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105 mk 1 crossover repair

 
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lt2525
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Joined: 14 Oct 2009
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Location: Canada

PostPosted: Sat Nov 06, 2010 9:32 pm    Post subject: 105 mk 1 crossover repair Reply with quote

I am recovering from an unfortunate sound system mishap. ('Sad')
I think the original cause was a floor standing lamp with a metal base which has that touch control (touch the metal and it turns on/off) which came in contact with the metal chasis of one of my power amps.('Embarassed')

Lots of blown fuses in amps and speakers which have all been replaced.

Now looking at one KEF 105 -1 speaker that has a problem. Both the LF fuse and the HF fuse had blown. One capacitor in the HF crossover was shorted out (C10 which is 3.3 uF). That can be replaced [actually might be the incentive to recap the system].

The thing that has me really concerned though is that one of the inductors looks pretty fried. This would be L7 which I believe based on threads elsewhere on this board is rated at 358 uF, 0.34 ohms. How does one go about finding a replacement for something like this? I have done some searching on the web and not really seen anything that might serve. Any one with any thoughts?
('Question')

Maybe I should try and rescue the ferrite core and rewind it? Any hints on what type of wire might work? Number of turns necessary?

Any input appreciated.

Thanks
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audiolabtower
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PostPosted: Sat Nov 06, 2010 11:14 pm    Post subject: Reply with quote

If it's the parallel coil in the tweeter crossover its 0.35mH (milli Henry) plus/minus 2.5%, but the selection value depended on the actual measured value of the first 3.3uF capacitor in the tweeter section. (KEF even wound to suit in the Reference series). The values were all measured and selected and then matched to the measured drive units by computer, for example my "3.3uF" actually measured 3.5uF matched to less than 1% between channels.

I would unsolder one end of the coil and see what a multimeter says.

If you do replace the caps without measurements the response will almost certainly not be as flat as it was when they left the factory, but if replacing with solid cores will need something around 1 ohm in series with the tweeter and around 0.5 ohm in series with the midrange to avoid unbalancing the system too much.

Bad luck and commiserations on the earthing accident Shocked
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lt2525
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Joined: 14 Oct 2009
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Location: Canada

PostPosted: Sun Nov 07, 2010 1:03 am    Post subject: Reply with quote

Well I think the capacitor values have changed over the years, for example, I did measure the first 3.3 uF cap in the tweeter section, and I read 5.2 uF.
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ColinR
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PostPosted: Sun Nov 07, 2010 12:14 pm    Post subject: Reply with quote

Ask Jerry at www.falconacoustics.co.uk Very Happy .
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lt2525
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PostPosted: Sun Nov 07, 2010 9:02 pm    Post subject: Reply with quote

That's a great source... would there be anyone like that on this side of the pond?
Question
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Lee in Montreal
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PostPosted: Sun Nov 07, 2010 10:46 pm    Post subject: Reply with quote

lt2525 wrote:
That's a great source... would there be anyone like that on this side of the pond?
Question


www.solen.ca for high quality caps. But beware. Better quality doesn't necessarely mean better sound as there's more than just capapcitance to a cap... Using more efficient caps, means reducing the resisitance in the system and unbalance will follow. Most likely, at least the medium channel will need recalibrating.
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speakerguru
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PostPosted: Mon Nov 08, 2010 11:56 am    Post subject: Re: 105 mk 1 crossover repair Reply with quote

lt2525 wrote:
The thing that has me really concerned though is that one of the inductors looks pretty fried.
Maybe I should try and rescue the ferrite core and rewind it? Any hints on what type of wire might work? Number of turns necessary?


If the wire has not fused, then you could carefully unwind it and count the number of turns. +/- half a turn, over the same winding length on the core, should be OK.

You could also use calipers to measure the original wire diameter. Do it at the ends where the wire was hopefully not so overheated. Try to get hold of self-fluxing enammelled copper wire. (quoted wire diameter is the copper diameter, not including the insulating enamel). You'll need to get the exact same (metric) wire guage in order to end up with the correct inductance and resistance..
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