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Kef 105.4 crossover refurbish & new bascabinets (p-6)
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willem-57
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PostPosted: Tue Jul 22, 2014 5:52 pm    Post subject: Kef 105.4 crossover refurbish & new bascabinets (p-6) Reply with quote

As you know i bought few weeks ago a set 105.4 of 30+ years old. The set is in very good condition and sounds awesome.
I read on several sites that the crossover filter should be recapped or rebuild? What should i do, i mean, the sound is very good and i cleaned the contacts with a substance that is called KONTAK electrical cleaner

Here's a site from a store nearby that is selling quality products, they also rebuild Kef filters. http://www.plsaudio.nl/index.php/Modificatie-KEF-Cresta/Bekijk-alle-producten.html

And the cap's, they have several quality's, whitch is the best for the 105.4 filter: http://www.plsaudio.nl/index.php/Condensatoren/Bekijk-alle-producten.html

Or should i do nothing and just enjoy?


Last edited by willem-57 on Sat Apr 11, 2015 2:32 pm; edited 5 times in total
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speakerguru
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PostPosted: Tue Jul 22, 2014 6:31 pm    Post subject: Reply with quote

As we say in the UK; if it ain't broke - don't fix it.

Capacitors can deteriorate with age but they can also last for a very long time. I would wait for some obvious change in sound quality or difference between the pair before changing any components. You would need to identify which capacitors are "regular" and which are "low loss" when replacement time eventually comes around. I think low loss ones ELCAPS of that era were marked "LL".
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willem-57
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PostPosted: Tue Jul 22, 2014 9:34 pm    Post subject: Reply with quote

Thank you again, i will leave it and if anything chances i will rebuild.

Today i took the bass chamber, and made it 100% airtight with new draftband and silicone. I also did some cosmetic surcery, i'm very happy with these speakers, very good sounding and a real good match with my ampiflier.





Last edited by willem-57 on Fri Aug 01, 2014 3:36 pm; edited 1 time in total
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willem-57
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PostPosted: Sat Jul 26, 2014 4:23 pm    Post subject: Reply with quote

Today i had contact with the refurbish shop about my 105.4 crossover and he told me that the value's of some capacitators are quit high. So they become very big because he use's MKP-QS capacitators, there mutch bigger as the Kef standard ones, so i have to figger out how we can do it, because some will not fit on the standard lay-out board. Also i found out witch crossover i have, kef made different ones for the 105.4, i have SP 1129. Or is there another brand whith are smaller? Still don, t know or i really gonna do it.

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audiolabtower
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PostPosted: Sat Jul 26, 2014 7:38 pm    Post subject: Reply with quote

Modern crossovers use polypropylene or polyester capacitors. These are more stable than electrolytics long term but have much less of a loss factor (and are much bigger and expensive for the same value). The loss factor of the electrolytics was taken into account by the Kef designers (tweeter electrolytics were higher cost lower loss than normal generally), so replacing electrolytics with polyprops generally require a power resistor of e.g. around 1 ohm total in series with the drive unit to maintain the design response. Otherwise the crossover could well end up with a peak and not flat.

The other problem is original value of uF. Kef selected the reference series caps to batches of 1% tolerance and matched drive units, inductors and caps to give a "centre response" by pairing a high inductor with a low capacitor to get the centre. So if a cap has 3.3 uF written on it, it could deliberately be 5 or 7% higher or lower than that to compensate for the rest of the circuit.
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willem-57
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PostPosted: Sat Jul 26, 2014 8:04 pm    Post subject: Reply with quote

Thx for your reply, only i'm not e real technic man, so your story i only understand partly, my english is reasonable, but technical talk is difficult to understand.
The filters where designed as a whole, they measured everything and paired? Well in that case it becomes difficult to replace the capacitators.


But is there a brand that i can use, or can i buy the same ELCAPS new? Than my problem is solved. Off course their will always be a small difference with new components.
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willem-57
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PostPosted: Sun Jul 27, 2014 8:43 am    Post subject: Reply with quote

I,ve read this topic about recapping an RR104: http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1203
And it is not easy to do it the right way. I think i must leave it for now, the 105's playing very well. What i maybe can do is buy the right alcap's for the future. I, ve have send a mail to Falcon, witch i found on the web.
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speakerguru
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PostPosted: Sun Jul 27, 2014 10:23 am    Post subject: Reply with quote

audiolabtower wrote:
The loss factor of the electrolytics was taken into account by the Kef designers (tweeter electrolytics were higher cost lower loss than normal generally), so replacing electrolytics with polyprops generally require a power resistor of e.g. around 1 ohm total in series with the drive unit to maintain the design response. Otherwise the crossover could well end up with a peak and not flat.

The other problem is original value of uF. Kef selected the reference series caps to batches of 1% tolerance and matched drive units, inductors and caps to give a "centre response" by pairing a high inductor with a low capacitor to get the centre. So if a cap has 3.3 uF written on it, it could deliberately be 5 or 7% higher or lower than that to compensate for the rest of the circuit.

This is true and very important. It could be that all your inductors are +10% of their nominal values and the capacitors all -10% (although things were never as simple as that). Without the original KEF drawing which contained both schematic and tolerance matching chart the only way to recreate the original would be to use a simulator.
If you put the original crossover with nominal values into a simulator with the correct driver equivalent circuit as loads, then you could calculate the nominal terminal volts for each driver. You could then change the inductors in the simulator circuit to the measured values from your speakers and then see what value capacitors were needed to produce the nearest terminal volt response to the"nominal" one. Quite a project, I think.
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willem-57
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PostPosted: Sun Jul 27, 2014 10:28 am    Post subject: Reply with quote

I mailed Falcon and got response inmediatly, recapping is no problem but he need's the value's. So i got the crossover out again, this time from the other speaker, and saw a hot spot on the back of the print. It is from a small item between 2 capacitators witch i can,t read the valeu.

Last edited by willem-57 on Sat Dec 03, 2016 9:56 am; edited 1 time in total
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willem-57
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PostPosted: Sun Jul 27, 2014 11:03 am    Post subject: Reply with quote

speakerguru wrote:
audiolabtower wrote:
The loss factor of the electrolytics was taken into account by the Kef designers (tweeter electrolytics were higher cost lower loss than normal generally), so replacing electrolytics with polyprops generally require a power resistor of e.g. around 1 ohm total in series with the drive unit to maintain the design response. Otherwise the crossover could well end up with a peak and not flat.

The other problem is original value of uF. Kef selected the reference series caps to batches of 1% tolerance and matched drive units, inductors and caps to give a "centre response" by pairing a high inductor with a low capacitor to get the centre. So if a cap has 3.3 uF written on it, it could deliberately be 5 or 7% higher or lower than that to compensate for the rest of the circuit.

This is true and very important. It could be that all your inductors are +10% of their nominal values and the capacitors all -10% (although things were never as simple as that). Without the original KEF drawing which contained both schematic and tolerance matching chart the only way to recreate the original would be to use a simulator.
If you put the original crossover with nominal values into a simulator with the correct driver equivalent circuit as loads, then you could calculate the nominal terminal volts for each driver. You could then change the inductors in the simulator circuit to the measured values from your speakers and then see what value capacitors were needed to produce the nearest terminal volt response to the"nominal" one. Quite a project, I think.


Indeed, undoweable for me. I have not the knowledge for it.
I think i will leave the filter but buy reserve capacitators from alcap, so when the need is there i can restore the crossover, of course, as descripted above, it will be a bit different.
Thank you for helping me out Exclamation
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speakerguru
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PostPosted: Sun Jul 27, 2014 12:05 pm    Post subject: Reply with quote

willem-57 wrote:
... saw a hot spot on the back of the print. It is from a small item between 2 capacitators witch i can,t read the valeu. It has been very hot, who know's what this is?.....

The overheated component is a 5W wirewound resistor, in the zobel network in parallel with the tweeter. I do not know the value. If it is not open circuit it is probably OK. Wirewound resistors are pretty stable until they fuse. If the tweeter has not burnt out, the most likely cause of overheating is high frequency oscillation of the amplifier. If the resistor has been overheated for some time, check the value of both the capacitors close to it. They may have dried out by the heat from the resistor.
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willem-57
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PostPosted: Sun Jul 27, 2014 1:12 pm    Post subject: Reply with quote

Thx, i,ve got an answer of the value tomorrow, Jerry from falcon will contact Kef about it. I already measured and photografed it, 20 ohm,s.

Answer from Falcon: Willem, W22 is a Welwyn Resistor Range type. The 5% is what I would expect the tolerance to be. I suggest we try KEF tomorrow for you, it probably is 20 ohms but best to check.

Fine you al help. Very Happy
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willem-57
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PostPosted: Sun Jul 27, 2014 1:22 pm    Post subject: Reply with quote

Here the pictures i took from a resistor:



Last edited by willem-57 on Sat Dec 03, 2016 9:57 am; edited 2 times in total
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speakerguru
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PostPosted: Sun Jul 27, 2014 5:21 pm    Post subject: Reply with quote

That looks fried as well. Must have been a lot of HF output from an amplifier at some point in the past.

Under the W22, it looks like 22R to me, which would make it a 22 Ohm resistor (much more common than 20 Ohm). If you can get the correct value from KEF via Falcon so much the better.
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audiolabtower
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PostPosted: Sun Jul 27, 2014 5:54 pm    Post subject: Reply with quote

The 105s with T52 tweeters had 47 ohm in the zobel network across the tweeter, so the 105.4s with the T33s? almost certainly would have been the 22 ohm mentioned, sounds about right.
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