2245H foam pole vent cover?????

stole this from another (Altec users) BBS.... thought you guys could help....
http://www.hostboard.com/cgi-bin/ult.../f/3729/t/1771


Can anyone shed some light on JBLs' purpose for using a foam cover on the interior end of the pole vent in a 2245H, in lieu of the more common screen? I am assembling a pair from some E155 baskets and OEM 2245H kits. When I stripped and cleaned the baskets (which both had burnt out coils), only the melted foam where it had been glued to the pole remained. Inspection of an original 2245H through the rear vent screen verified that the foam is installed as a covering for the interior end of the pole vent, just behind the dustcap.
Other than giving an indication that the driver had been overheated at some point, I can see no reason for using foam instead of the standard screen, which they do use on the outer end of the pole vent, unless it's for damping to eliminate 'chuffing' on long excursions. Also I'm concerned that the inevitable deterioration of the foam could cause future problems, by "gumming up" the coil/pole gap. (The lower third of the original burnt coils was black and gummy, with deposits on the pole also).

The open cell foam was used to keep debris out of the voice coil gap...You've discovered that it rots and makes a gummy mess. An Alternative to the foam or screen is: cut a circular peice of double-knit grille-cloth and glue in place of the foam plug. It breaths and filters fine, is quiet and won't rot...Some trivia on older JBL's with the stiff cloth screen: even though it might look fine, after it's stretched out a bit from years of use, it needs to be replaced otherwise it can "oil-can" and make fluttering noises at low frequencies, almost as bad as a coil rub.


Edgewound

thanks a bunch...

and my wife thinks I know too much about antiquated audio gear! I got nothing..... but slowly that is changing....


now if only I can break it to her that I want to quit my day job and tweek speakers and cabinet all day long...... ;) (without being :die: )