff, maybe overkill, but I'm not an engineer,
so when the marginal cost is low, I go for overkill, unless
I am causing _new problems_. (Jump in here, please!)

A plate this size allows for the use of more, but shallower screws,
and lots of glue between plate and MDF surface. Some of my reasoning
is due to a distrust of MDF, maybe unreasonable. But really,
it's just compressed sawdust and glue, right?
Plus the thickness of the plate will allow for counter-sunk screws.

I will bevel the edges which will contact the covers.

I will disassemble for this process- which I must do in order
to replace the covers in any case.

Thanks for the attention.

Clay Turner