crooked magnet- jbl k130

[crumb]

hello everyone, you have a very excellent forum.

today i have a question regarding a crooked magnet on a k130.
I had to take the dust cap off this thing as i could hear a loud buzz every time i played a 'g' note on my guitar.
i blew the voice coil gap clean with an air nozzle, and my problem was gone.
while I was doing this, I noticed that the magnet was not exactly aligned under the top plate. I estimate that it is out by two or three mill.
Wary of bent frames and bad shipping practices, i carefully checked the frame for any warpage, i carefully measured the speaker on top of a flat machined surface, and it is flat and straight, unbelievably so actually.
The speaker has been reconed, and is 4 ohms. This work was done in 1990.
It sounds really good (now that buzz is gone) and im wondering if this thing must have recieved a knock, or whether the guy on the production line just slapped it in there crooked.
Another observation..the speaker has more highs and sounds more transperant with the dust cap off. I know this sounds erroneous, but im sure of it.

regards
crumb

[speakerdave]

Hi,

If I understand what you are looking at, the offcenterdness you are seeing could be due to the cone and voice coil being out of position as much as it could be the pole piece, and I think it is more likely that, given that it has been reconed.

In any case, you seem to have found a way to be able to use it the way it is. The pole piece can be recentered, but I believe the motor would need to be demagnetized first, and then remagnetized when the work is done. I don't know if this can be done with the cone and voice coil in place--I would doubt it.

As to the subjective change in sound--I wonder if you are hearing a diffraction resonance across the voice coil former.

David

[boputnam]

The pole piece can be recentered, but I believe the motor would need to be demagnetized first...Yes it can, and no it doesn't. A qualified reconer can do this (but they don't give-up their secrets easily... :scold: ).


I don't know if this can be done with the cone and voice coil in place--I would doubt it.No, it cannot - you are correct.

The motors on vintage JBL transducers are not bolted, they are merely glued, in-place. Over time, that glue becomes brittle and any sharp knock/drop can result in the motor being dislodged off-axis. As a matter of fact, the magnetic pull wants the motor to be anything but centered... :p

For repair, the motor is disassembled - the magnet is removed from the plate. Shims are used to re-center the pole piece in the gap and held there while the new glue sets. This is tricky. Then, the basket must be reconed.

cf. Re-aligning the pole piece

To the sound difference, I attribute the all-powerful psychoacoustics... ;)

[edgewound]

Alnico motors are held in place by the sheer strength of the magnet. the later ferrites are glued together.

It's highly unlikely that the alnico slug being out of position by 2 or 3 mills will cause any problems....especially in a guitar speaker.

To realign the motor, it would have to be pried apart....the easy way would be to demag it first. If you somehow pry and muscle it apart the magnetic charge will go away significantly with out machine demagging. The the parts can be manuevered with a mallet. then it would have to be recharged.

The the of the K130 with the aluminum dome removed will be different...but describing it could be seen as subjective.

But....I wouldn't lose too much sleep over it.

[boputnam]

To realign the motor, it would have to be pried apart.... If you somehow pry and muscle it apart the magnetic charge will go away significantly without machine demagging. Then the parts can be manuevered with a mallet. Then it would have to be recharged.I've not mussed with AlNiCo motors, but the ferrite was indeed able to be pulled apart (see that "realigning the pole piece" thread...) for cleaning and prep.

Once apart, it is damned difficult to re-assemble as the magnetism did not dissipate / lessen to any degree by disassembly, and the motor keeps trying to glom onto the plate. Very focussed forces! Watch your fingers...

The difficult part, as you know, is getting the shims into place and re-establishing a centered pole piece. Tedious and frustrating, but it can be done.

[crumb]

hello again,

pls note here that the coil cover is paper, not aluminium.
my query was only for the purposes of understanding, this speaker works too well to be further tampered with.
the cone has 58057 stamped on it, and has the cloth surrounds.
i understand about costing and so forth and so on, but it seems to me that the magnet could have been held in place by a better moulded magnet cover.
The speaker was designed after all for guitar cabinets which are bound to be thrown around.
Just looking at this speaker, i cant help but be amazed at the quality of it.(apart from the floating magnet)

[subwoof]

There is actually a small recess in some versions of the iron "cup" that surrounds the magnet to allow it to be centered. When these magnets shift, it's ALL the way to one side of the gap ( the opposite magnetic polarity ) and binds the coil. It's never half way.

I have seen this apparent "shift" on a number of speakers and 99% of the time it is NOT a shifted magnet but an offset caused by the cone/coil/former/etc ( the kit ) being slightly tilted due to paper warpage or glue slippage.

If you set the speaker face up on a table and get down and look from one side, you might see a "tilt" on the whole cone or just on the spider. This is a VERY light kit with VERY thin paper and it is easily "warped"

If it works OK, Have a service center put a new dustcap back on and play on. This is NOT worth all the mucking around and unless you drive this cone *backwards* to full excursion ( which is damn rear impossible with a standard guitar amp ), there will not be a mechanical conflict.

sub

[crumb]

nah, its crooked.
imagine the dust cover missing. you are looking straight down at the speaker.
there is a hole, about one inch, in the centre of the top plate.
underneath this top plate, the magnet resides. you can clearly see that the magnet has either dislodged, or was put in crooked.
the magnet is flush with one side of the hole in the top plate, and 180 degrees away it lies about three mill 'in'.
i put the dust cover back on myself, and by methuslah i swear the really high clear tops have gone to tasmania (thats south here).
you know, i dont think its shifted at all. i think it was put together like that.
perhaps this 'flaw' will give me a unique tone, i really need all the help i can get regarding my playing.

regards
crumb