Sunday, September 09, 2012

Dusting off the PA since it's going to see some use this fall, and I'd really like to dodge the last minute cram if at all possible.

While I was tooling about that corner of the garage I figured I'd give the pile of dead speakers a more thorough examination so I can start chewing on my plan for them as well. Enter a Cerwin Vega 154FM that was beaten into submission at Eugene Noise Fest 2007. If I had to wager a guess, I would say it was during The Sunken.

Which led me to this observation. Furthermore, clamping a battery lead onto that little sliver of wire confirmed that everything else was intact and functioned as normal.

Upon removal of the dustcap, which is a two layer fabric mesh over aluminum assembly, I was greeted with less than optimal news. Judging from the concentric creases, it would appear as if the tinsel fell out to prevent the voice coil from punching through the cone.

The eyelets that tie the tinsel to the voice coil wires are on the verge of punching through as well, observe the discoloration of the VC lead terminating into the empty eyelet, that'll be explained in a moment.

Due to the weakened cone structure, the voice coil assembly and immediate surrounding area could close up to the aluminum dustcap when the speaker was in it's positive excursion. This effectively shorted the output of the amplifier across the dustcap, producing the spalling seen here and heating the joint up enough to cause the solder to flow, whereupon that connection acted as a fuse and opened up eliminating further damage.

Repairing the electrical connection was easy. Reinforcing the integrity of the cone to avoid repeating the failure, however, resulted in the plastic surgery disaster seen here.

I'm not ashamed of this, it is ugly and I would not endeavour to recreate this, but honestly this driver has been sitting around awaiting a recone for 5 years, and if this botch-work quilt of a fix extends the use of an otherwise trashed object, I'm all for it. I realize that with the pooled glue splints the delicate nuances of whispering sweet lower mids have been pounded into the bog, but since this is a piston being deployed in the 40 to 100 hertz range that wouldn't have come up anyway.


MKULTRA said...

That is a hall of fame repair.

MKULTRA said...

That is a hall of fame repair.