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Kef 105.4 crossover refurbish & new bascabinets (p-6)
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willem-57
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PostPosted: Sun Jul 27, 2014 6:30 pm    Post subject: Reply with quote

But my sound of the 105.4 is very good, i have the impression that the T33 is very open and beautiful. I know the sound of the high and mid because i had in the past the CS7 kit, who has the same B110 and T33.

Last edited by willem-57 on Sat Dec 03, 2016 9:58 am; edited 2 times in total
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willem-57
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PostPosted: Mon Jul 28, 2014 8:01 am    Post subject: Reply with quote

audiolabtower wrote:
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.


I measured it on an old and simple analog multimeter, it was 20 or 21 ohm,
so 22 ohm could be.

Answer form Falcon about this matter: Willem, I think it's probably 22R0 ( that's a typical KEF value used in many designs), but let's wait and check to see if KEF can help tomorrow.
I'll do the quote when I hear back from them.
Kind regards
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willem-57
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PostPosted: Mon Jul 28, 2014 6:42 pm    Post subject: Reply with quote

When i removed the burned resistor, underneath was to read: R3
I got an answer today: Willem, hi, KEF have very kindly supplied us a schematic and layout drawing. R3 is 15 ohms.

Good that we first asked Kef. Shocked
By the way, what does that piece of network, it has to do with the tweeter, but what exactly Question
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willem-57
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PostPosted: Tue Jul 29, 2014 9:46 am    Post subject: Reply with quote

I think i found the 15 ohm resistor that i need, including datasheet: http://uk.farnell.com/welwyn/w22-15r-ji/resistor-ww-7w-5-15r/dp/9504575
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audiolabtower
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PostPosted: Tue Jul 29, 2014 10:51 am    Post subject: Reply with quote

willem-57 wrote:
but what exactly Question

The drive unit is an inductor, and simple crossover equations assume a resistor as the load. By putting a resistor in series with a capacitor across the tweeter it compensates for the inductor giving a more flat impedance above the resonance frequency. It is not always necessary, but sometimes cheaper than other crossover components to get the design response.
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audiolabtower
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PostPosted: Tue Jul 29, 2014 10:54 am    Post subject: Reply with quote

willem-57 wrote:
R3 is 15 ohms.

Hmm, I wonder if there was a production change along the way? The burnt out numbers (under the W22) do look like 22 in your picture.
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willem-57
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PostPosted: Tue Jul 29, 2014 6:53 pm    Post subject: Reply with quote

Yes, i think you can be right, but in the Kef lay out of the SP1129 crossover R3 is 15 ohm's ..... Rolling Eyes

Another question: what happends with a resistor that has been overheated, does it in- or decrease in value, or does nothing chance?
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audiolabtower
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PostPosted: Tue Jul 29, 2014 8:12 pm    Post subject: Reply with quote

These are wirewound, so unless they fail open circuit they would not drift much, I have seen burnt ones in eg valve guitar amps that were still in spec. The look of them after 35 years is not even that strange, they can go white powdery just from atmospheric oxidation. What is strange is the heat spot on the board - that is wrong. If you were replacing them in an amp I would advise to solder them in a couple of cm off the board, so air is all around for dissipation. But on a speaker board that should not be necessary since you would assume that Kef rated them well within the design voltage input of the drive unit, even under continuous use.
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willem-57
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PostPosted: Tue Jul 29, 2014 8:38 pm    Post subject: Reply with quote

There was some white powder that i brushed away, before i photografed them. When i solder them new, i will give them a little more space around so that they can cool better.

By the way, Jerry from Falcon also advised to give them space from the board.
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willem-57
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PostPosted: Tue Jul 29, 2014 9:36 pm    Post subject: Reply with quote

Ok, i'm ready to order, there is a difference, the 150V are not made anymore, that will be now 100V, and the capacitators will be matched pairs.

C1 - 120 MFD 150v become 100 v
C2 - 120 MFD 150v become 100
C3 - 120 MFD 150v become 100
C4 - 120 MFD 150v become 100
C5 - 72 MFD 150v become 100
C6 - 30 MFD 150v become 100
C7 7.0 MFD 50v LowLoss
C8 - 50 MFD 50v
C9 - 12 MFD 150v become 100
C10 3.3 MFD 50v LowLoss
C11 4.2 MFD 50v LowLoss
C12 4.2 MFD 50v LowLoss

R3 15R0/5/W22



Last edited by willem-57 on Wed Jul 30, 2014 7:57 am; edited 5 times in total
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speakerguru
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PostPosted: Tue Jul 29, 2014 9:48 pm    Post subject: Reply with quote

willem-57 wrote:
..., Jerry from Falcon also advised to give them space from the board.

Good idea. Use ceramic or ptfe spacers on the resistor leadout wires.

When I designed the Model 1,2,3,4 and "Maidstone 109" XO pcbs I had the board under the power resistors routed away completely to increase ventilation.
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willem-57
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PostPosted: Tue Jul 29, 2014 10:25 pm    Post subject: Reply with quote

Good idea, i shall also try to give R3 more space by putting C9 as far down as possible, making the space between C9 and C12 bigger.

Is there a problem that the 150v type's becoming 100v types Question
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speakerguru
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PostPosted: Wed Jul 30, 2014 7:43 am    Post subject: Reply with quote

willem-57 wrote:
Is there a problem that the 150v type's becoming 100v types Question


Only if you use an amplifier large enough to produce peak voltages >100V and you regularly use its full output capability.
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willem-57
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PostPosted: Wed Jul 30, 2014 8:03 am    Post subject: Reply with quote

I got an 20 year old Accuphase E405, 2 x 170W RMS 8ohm load, 2 x 250W RMS 4 ohm load, 2 x 350w RMS 2 ohm's load. http://www.accuphase.com/cat/e-405en.pdf

I never play very loud.
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willem-57
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PostPosted: Wed Jul 30, 2014 8:52 am    Post subject: Reply with quote

By the way, found a topic of another guy who has recapped his 105.4 crossover with Alcap, he is very pleased with it. If you look carrefully you can see that his crossover is a little different than mine, mine has 4 x 120MDF ( C1-2-3-4) He has 3 x 120 MDF and 1 x 80 MDF, another SP number, i think Question
http://www.audiokarma.org/forums/showthread.php?t=576854
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