question about diagnosing speaker problems

[Jonk]

I have a pair of JBL 4412's that have some problems. (very dark and muddy sounding, and one of them distorts at certain frequencies.)

So, I started running some test tones through them.

First of all, the signal is almost non-existent before about 60Hz. It quickly gets stronger after this. But the right speaker distorts until about 120Hz. Now, there's not a lot happening in most music down below 60Hz I know. But I thought I'd mention these symptoms.

The real noticable problem is that both speakers start to roll off sharply between 5K and 6K and by 10K there is virtually no signal at all again.

My question is,

What components do you guys think need to be replaced? The tweeters seem like a possibility. Drivers too?

And the right bass speaker cone? (would the distorting up until about 120 Hz indicate a possible dried out cone?)

???

[Wavebourn]

Originally Posted by Jonk

I have a pair of JBL 4412's that have some problems. (very dark and muddy sounding, and one of them distorts at certain frequencies.)

So, I started running some test tones through them.

First of all, the signal is almost non-existent before about 60Hz. It quickly gets stronger after this. But the right speaker distorts until about 120Hz. Now, there's not a lot happening in most music down below 60Hz I know. But I thought I'd mention these symptoms.

The real noticable problem is that both speakers start to roll off sharply between 5K and 6K and by 10K there is virtually no signal at all again.

My question is,

What components do you guys think need to be replaced? The tweeters seem like a possibility. Drivers too?

And the right bass speaker cone? (would the distorting up until about 120 Hz indicate a possible dried out cone?)

???

Sounds like fried goffer.

[David Kulka]

With bad 2 and 3 way speakers it's very common for more than one driver to be blown. Visually inspect the woofer for a torn or frozen cone, or disintegrating surround. If you have a line level oscillator, you can check mid range drivers with 500 Hz. tones and tweeters with 5k tones, if you're willing pull the drivers out for a test.

Replacement parts may still be available from JBL but repairing these oldies *may* not be worth it, let your conscience be your guide.

[The Byre]

Originally Posted by Jonk

First of all, the signal is almost non-existent before about 60Hz.
???

I do not know these speakers, but this sounds like a design thing. I always feel that you need to be able to hear what is going on at below 60Hz. If there is something there that you do not know about, you could be producing mixes that are loaded with bass rumble without realising it. Adding a sub and an active cross-over at around 100Hz will give you the right freaquency balance.

Originally Posted by Jonk

I And the right bass speaker cone? (would the distorting up until about 120 Hz indicate a possible dried out cone?)

???

Cones do not get 'dried up' but they do tear and rip and this sounds like at is a possibility here. Another possibility is that dirt has entered the winding gap in the magnet and is rubbing between the speaker winding and the magnet. Yet another possibility is that the cone was placed into the maget and cassis assembly incorrectly and is touching the magnet.

You can fix a tear or rip with silicon by rubbing it gently into the tear with you fingers. Use very little as any additional weight will reduce the resonant freaquency of the driver and alter the sound.

If dirt is touching the windings or a construction fault has occured to make the windings touch the magnet, you can hear this when you push the cone up and down (gently!!!) as a slight scratching noise. Sometimes you can hear this without even taking the driver out of the speaker box, but only when it is very accute. This fault can be repaired by either replacing the cone (after you have cleaned out the magnet with Gaffa-Tuck tape!) or replacing the whole driver. JBL should be able to advise you here on which is the best way.

If you do replace the cone, before gluing on the center dome, remember to 'centre' the cone by putting paper between the windings and the magnet. Then glue the cone at the edges with silicon and test it for free movement without it touching the magnet. When the glue has set and it still moves freely without rubbing, glue in the done.

You can test the drivers for function (i.e. are they working at all!) with a multimeter. Each one sould have a nominal resistance of about 6-8 Ohms (the tweeters may be higher, depending on the design) and should produce a slight click when you place the leads to your multimeter. You will need to disconnect at least one lead to each driver to test them this way.

I used to build and repair speakers so if you (or come to that, anyone else with similar problems) have any problems or further questions, you can contact me via email at mail@the-byre.com

[Jonk]

Wow,

Thanks for all the help. That was a bunch of great info. I'll get to opening them up and testing the drivers, and I'll report back.

Jon