Centering a new fram via tone

**Steve Schell** · April 15th, 2010, 02:00 AM

Haven't we all had similar battles? If the driver is in good shape with a clean concentric gap and the diaphragm is good, centering should be simple, often just popping it on the locating pins. In my limited experience with GPA diaphragms they have had correct tolerances and installed easily.

Panomaniac I have been through the agony you describe, and from past experience I suspect some undiscovered metallic debris in the gap. These tiny demons can cause what you describe, i.e. every micro movement changes the sound and spectrum and it is all incredibly "touchy." I recommend inspecting the gap with a magnifying glass and a really strong flashlight, and cleaning the gap with sticky tape for another ten minutes even after it appears perfect. Then if the diaphragm still rubs, some tolerance is seriously off. Could be a non-concentric gap or (less likely) an out of round voice coil.

My final test is to sweep the driver (at very low power, just barely audible) between 100Hz. and 500Hz. It should sound perfectly clean with no funny noises.

**Panomaniac** · April 15th, 2010, 10:05 AM

Thanks Steve, very good advice. Nice to hear from someone who has been thru the ordeal.
I will get in there and clean again. Wish I still had a good source of compressed air.

The sticky tape is really hard to do - it sticks to the sides!

**rontec** · April 15th, 2010, 06:11 PM

Originally Posted by Panomaniac

Wish I still had a good source of compressed air.

The sticky tape is really hard to do - it sticks to the sides!

I've had the same experience with sticky tape! I wasn't sure about using a compressor but that's what I've done in the past. After using tape first then heavy paper I hit it with compressed air. The high pressure air seems to get everything out, but not sure if there is any negatives?

**bfish** · April 15th, 2010, 06:37 PM

The tape routine is easier if you fold it (inverted) over a business card or plastic shim stock to stiffen it. I've never used air, out of fear I'll just blow a chunk inside where it will come back later to haunt me.

In addition to the gap, look over the coil with a magnifier. I once had a thickened spot of coil varnish that rubbed.

**Panomaniac** · April 15th, 2010, 07:03 PM

Good advice guys. Will do tomorrow. Thanks!

**Earl K** · April 21st, 2010, 08:08 AM

Pano,

How goes your battle in centering up your new 288 diaphragms ?

Did you contact GPA to ask about obtaining those missing dowels ?

<> cheers EarlK

**Panomaniac** · April 22nd, 2010, 07:03 AM

Hey Earl, thanks for asking. Got really busy and didn't follow up.

I think I made pretty good progress. I put the horn on the floor and pointed the back of the fram up. Just right for working on it. Miked the back of the fram. Seems like it's good.

Two or 3 things I do want to check.

1) Run a feeler gauge around the gap to see if it's centered. It may not be.
2) Measure the thickness of the voice coil to find how much clearance there really is in the gap. Where the lead wires attach to the coil are by far the thickest points.
3) Stack the new fram to the old one - VC to VC. Are both round?

BTW, should have time today to send you that spider shim. As well as shipping A.B.'s driver back to him.