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Kef 105 crossover and peak-level protection
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bawnfree
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PostPosted: Sat May 05, 2012 9:08 am    Post subject: Kef 105 crossover and peak-level protection Reply with quote

Hi, this is my first posting to this forum and I guess I'm in at the deep end.
I have recently acquired a beautiful pair of Kef 105s (SP 1059 - Serial No 4515 A/B) from a daft old BBC geezer who smuggled them out of work in his lunchbox back in the day. They were in excellent nick except for a lack of tweeters. I manageed to source a pair of T52s in "perfect" condition but when I went to wire them in I found that the previous tweeters were wired directly to the HF output of the MF crossover board - bypassing the SP1062 circuit completely.
So....I called the original owner re this and he said that that old HF system was noisy and kept tripping-out on him and to just wire the T52s direct to the MF board,- So I did - and they sound fine - but they shouldn't,- should they? I can understand bypassing that transistorised protection and that horrible fuse and that rotary-switch from hell but don't I at least need C13 and R3 across these tweeters? (I'm looking at an early Mk1 circuit here).
Can anybody point me in the right direction here.?
Much obliged in advance - Denis.
[/b]
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ColinR
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PostPosted: Sat May 05, 2012 1:28 pm    Post subject: Reply with quote

Quote:
the original owner re this and he said that that old HF system was noisy and kept tripping-out on him


Looking at fig6 page6 KEF Topics Vol3 No3, KEF 105s consist of 3 drive units, two crossover boards and an electronic "peak indicator" board.

The peak indicator board will be in parallel with the crossover and should not affect the sound, can be removed and be "jumped" if required.

So how much of the fuse protected mid/treble PCB do you have?
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audiolabtower
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PostPosted: Sat May 05, 2012 2:38 pm    Post subject: Reply with quote

Strange... Colin is right, the Mk2 had an S-Stop circuit which was designed to "cut-out" under overdrive but the Mk 1 just had the led power indicator circuit across the amp output (crossover input) so should not be affecting the sound unless faulty and low impedance for some reason?

But that would not be a reason to take out just the treble crossover circuit? and if connected to the midrange would not have an output above 2.5kHz?

I've run mine for years without the fuses (with three 100W+ amps) with no problems but cannot tell any difference with the led circuit connected or open circuit.

By the way, I don't have a circuit for the Mk1 led indicator, can anyone help?
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speakerguru
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PostPosted: Sun May 06, 2012 12:43 pm    Post subject: Reply with quote

The primary function of the LED was to confirm that the heads were pointing in the right direction. If you could see both lights from where you were sitting, you were in the right "listening window" and not too far off axis. Once that was set up, I thought you could switch it off on one of the switch positions.
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audiolabtower
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PostPosted: Sun May 06, 2012 6:40 pm    Post subject: Reply with quote

speakerguru wrote:
I thought you could switch it off on one of the switch positions.


Absolutely correct, thus no reason to interfere with the tweeter wiring.

Anyone got a circuit of the peak indicator?
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speakerguru
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PostPosted: Sun May 06, 2012 10:52 pm    Post subject: Reply with quote

No, but it's all 70s discrete components. Shouldn't be too difficult to trace it out.
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bawnfree
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PostPosted: Mon May 07, 2012 10:26 am    Post subject: Colin wondered..... Reply with quote

So how much of the fuse protected mid/treble PCB do you have? Good Question Colin,
A couple of photos of the PCB ......Um .No can't do that
OK : PCB is the P 100515 and has vacent locations for C9, C11, C13 and R3 - with all other spaces in use.
There are 5 coils of various size but I can't make out what "L" designation (if any) they have. - Both MF + HF fuses are on this board and both operate correctly.
I will suss-out a good hosting platform (any sggestions) and u/l some photos when I get some time.
Thanks for the interest/replies guys, -Denis,.[/img]
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ColinR
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PostPosted: Mon May 07, 2012 11:05 am    Post subject: Reply with quote

So you have the following parts "missing" Sad
C9 30uF
C11 0.2uF
C13 2.2uF
R3 47R
I think a little bit of shopping is in order.

With the extra components will your PCB look like the one in the KEF Topics photograph?

http://www.kef.com/uploads/files/en/kef_topics/KEFTOPICS_vol3no3_maintaining%20quality%20standards%20in%20production.pdf
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bawnfree
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PostPosted: Mon May 07, 2012 11:32 am    Post subject: Reply with quote

Yep Colin, and would these missing componants happen to have been re-located by Kef onto the (now disconnected) SP 1062 board which is mounted directly onto the "peak-level" switching knob? I can see plenty of passives on the board but have not de-soldered it from the rotary switch yet.
On listening, thw T52 is fine but the overall sound is a bit harsh in the mid-range by comparison with my other speakers (Castle Harlech S1).
So, should I just hook-up the small (SP1062) board but turn the knob to the "Off" position - thus bypassing the transistor/diode section which may have been causing the previous owner's troubles.
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ColinR
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PostPosted: Mon May 07, 2012 12:00 pm    Post subject: Reply with quote

I'd be inclined to relocate the capacitors back to the mid/treble crossover board and disconnect the peak level board.
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audiolabtower
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PostPosted: Mon May 07, 2012 12:56 pm    Post subject: Reply with quote

I believe there is nothing wrong with your crossover, unless caps have drifted or some other fault and this is the reason for your subjective impression.

My versions are 3067A and B and the same components are missing from new. The C9 and C11 are acoustic butterworth caps and the other cap/resistor a zobel across the tweeter. I was told that they were put in depending on the tweeter selection - some with low resonance freq and good damping/rolloff did not need the butterworth components to meet the design spec. The remaining series caps should both be marked 3.3uF with 0.35mH down to ground and a series 0.028mH.

If you put the butterworth caps in then the one series cap will need a different value so not a straight insertion. My series caps were selected at the factory to be 3.5uF and 3.8uF matched across channels so a deliberate design intent.

The difficulty with all of this is the caps were selected at the factory to 1% out of 10% or 20% tolerance components to match the measured drive units spreads so the printed value may not be an accurate indicator.

I'm pretty sure someone had an earlier serial than mine with the butterworth etc components in, so it was a factory decision depending on the drive unit selection.
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bawnfree
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PostPosted: Mon May 07, 2012 2:31 pm    Post subject: Reply with quote

Thanks Colin - Those crossovers as in Fig6 look exactly the ones. I'm going to re-hookup that small board and assume that with the knob in the off position that only the passive componants are in play. (I can definatly make-out C9/30uF as the first componant on this board)
Thanks Audiolabtower, - I have new T52s anyway and no way of getting back to design-stage componant matching and even if I could my trashed ears could never hear the differance anyway but as for cap drift, - Do you think I should replace all the 30yr old slightly pregnant-looking elcaps and if so where is a good source? I'm in the back of beyond here in Dingle so any bits'n'bobs need to be ordered online.
Thanks again guys, I'm getting my head around these beautiful loudspeakers finally. -
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ColinR
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PostPosted: Mon May 07, 2012 3:02 pm    Post subject: Reply with quote

"Locally" there are a couple of vintage semiconductor and IC vendors in Cork they may be able to supply new bits so check your Yellow Pages and ie.farnell.com .

Contact Jerry at www.falconacoustics.co.uk if you want a set of 2% Alcaps to replace your pregnant Elcaps.

For pure diy, google Wilmslow Audio, hificollective, Madisound US, plus various online .fr and .de sites for € pricing.
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audiolabtower
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PostPosted: Mon May 07, 2012 3:07 pm    Post subject: Reply with quote

Colin's given you good info. Yes I would replace the elcaps at this length of time, getting them back near original spec will be well worthwhile if the drivers are still ok. Yes, the off position on the led control should be open circuit.
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bawnfree
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PostPosted: Mon May 07, 2012 3:55 pm    Post subject: Reply with quote

You guys are great. Falcon have a complete +/- 2% set for£60 .
While I'm in there I was thinking of replacing all internal wiring with some heaver gauge ofc left over from my speaker runs and bypassing the fuses with same. My Linn amp only puts out 75W bi-wired to the bass-cab / top-unit and has it's own o/p protection circuitry so I should be safe enough here, Any thoughts?
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