Voice coil overheating 128H (L150)

[Cliff A.]

Hi, I've just discovered the reason that one of my 128H drivers (in L150) makes irritating quiet buzzing noises sometimes. The voicecoil has overheated and cooked the bobbin, causing bubbles in the material that rub slightly on the magnet assembly sometimes. The good driver sounds perfect but maybe I should get the dust cap taken off and look inside that too! Now I have the predicament of deciding what to do. Apparently the re cone kits for the 128H are black these days, so should I buy: 1) 2 new matching black cone kits, 2) 1 new cone, save cash but have odd looking speakers and hope that the other coil is not cooked too. 3) search for white cones somewhere, or 4) ask the repair technician to stick a new voice coil onto the old cone, which will cost plenty for the work time.
What would you guys do? Any good ideas appreciated!
Its a mystery how this overheating has happened, the speakers have not been driven at very high levels in the last 5 years. Amp used was Kenwood 650 which goes to 148W per channel and L150s are rated at 300W.

[Earl K]

Hi ,

Its a mystery how this overheating has happened, the speakers have not been driven at very high levels in the last 5 years. Amp used was Kenwood 650 which goes to 148W per channel and L150s are rated at 300W.

- The "Pe" for the 128H is 100 watts ( rms ).
- Thermal destruction of the voice coil, looks to be around 150 watts ( pink noise / which roughly equates to rms watts ) .
- A "148W" amp has the ability to cook those voice coils ( even if it's actually a 75W rms amp / rated as "150 Program Watts" ) if it's allowed to go into clipping .

1) 2 new matching black cone kits,

- If you truly love these speakers and have the money , this is your best option .

2) 1 new cone, save cash but have odd looking speakers and hope that the other coil is not cooked too.

- If you love these speakers but don't have " all " the funds available , this is your next best option . Do you have grills ? If so, use them . If not, make some .

3) search for white cones somewhere,

- 128H woofs , do come up on eBay from time to time .

or 4) ask the repair technician to stick a new voice coil onto the old cone, which will cost plenty for the work time.

- Definately your worst option .
- Ask your reconer these questions;
(1) How many "turns" of wire make up his replacement coil ?
(2) How many inches (mm) deep is the overall winding of the voicecoil ?
(3) Can he measure and guarantee the BL figure for his "aftermarket" voice coils ?
(4) What's the Re of this replacement coil ?

The answers to these 4 " test " questions are posted in the tech support area of this fourm . If your reconer looks "blankly" at you when you ask him these questions / well then / the answer to this option should be clear .

[Cliff A.]

Thank you very much for your reply. It looks like I might have been regularly baking them in the early years. The amp is 148 Watts sinus wave per channel and although never run into clipping I guess it must've often been over the limit of the drivers. JBL recommended a 300 Watt amp for these speakers in their 80s brochure so I've presumed all was well.
For one month I've been using a McIntosh 250 Watt amp, but never over about 30 - 35 Watts output, it has meters to indicate power level and I don't listen to very loud rock as I did when I was young. The defect was first noticed about 12 months ago, but original damage I suspect was much earlier.
I'll probably have 2 new cones fitted. I've got till next Monday to think about it, as the technician is away in Scotland until then.

[Chris Brown]

Damn that sucks

Is removing the dust cap the only way to check the Voice Coil?

I’ve had my L150’s for a little over 4 years now; they were refoamed at that time. I’ve powered them with a 120wpc Pioneer SX-1050, followed by a 125wpc Fisher RS-1060, and now my 240wpc Yamaha M-2.

The only problem I’ve ever had was a few times where the woofer would make a “pop” sound, which I assume is the voice coil hitting the magnet during high excursion but I’m not sure.

I hope to have my speakers for a long long time but I love playing them loud and my tone controls aren’t exactly flat. :|

Let us know how it turns out

[Anthony L100]

Hi Cliff, although the current replacement cone kit (C8R 128H1) is black you might be lucky enough to get hold of an old stock C8R 128H right here in the UK. I've never dealt with these people but I know they recently updated there website inventory. As they list both variants, it's gotta be worth a call. Good luck

http://www.cimplespares.com/search.p...h&xSearch=128H

[boputnam]

Notwithstanding the cosmetic change, you ought to consider reconing, anyway. Due to age and use, those clearly would benefit from being refurbished. You should be very pleased with the result.

[Don C]

They are not going to keep making the kits forever. I suggest to get them reconed now, while you still can.

[macaroonie]

If your drivers are all original from new you may well find that the cones have sagged due to the softening of the roll surround. You assert that the coil is cooked on one driver, well try this....... mark which way is up on the frame and remove from the cabinet. Put it down cone up and gradually manipulate the cone to emulate the cone excursion. rotate the driver 10 deg at a time and repeat the process. try to initiate the rub and then see if the opposite bias on the cone removes the rub. If you find A pattern to all this then your cone has probably gone off center due to age / gravity. You may also find a distinct piont at which the cone rubs. Try to increase this manually and you may find that the bubble of resin will erode within the gap or there may just be a speck of crud that has decided to mess with you. Quite often you will clear the prob this way. You have nothing to loose here coz the recone guy is going to tear them out anyway. Cones are really quite robust although be careful around the foam if it is aged. If it works put them both back top to bottom. Trust me I'm a semi functional scottish idiot. best o luck.

[Cliff A.]

Hi Cliff, although the current replacement cone kit (C8R 128H1) is black you might be lucky enough to get hold of an old stock C8R 128H right here in the UK. I've never dealt with these people but I know they recently updated there website inventory. As they list both variants, it's gotta be worth a call. Good luck

http://www.cimplespares.com/search.p...h&xSearch=128H

Thanks very much for this link.

And thanks to everyone who encourages me to get both drivers re coned. This looks like the best route.
Although it may be possible to do some other things and save cash - in the long term I want to listen to these speakers for many years to come.

[Cliff A.]

Damn that sucks

Is removing the dust cap the only way to check the Voice Coil?

I’ve had my L150’s for a little over 4 years now; they were refoamed at that time. I’ve powered them with a 120wpc Pioneer SX-1050, followed by a 125wpc Fisher RS-1060, and now my 240wpc Yamaha M-2.

The only problem I’ve ever had was a few times where the woofer would make a “pop” sound, which I assume is the voice coil hitting the magnet during high excursion but I’m not sure.

I hope to have my speakers for a long long time but I love playing them loud and my tone controls aren’t exactly flat. :|

Let us know how it turns out

Well I just got my drivers back from the workshop. For some reason the guy there decided to save the white cones by fitting a new voicecoil (the correct one) to the cone of the defective driver. Don't know why he didn't tell me about it. We'd already agreed that he would do a full recone on both units with the 128H-1 kits that he's got in stock, but .....
The good news is that they sound PERFECT.
The cost for fitting 4 foam surrounds plus one spider, one voicecoil and 2 dustcaps: $740. Thats about the same as the pair of speakers cost me back in '81.

[Cliff A.]

I meant to say something about your popping noise.
I used to get that a few years back when I used to play extra loud dance music. The thing is I think this could damage the voicecoil if it either bottoms out or clears the pole at the other end of the excursion. That might be one reason for the delamination areas on my coil which were right at the bottom of the bobbin. I hope you don't get irritating high pitched noises one day as a result! The best thing is to control the super low frequencies, so a 20 or 30 Hz filter would prevent the excessive excursion and the accompanying popping sound.
I guess that if we want to put more than 150 Watts continuous through L150s then we might be driving them to their limits or beyond. Now I've got output meters on my amp so I can watch whats going on. 50 - 70 watts seems reasonable to me these days, but I've got my sound system in a smaller sized room than I used to have.
If I get the time and the place then one day I'm going to build a nice pair of speakers with JBL 18" bass units.

Cliff A.

[edgewound]

The white cone C8R128H is still available from JBL Pro

Kit price is $176.00usd

[boputnam]

For some reason the guy there decided to save the white cones by fitting a new voicecoil (the correct one) to the cone of the defective driver.

Hey, edge...

What's your opinion of this, vs a full recone...?

Thanks.

[edgewound]

Hey, edge...

What's your opinion of this, vs a full recone...?

Thanks.

Total waste of time and money when the right kits are available...I'd question his motives and ability to get the right parts.

Of course...we're in California, and they're not. Maybe there's an availablity issue overseas.

That repair seems like an awful lot of money for surrounds and dustcaps.

[Cliff A.]

If I hadn't been mis-informed about the parts in the first place it would have been a straight forward recone. The workshop I went to is famous in London and has been doing business for something like 40 years. I thought they might know what they were talking about.
The parts from JBL are all available in England but take 6 weeks to get here and cost from 40% to 80% more than in the USA.
The charge for refoaming one 128H was £65, which is about $120.
Looking on the positive side - at least my speakers sound good again.