DIY 904-8A conversion to 604-8H

Do It Yourself
I’ve had a pair of 904’s for many years. I honestly don’t know how long. I believe I bought them from a forum member on the Altec Users Board. (So if it’s one of youse guys, chime in)

Anyway, measured the woofer section, and built boxes with the help of BoxPlot software.

I essentially built rectangular Stanley Screamers….stage monitors.
They never sounded good.

I had planned to send these to GPA for conversion, actually talked to Bill about it. But I never did.

So I’ve started the process of doing it myself. I have reconed 10 Altecs since last year with good results, so why not?

Unglued the center dust cover using Acetone. Then remove the cone assy and clean everything.
In the voice coil photo, the bottom is this duplex. The top one is from a 418-8H guitar woofer. This explains why thy sound like…well, music played through a guitar speaker. 😊

I’ll be using cone kits from The Speaker Exchange. MWA parts.
I’ll stick with the mantaray horns.
I’m considering maybe using felt in the center. I’d be interested in you duplex guys opinion on that?

Should be a fun project
 

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Got the cone kits today.. Two 604-8G and one 416-8B.
Identical parts for the 416 and 604. I figured as much since the frames, distances, pole piece etc all have the same dimensions.

I’ve no doubt this will transform the 904’s sound.

Also got “sticky surround” dope from Springfield Speaker. It looks and feels just like the original.

For cabinets I’m re-purposing some 9846-8A units I’ve had for years. Just a test. They will be on the order of 9 cubic feet. And will be ugly!
 
Ok, just getting started with this.

First thing I do is test fit the parts.

Voice Coil in the gap.
Spider to the voice coil.
Cone to the voice coil.

Sadly one voice coil was out of round. Not the first time.

I measured the gap on these with a drill index. They are .003” larger than the 416’s I’ve done. So I used .010” shims instead of .008”. You don’t want the VC to move easily.
The photo shows the VC shimmed so the coil is sticking up out of the gap. Shims are brown, a little hard to see.

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Next I add my fixture.

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The fixture allows me to get the coil positioned correctly in the gap. I had to draw it all to 4x scale because I need visuals.

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The MWA winding is taller than the Altec. .300” vs .250”.
The coil forms are different as well.
So from my drawing, the top of the coil form needs to be .8125” from the pole pice to center the coil in the gap. I made it out of a Chevelle Alum control arm bushing….my buddy let me use his lathe.

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Next I just use a flat piece of alum to push the coil in. Sets the position and keeps it parallel to the pole piece.

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More to follow
 
The next step is to install the spider once the voice coil is in place.
It glues to the frame with speaker rubber cement.
Next, 2 part epoxy to glue the spider to the voice coil.

They provide 5 minute epoxy. Self mixing. So if using that, you better be ready to install the cone. I never am, so I use long cure two part, U-mix. I invented a syringe method to apply it.
I thought I was buying blunt nose needles, I got real syringes for medical use. Oops. So I improvised with some brake cleaner spray tubes. Worked really well.

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Tomorrow the cones. The surrounds are pre-attached.

Below are the 9846 boxes I’ll use. Ugly.


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Ron
 
The next step is to install the spider once the voice coil is in place.
It glues to the frame with speaker rubber cement.
Next, 2 part epoxy to glue the spider to the voice coil.

They provide 5 minute epoxy. Self mixing. So if using that, you better be ready to install the cone. I never am, so I use long cure two part, U-mix. I invented a syringe method to apply it.
I thought I was buying blunt nose needles, I got real syringes for medical use. Oops. So I improvised with some brake cleaner spray tubes. Worked really well.

View attachment 79028

Tomorrow the cones. The surrounds are pre-attached.

Below are the 9846 boxes I’ll use. Ugly.


View attachment 79029

Ron
How did you arrive at the vc height/depth dimension?
 
How did you arrive at the vc height/depth dimension?
I measured the position of the original VC in these. That coil length is .350" and was centered in the .475" long gap. (Top plate thickness)
I simply did the same with these .300" long coils.
I showed that in the drawing above.
 
I measured the position of the original VC in these. That coil length is .350" and was centered in the .475" long gap. (Top plate thickness)
I simply did the same with these .300" long coils.
I showed that in the drawing above.
Gotcha, thanks Ron.
 
I was going to go drag racing today. But wanted to continue with the re-cone while I was on a semi roll.

Last night I had both spiders installed and glued. I also had marked the cones for the leads and installed ahead of time.
On a 416 you want these hidden by the dust cap. So I just did these the same.

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Next, set the cone in to see if it’s going to sit at the right height. I use this straight edge to check that the cone edges are at the same height in 4 positions. It’s possible for the cone to tilt.
Also, depending on how snug the cone fits to the VC, it might be too low, or sunken. That was the case here.

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I also measure the surround to frame distance to make sure it’s on center.
Because this cone wanted to sit too low, I made this fixture.

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Goofy but worked excellent!

Once happy with the cone position, dress the VC leads and apply the 5 minute epoxy. W/O touching the cone. It’s tricky. I let the epoxy hit the cone first and it rolls onto the VC. I use reading glasses, a lot of light, not too much caffeine, and spit out my gum….

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I let it sit for 20 minutes and then glue the surround. Clear rubber speaker cement. Just lift the edge up, lay the bead as you go around, press it down with a spudger or whatever.

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Let it sit 1/2 hour. Then dress and solder the VC leads. These get covered with black glue.

Next solder the tinsel leads.

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Ok, now it reveal time. Nervous time. Remove the shims. I just remove every other one then the final 4 the last two at the same time. The feel will tell you if it’s centered. It was.
Cone moves perfectly, it feels right. And, the coil height did not change. So no tension in the spider or surround.

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Now, run DATS to measure the T/S parameters.

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This actually looks excellent. The surround isn’t doped yet, I’m thinking a thin coat will suffice.

Last step is to glue the original gaskets back on.

Hope you are enjoying this.
 
Thanks EarlK.

The second one is done up to the same point and happily measures nearly identical to the other one.
Very happy about that.
I actually measured a pair of GPA 416-8C re-cones and they were quite different from each other. So this is good.
 
Nice write up and detailed pictures. Thanks for posting.

How did you deal with the out of round voice coil?
 
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The surrounds treated, gaskets on.
Treating the surrounds is an odd process. It takes a few coats, or at least that’s how I do it. The issue is, the liquid material distorts the crap out of these surrounds. Some of the ribs sink, some rise up. After the stuff sets, they do calm down. But never look perfect. You do have to look close to see it.
All the 15’s I’ve done behaved the same way. The 12’s didn’t do this at all.

The one horn is just sitting there.
I ordered felt parts from Simply Speakers. They fit the multi cell -8G horn, but I think they will fit these as well. Arriving next week so I’ll show you that stuff then.
 
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