Good work Coen! Waiting for the new graphs
@ Earl, I will logon DIYaudio to get the cookie.
Just a little note from the centering activity of tonight.
The 288-Ks don't have the dowel pins. There are the two little bolts that have an oversized washer around them that is part of the diaphragm. I think the adea is to get audibly rid of the nasty resonance caused by the scraping of the voicecoil against the sides of the gap and then fix um.
I had a little rattle on one unit (the measured one) and the other is 'totally' clean. 200Hz at 2V works fine, you have a lot of movement in the diaphragm and any misalignment is easily observed (heard).
It was a hell of a job to get rid of the rattle, there is only one position that the rattle disappears (almost). I have the impression that the gap isn't evenly distributed, iow the phaseplug isn't totally in the centre. I have to measure to be shure.
Sorry no measurements yet of the recentered unit with the horn in place, next week more results in this theatre.
At last, I checked the other 802 and it exibited a higher distortion. After a lot of searching it turned out that the metal strips on the diaphragm (where the Voice Coil is soldered to) stuck out a little. I carfeully scraped the excess away with a watchmakers tool. Putting it back it didn't work out completely satisfactory. A definite improvement yet I discovered that I had to play with the pressure on the fixing screws to make distortion disappear. Thia was also the case with the better diaphragm of the other unit when inserted this one, so this is an imperfection of the unit itself (I should check the gap here too!).
A very instructive night!
Coen
Good work Coen! Waiting for the new graphs
When you go Back to the 288's.You should really try fixing/replacing the locating dowel pins on those 288-8K's.As Pano & GM have said you need those to center the diaphragm easily and in a timely fashion.Once you have the pins you will make quick work of it before it drives you crazy or just takes too much of your time.. My point is it can become tedious..But your doing a great job ! Keep up the great work Coen.
Fitting old drill bits in the holes and then cutting them to size is a great idea as GM posted.If not GPA should be able to provide them for you.
Sheesh AB,Originally Posted by jOHN
Back to the 288's.You should really work on fixing the locating dowel pins on those 288-8K's.As Pano & GM have said you need those to center the diaphragm easily and in a timely fashion.Once you have the pins you will make quick work of it before it drives you crazy or just takes too much of your time.. My point is it can become tedious..
Fitting old drill bits in the holes and then cutting them to size is a great idea as GM posted.If not GPA should be able to provide them for you.
The newer series 288s include a 2-piece dowel system ( which includes a hollow over-sized sleeve & a small bolt that holds that sleeve in place ) .
It is not, a solid metal pin that can be simply replaced by a small piece of metal rod ( cut to length and stuck in the threaded hole as you & GM suggest ) .
BTW, Pano is missing the 2 over-sized sleeves that are snugged-up inside the 2 locator holes within the diaphragm & held there by the set-screw . ( Look at the picture I posted of Panos open-backed driver to see the set-screws ) .
The center "hole" in the sleeve is larger than the diameter of set-screw, essentially allowing the sleeve to "float" or slop around a bit ( when the set-screw is loose ). This "play" when used in conjunction with the spider shim is what allows for an accurate fitup in the new system .
Coen's description of what he has seen in his 288s suggests he might have a fully functional ( adjustable dowel ) system ( compare to the pic above ) .
He simply needs to read the instructions on how to use it ( along with the included spider shims coming from GPA ) .
DownLoad & Read ;Altec Lg-Format Driver Diaphragm Replacement Instructions
Just to be clear, I was responding solely to Pano's post and did not say nor imply cutting the drill bits to suit [though of course one can do so when appropriate if they have the means to cut such a hard material!], but merely to use them to quickly align the VC for fine tuning as I've found that while the positioning may be geometrically correct, it isn't necessarily the best performing location IME with small format drivers.
Note too that my Altec large format driver 'hands on' experience is limited to just the 288B and even that was at a time when I assumed that the factory assembly was already 'optimized', so in retrospect shouldn't have posted on this thread and have deleted my misleading post.
GM
Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
Thank-you GM ! for deleting your misleading post ( now maybe AB will follow suit ) .
Since Coen is going to use drivers for home listening, once he determines and fixes position of the diaphragms, the drivers will be good for the next few decades. So, adjustable dowels, although nice and convenient detail, are not indispensable in this case.
Are you kidding ? I was only talking as a temporary means to align the diaphragm while he is tighten the diaphragm screws to help keep the diaphragm centered and from shifting.My advice as you know is to always Contact GPA first for obvious missing parts.! As I will always recommend buying original replacement Altec parts first... Bill/GPA I'm sure probably has these laying around. At the end of my post I said to call GPA for the missing pins...If not GPA should be able to provide them for you
BTW - As I previously posted I think Coen is doing pretty darn well himself IMHO ! Keep up the Good Work Coen ! Just keep measuring everything you can,practice will make perfect.I'm pretty sure you have a good handle on things.
On mine, it was the result of an impact, and the site was clearly externally noticable.
Of course, if it impacted the cover, the cover could be switched around, but only in so many possible orientations.
But of course maybe there are other reasons, pole pieces shift? idk.
* Ahh - and editing. I see you mention the phase plug specifically. I am sure we have seen mention of them coming loose/regluing in other posts, but I have not experienced this myself.
Well they were hesitant to send me some, so suggested I try the 200Hz tone. If that didn't work, I think they'll see some. I'd be happy to buy a set and pay all postage, so I'll let GPA know that. I really don't know if they have a bunch of them hanging around. I think they were afraid of charging me too much postage for a few little pins.
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