Fraltecstein

[bfish]

Looking good David!

By now you've figured out how little difference there is between building and re-building these things.

I noticed your "stacked" driver baffle is cutout as an entire circle, rather than the normal sorta-oval. Whether that'll matter much, I know not, but it'll increase the front chamber volume, and appears to locate the driver farther back, which will mean a need for equal offset of the HF horn. The pictures may be tricking my tired eyes though.

For the cavities, hard is what you want (you're bracing, right?). I used spray foam, but be aware it's risky not knowing the expansion rate. Had I not stuck around, it would've destroyed the horn. By immediately drilling several 1/2" relief holes in the rear, and strapping each side in place with a couple of 2" HD rachet straps, I was able to keep everything in place...barely. The flares are rock solid now...(oh yeah, there's an inch of reinforced concrete backing each flare too...). I'd recommend spending a little more for 2 part expanding foam (like that used for floatation in docks) since the expansion is predictable.

<font color="#FFFFFF" size="1">[ April 06, 2008 06:48 PM: Message edited by: bfish ]</font>

[Audio_by_Goodwill]

Originally posted by Vathek:
[QB]OK, so this is what I've done, the only drawback I can imagine is that the whole bracing might have reduced the volume too much..

<font size="2" face="Verdana, Helvetica, sans-serif">Don't worry about it. Many people think the cabinets are a bit too big, to begin with.

The bottoms were totally bent and really not usable anymore from standing on those legs for too long. So I removed them.

<font size="2" face="Verdana, Helvetica, sans-serif">Wow, thats pretty bad! One of the drawbacks to MDF, I guess.



Very nice!

Which braces are one piece all the way across?

That's what they look like now. All the bracing is done with 19mm MDF, the bottoms are 22mm MDF.

<font size="2" face="Verdana, Helvetica, sans-serif">Wow, 19mm, and 22mm! I think the stuff we get here is only 18mm.

Where the ports are, the brace is 9mm from the front, so I can try various port-configurations with 9mm plywood-boards.

<font size="2" face="Verdana, Helvetica, sans-serif">9 or 19? 9 seems very thin, to me.

[Vathek]

Thanks for the kind words, guys!

By now you've figured out how little difference there is between building and re-building these things.

<font size="2" face="Verdana, Helvetica, sans-serif">Well, yeah, sort of, it definitely was a lot of work. And I was thinking about starting from scratch, but decided against it mainly for two reasons. My limited woodworking skills and my limited tools. I only have a jigsaw and two large clamps, so I tried to not have to touch the top part with the non-right angles and the curves of the front horn.
But that leaves me with the one problem you mentioned: the mid-front-horn is not braced. I guess the best thing to do would be to brace it like a ribcage, but retrofitting that is again just beyond my capabilities. Maybe I should just fill those cavities with cement (just to wish I hadn't, everytime I have to move the cabs), but something has to be done. Do you think that foam-stuff wouldn't make it rigid enough by itself? (Oh, the cutouts are as they should be, they only appear to be a circle on those pictures.)

Which braces are one piece all the way across?

<font size="2" face="Verdana, Helvetica, sans-serif">The ones going from left to right are one piece and the one going from front to back is in two pieces.

9 or 19? 9 seems very thin, to me.

<font size="2" face="Verdana, Helvetica, sans-serif">The idea was, that they're just a cover for the front cross-brace which gives the cab the stability, so the thin 9mm boards won't have to add anything stability-wise and are just screwed to the front brace. In a way that makes it 28mm but now that you've mentioned it, if those panels start to resonate over the large cutouts... Oh well, I'll see.

[Vathek]

Just for the record, maybe this helps anyone who's thinking about restoring some old 828s. I've used two-component foam to fill the cavities behind the fronthorns, because it doesn't expand as much as the normal stuff and mostly because it doesn't depend on moisture to harden. Worked out very well, everything is hard as a rock now, so I can only recommend this. But if anyone plans to do this, be smarter than I was and fill the cavities before you seal them. Stupid me! But it did work out in the end.




This is what they look like now. I may at one point paint the inside black for aesthetics and will add something to the back panel to minimize direct reflections to the backside of the driver once I get some proper drivers.
The hockey pucks are just for protection of the corners of the cabinet plus I like the look and it reminds me of my youth spent on ice.
Thanks everyone for the advice and support! It's very much appreciated!

[Cal Weldon]

Very nice indeed.

[bfish]

Glad you got there, David. Looks nice and solid!

Also glad you included the pictures with the straps in place. Foaming-filling an enclosed space can yield the best ultimate rigidity...as long as you know the risks and are ready to deal with them.

When I did it, I was too busy wiping a big "whew, close call" from my forehead to think about snapping a picture.

[timofred]

Great job Vathek

[Audio_by_Goodwill]

Originally posted by Vathek:.... I've used two-component foam to fill the cavities behind the fronthorns, because it doesn't expand as much as the normal stuff and mostly because it doesn't depend on moisture to harden.

<font size="2" face="Verdana, Helvetica, sans-serif">Could you provide more details on the foam? What's it typically marketed for?

[bfish]

Don't know what David used, but locally it's common in marine/flotation apps. Evercoat is a common US brand, but there are many others. The real beauty of the 2 part foams is predictable expansion. Haven't looked lately, but there used to be small qtys to be had on eBay.

Foaming the cavities is walking a fine line. You want enough overfill to put signfigant pressure every inch of all surfaces, but a little too much will literally blow things apart.

[Vathek]

The stuff I used is called 'forbo' which I'm sure is only useful for the europeans here, but it's just standard 2-component foam from my local hardware store ? la home depot. It's typically used for filling door- or windowframes, sealing pipe entries in walls or even building mountains for model railways, anything really.
I've chosen that for two reasons: (1)a 500ml can expands to about 15 liters, whereas one component foam expands up to 40 liters out of the same size can, so the less expanding is easier to control. (2) one component foam relies on moisture to harden and the instructions advise you to moisten the surfaces of the cavity you want to fill. I didn't really like the idea of moistening/weakening the already thin bent plywood. And the surface to volume ratio is really low, so I was worried that the foam in the middle would not properly dry/harden or at least not in a predictable way and the one thing you don't want is continuing hardening/expanding in the middle while the outside has already become hard, kind of like a pipe bomb.
The downside is that the two component stuff is more expensive and it also takes more because it expands less. If you don't seal the cavities before you decide to fill them you could probably use one component foam too if you fill it in slowly and let it dry layer by layer.