Centering compression driver 'frams by tone.

[Pano]

Need some help centering CD diaphragms.

I have a pair of 291-16K Altec compression drivers for which I have bought new GPA aluminum frams. (288 style)

Unfortunately the 291s do not have the adjustable locater pins. Just a pair of small screws. So even though the new frams came with the spider shim, they are giving me a headache getting them centered. Without the pins, the shim is no use.

I can play a 1KHz tone and look at it on FFT (Arta). As I shift the fram around, the harmonics change. Even tho I can get to a spot with low distortion, or a nice even fall-off, once I start to snug the screws the 3rd harmonic shoots up. I've tried and tried, but every time I tighten even a bit, the distortion shoots up.

Any hints or tips for doing this right. I know some of you guys have practice at this. What worked for you? Thanks!

(P.S. Also posted this on the Altec board. Will post here if the solution is found over there)

[Robh3606]

Hello

Try over on Lansing Heritage as well. There are some smart reconers over there that might be able to help you. I have had problems similar to that but was always able to figure out a screw sequence that worked. I had problems with 2 particular drivers that always were a headache. Did you try calling GPA??

Good Luck

Rob

[CLS]

Adding 2 flat washers under each screw might help.

[Pano]

Hi guys. Thanks. I will try the Lansing Heritage forum.
Did speak to GPA but may have to again. Because I can get low distortion until all the screws are snug. Then 3rd harmonic shoots up. Was not expecting that! And it's audible.

Hmmm.... washers.

[CLS]

Which direction do you think (see) the diaphragm goes when the screws are tightened?

Dose it move along the plane of top plate by the torque (twist) of screws, or move along the axis of VC (up/down)? Or both?

Flat washers help in providing a proper 'slide' between 2 twisting surfaces, so they can be secured with much less twist from the screw or hopefully pure vertical pressure is left. A very thin layer of oil between the 2 washers, or one more nylon washer in between them might be even better in this regard.

If it moves along the axis of VC, then maybe the surround of surround is pressed or distorted and make the VC get out of the gap by some degree. But is that possible?

[Pano]

Hey CLS. It's hard to say. And it seems to be the "last screw" that does it. All others can be tight, but the last one will cause 3rd harmonic distortion to rise when tightened. Drives me nuts!

So perhaps it's the way that the coil sits in the gap. Maybe the fram and coil need to be just slightly off level to be best placed?
I am inclined to put thread lock on that last screw and leave it just barley tight.

[Cal Weldon]

Originally Posted by panomaniac

barley tight.

Does that mean we are going to run out of beer? Sorry pano, couldn't resist.

[Pano]

Not if I "Hop" right to it!

[bear]

you do not want to make all three screws "tight" by tightening one at a time. You need to do the same trick that is used on things like engine heads and other things where the stresses need to be even. So, tighten slowly, rotate around the diaphragm if there are only three screws. For 5, use a star pattern, etc...

this might help.

_-_-bear

[Pano]

Yep, tried that. Used a star pattern. Even got them almost tight and tapped the edge of the fram (plastic rim) into place. But if I try to tighten, things change for the worse.

But here's the rub (excuse the pun). No 2 measurements show me the same thing. :-/
Single sine and swept are different. I'll try to post some of these later. According to the swept or stepped sines, 1K is not my problem area. 3K-5K is.