advertisement


Kef KM1 in need of repair.

Paul Marchant said:
And so it begins. They have arrived with me.

First impressions are better than I feared. There's no voice-coil rub on the mids. The tweeter domes are indeed undamaged. Woodwork is a little scraped, but not deeply gouged. They appear complete.

I'm feeling optimistic about this. I've tracked down one of the original designers , and as long as my luck holds with the forum he posts on a lot, I hope to be able to ask him some useful questions (types of adhesive, types of ferrofluid etc). I'm awaiting a reply from him.

https://imgur.com/a/O9F3oIo
Click to expand...

I didn't know they came apart. Cool.

Is that a drawer under the mids?
 
Update:

OK. So, I've had a little bit of time to see where I'm at with the drivers, and it's looking promising.

All the bass drivers show a very similar DC resistance, the mids are all the same but one is a little higher.

The two tweeters are the same as each other.

I bought a really cheap and nasty battery-powered amp off Amazon for this moment, and I've put tone and program (music) into each driver.

The mids all seem to be OK, and sound equally loud.

The tweeters seem to be OK, and sound equally loud.

One bass driver was lacking LF, and upon giving it a poke it seems the cone has sagged and the voice coil's rubbing on the magnet. I'm reasonably optimistic that rotating the driver through 120' will get me out of trouble there.

Curiously, after putting music (at perhaps 1/2 a watt) through all the drivers, when I re-measured the DC resistance of the mids, the one that was reading a bit high was closer to the others. I think it's perhaps a combination of tarnished pins on the plug, kneeling on the floor in a very confined space and trying to hold the plug and the meter probes with one hand whilst using the other to lean on a bass cab to keep my balance, in other words, I think - leaving aside the centre domes and whatever ferrofluid fun lies ahead, the drivers will be OK.

I only had a few minutes, so I've done nothing more, but I'm greatly encouraged by the results of my half-baked prelimenary tests.

Next thing to do is to get the mid / HF enclosures out, and on my bench where I have proper amplification, an Audio Precision test set and somewhere a decent measurement microphone. I'll do a fairly high-powered frequency sweep on all the drivers in case there are rattles or other things I didn't hear today.

But... so far... it's looking promising.
 
Nero said:
That’s a lovely looking bit of kit
Click to expand...

It's quite dense in terms of space around the boards, and nothings on unpluggable wiring. That sort of thing is a bit miserable to work on. That said, I've fixed much worse stuff, and it's just one more step in the journey to getting everything working.
 
In other news, I've been doing some digging about where some pairs of KM1s currently are to be found.

Of the three sets of BBC KM1s, I've got one of the black pairs.


The Science Museum have got the other black pair:

https://collection.sciencemuseumgro...355779/kef-km1-active-loudspeaker-loudspeaker


PMC Loudspeakers have the light ash veneered pair from Golders Green Theatre:

https://www.pmc-speakers.com/pmc-audio-museum


Beyond that, I've exchanged a few messages from a chap on Facebook, who's mentioned that in a studio he's involved with they have a pair. I don't have any precise details on where that is though.
 
robbyd said:
This of any use Paul?

https://www.ebay.co.uk/itm/33270985...d=link&campid=5338728743&toolid=20001&mkevt=1
Click to expand...

I've got a PDF of the circuit diagrams that are in that listing already.

That said, as a collector's item, the whole 'having original blueprints' thing is very attractive, along with a real paper version of the brochure for the KM1.

I'd seen it previously and had a think about it. On the one hand, it's an incredibly high price for what it is. On the other, I'd kinda like to own it.

The conclusion I came to was that if I succeed in getting my KM1s fully working (which I think I will manage), then screw it, if the Ebay listing's still there, I'll pay the price and buy it. But I've got to hold out until I've had the drivers repaired and re-ferrofluid-ed, I think.
 
This site contains affiliate links for which pink fish media may be compensated.
A couple of interesting facts in the brochure. The B110s are polypropylene rather than bextrene which I've not heard of before - was this standard by this period in its development, or unique to these?

Also the tweeter magnets are twice as powerful as the bass unit magnets to make them efficient enough to use a single one in each speaker; cripes.

The ferrofluid cooling in the B110 would have been another surprise had it not been mentioned already.
 
lawrence001 said:
A couple of interesting facts in the brochure. The B110s are polypropylene rather than bextrene which I've not heard of before - was this standard by this period in its development, or unique to these?

Also the tweeter magnets are twice as powerful as the bass unit magnets to make them efficient enough to use a single one in each speaker; cripes.

The ferrofluid cooling in the B110 would have been another surprise had it not been mentioned already.
Click to expand...

The Kef Reference stuff at the time was still using Bextrene for the B110. I am pretty sure by the time the Reference 107 showed up in 1986(?) that had polypropelene B110s.

I remember reading about the magnets in those special T52s and being a bit gobsmacked!
 
Minor progress update:

I've got a litre of MEK solvent on order to facilitate disassembly of the B110's

I've managed to get Ferrotec's UK distributor to sell me a bottle of the correct ferrofluid (79mL for £276 - smallest bottle they sell). It's got to come from the USA, so it'll take a little while to get here.

The amps and mid-range units are now safely hidden away at work. I intend to have a look at the faulty amp one evening in the next week or so to see what's up with it and fix it.
 
Last edited:
When I initially did a quick check of voice-coil resistance the other day (at the pins of the plug for the enclosures), one midrange driver measured a couple of ohms higher than the others. Uh-oh.... But putting test tone through them, one at a time, seemed to suggest it was outputting about the same level. Then, when I re-measured, the resistance seemed closer to what I was expecting, but still an ohm high.

This evening, I've had time (and a suitable working environment) to pull that midrange enclosure apart and measure directly at the solder tags on the drivers themselves.

https://imgur.com/a/FDBmiQM

All is good - the two in that enclosure are within 0.05 ohms of each other. There's a screw-terminal block inside which all the wiring passes through, and the screws weren't particularly tight. I've tickled them all with a screwdriver and now all the resistance measurements from the plug's pins are consistent and the two mids match each other.

Very good news.
 
Paul Marchant said:
When I initially did a quick check of voice-coil resistance the other day (at the pins of the plug for the enclosures), one midrange driver measured a couple of ohms higher than the others. Uh-oh.... But putting test tone through them, one at a time, seemed to suggest it was outputting about the same level. Then, when I re-measured, the resistance seemed closer to what I was expecting, but still an ohm high.

This evening, I've had time (and a suitable working environment) to pull that midrange enclosure apart and measure directly at the solder tags on the drivers themselves.

https://imgur.com/a/FDBmiQM

All is good - the two in that enclosure are within 0.05 ohms of each other. There's a screw-terminal block inside which all the wiring passes through, and the screws weren't particularly tight. I've tickled them all with a screwdriver and now all the resistance measurements from the plug's pins are consistent and the two mids match each other.

Very good news.
Click to expand...

Great news!! .. really jealous now, would have thoroughly enjoyed sorting these speakers out, as clearly you are.
 
Blue Belle said:
Paul, have you any idea further down the line which pre you will use, or other front end options?
Click to expand...

And now I will start the mother of all internet disputes... it'll be something (yet to be decided) inexpensive with a remote control for the volume.

Stand by to hate me forever more for the following words:

My view on this sort of thing, having played with vast quantities of equipment over the years (at home, in my time writing for one of the hi-fi mags, and in my career as a broadcast engineer) is that any half-decent pre-amp is as good as any other. I have similar feelings towards power amps (as long as it's got enough power that you're nowhere near maximum output) and CD players / DACs and the like. I'm not even going to comment on the cables / mains distribution / gold plated fuses etc.

Indeed, if you look at - for example - the Dynaudio M3A's that I have... the amps that Dynaudio supplied with their name on are re-badged Hill Chamelon PA amps. The final version of the M3's came with LabGruppen PA amps for getting on for £20k for the package.

Modern electronics is generally sufficiently good that in a properly conducted blind test, despite what people like to kid themselves, you won't tell stuff apart.

I've embarrassed plenty of 'golden ears' people proving this over the years.

There's a very telling test that Quad did, a number of years ago:

https://www.vintage-radio.net/forum...6a86c80200f7&attachmentid=236023&d=1623919412

which is quite compelling - look at the people who were the test subjects.

And Alan Shaw, of Harbeth loudspeakers made a public challenge that - if anyone could successfully prove him wrong, and identify one amplifier from another purely by listening, he'd give them a pair of any of the loudspeakers his company was selling (upto an including the £5k Monitor 40's):

https://pinkfishmedia.net/forum/thr...ho-can-identify-amplifier-differences.117491/

No-one succeeded.


Loudspeakers are the one thing where the difference is real and obvious, where the difference in price and size absolutely matters. I have a decent collection (Quad ESL63, Dynaudio M3A, B&W Matrix 801, Yamaha NS1000M, Rogers LS5/8 etc) and they are all very different.



I have a hankering to one day get around to building my own pre-amps and power amps (obviously not power amps for the KM1s), but it won't be for a while. It's for no other reason than to be able to say that I did it. There's no justification for the effort and expense it'll require.
 
Blue Belle said:
Great news!! .. really jealous now, would have thoroughly enjoyed sorting these speakers out, as clearly you are.
Click to expand...

It's been a bit of a roller-coaster so far.

Got them delivered, having paid a lot of money. OK... better condition than I expected. Good start.

Was going to buy the two tweeters and two mids off Ebay.com as spares. No drivers or parts are available from anyone to repair these KM1s if a driver is damaged. Someone beat me to it last week. No spare parts available. Uh-oh.

No information available from KEF... uh-oh. But then I tracked down one of the original design team on another forum who's helped me enormously with what I needed to know. Awesome.

Midrange driver voice-coil high resistance on one of the four... oh no, I'm really in trouble now... But it wasn't - it was a loose screw terminal inside the midrange enclosure.

Need to fix one of the amp racks...sigh.. but spotted this evening that the mains voltage selector was mid-position, between 240v and 120v. Didn't have time to check much further, but perhaps this is what the problem was with one amp rack not doing anything. Fixing them would be a pain in the neck, but nothing more. Parts are more-or-less available (by cannibalising a Quad 405-2 off Ebay). I do electronics repair for a living. But perhaps I don't need to fix any electronics here. More details soon...

At risk of incurring bad luck, things are currently looking very positive. The next few weeks will be a bit stressful as far as this project goes but I have a good feeling about it.
 
This site contains affiliate links for which pink fish media may be compensated.
Paul Marchant said:
And now I will start the mother of all internet disputes... it'll be something (yet to be decided) inexpensive with a remote control for the volume.

Stand by to hate me forever more for the following words:

My view on this sort of thing, having played with vast quantities of equipment over the years (at home, in my time writing for one of the hi-fi mags, and in my career as a broadcast engineer) is that any half-decent pre-amp is as good as any other. I have similar feelings towards power amps (as long as it's got enough power that you're nowhere near maximum output) and CD players / DACs and the like. I'm not even going to comment on the cables / mains distribution / gold plated fuses etc.

Indeed, if you look at - for example - the Dynaudio M3A's that I have... the amps that Dynaudio supplied with their name on are re-badged Hill Chamelon PA amps. The final version of the M3's came with LabGruppen PA amps for getting on for £20k for the package.

Modern electronics is generally sufficiently good that in a properly conducted blind test, despite what people like to kid themselves, you won't tell stuff apart.

I've embarrassed plenty of 'golden ears' people proving this over the years.

There's a very telling test that Quad did, a number of years ago:

https://www.vintage-radio.net/forum...6a86c80200f7&attachmentid=236023&d=1623919412

which is quite compelling - look at the people who were the test subjects.

And Alan Shaw, of Harbeth loudspeakers made a public challenge that - if anyone could successfully prove him wrong, and identify one amplifier from another purely by listening, he'd give them a pair of any of the loudspeakers his company was selling (upto an including the £5k Monitor 40's):

https://pinkfishmedia.net/forum/thr...ho-can-identify-amplifier-differences.117491/

No-one succeeded.


Loudspeakers are the one thing where the difference is real and obvious, where the difference in price and size absolutely matters. I have a decent collection (Quad ESL63, Dynaudio M3A, B&W Matrix 801, Yamaha NS1000M, Rogers LS5/8 etc) and they are all very different.



I have a hankering to one day get around to building my own pre-amps and power amps (obviously not power amps for the KM1s), but it won't be for a while. It's for no other reason than to be able to say that I did it. There's no justification for the effort and expense it'll require.
Click to expand...

Oh, Paul, that would be fantastic. Give it a shot, we are a long time dead, my friend, and in life it's always better to look back on trying, than regret not trying. As for things being better or not, it's a subjective thing mostly for me, if upgrading and spending more money on 'better' equipment tickles your pickle, then who are we to judge? Do whatever makes you happy, is my belief. It's an interesting argument though, are we hearing better, or just different?
 
Blue Belle said:
Wow.. now that would be a good news day, if the voltage switch was a bit squiffy. :)
Click to expand...

Yeah. Even though I do board repair for a living, I don't particularly relish the thought of having to work on this:

https://drive.google.com/file/d/1ZlBcMyZLyfoFcZnuTQeK-DB3SwyEVV25/view?usp=sharing

A bit crowded inside. No unpluggable wiring to get boards out. Non-standard output connectors which would make assembling a dummy resistive test-load difficult. Unpleasantly heavy to move around from where it is now to a workbench.

It'd be nice if it was just the voltage selector switch.
 
Paul Marchant said:
Yeah. Even though I do board repair for a living, I don't particularly relish the thought of having to work on this:

https://drive.google.com/file/d/1ZlBcMyZLyfoFcZnuTQeK-DB3SwyEVV25/view?usp=sharing

A bit crowded inside. No unpluggable wiring to get boards out. Non-standard output connectors which would make assembling a dummy resistive test-load difficult. Unpleasantly heavy to move around from where it is now to a workbench.

It'd be nice if it was just the voltage selector switch.
Click to expand...

Hmmmmmmm.. looks like a can of worms to me. :D
 
You must log in or register to reply here.


advertisement


Back
Top