SpeakerTalk

[Paul Marchant]

So, some of you might have spotted a pair of KM1 loudspeakers on Ebay recently. I did, having been after a pair of them since I heard them twenty years ago.

I'd hoped to find a fully working pair, but my luck wasn't quite that good.

Not being one with adequate common sense, I've managed to buy them. I've rented a lock-up to keep them in with Big Yellow Storage to avoid being lynched by the rest of my household.

They've had a hard life. The woodwork is scuffed (but not destroyed).

The dust-caps have been pushed in / knocked off of all four midrange drivers and two of the bass drivers.

There is, apparently, an electronics fault with one of the amp-packs.


I fully intend to get these working. It's going to be somewhere between 'difficult and expensive' and 'very difficult and very expensive'.

The woodwork is, in many ways, trivial. I can have a go at it, or there are plenty of 'furniture repair / refurbishment' companies who could doubtless do a good job.

The electronics fault won't be a problem. I've worked as a repair engineer in the TV industry since I left college twenty five years ago, fixing broadcast gear, both audio and video.

The drivers are what I'm worried about the most. When I contacted KEF about them, the best they could do was encouraging words. I've got a guy who has repaired drivers for me in the past, who is well-regarded but doesn't have any information on them and didn't seem super-optimistic. My research has led me to know that the drivers are unique to the KM1s, and the only hope for parts is scouring the internet for someone who's breaking a pair (I've found two mids, two HFs and two amp-packs so far).

I'm aware that the mids and the HF drivers will need a ferrofluid change - a process fraught with risk in the disassembly of the glued joins between parts.

I only had a few seconds to inspect them upon delivery (had to get back to work). Thus far, the positive comments are:

HF drivers are cosmetically undamaged - the domes are fine.
MF drivers don't have coil-rub, nor are they locked up. The surrounds and cones look OK.
The KM1s as a whole appear to be more-or-less complete.
The woodwork isn't as bad as I was expecting.


The unknowns are the bass drivers (I didn't have time even to pull the grilles and look), and the electronics. I haven't had a chance to check voice-coil resistance on any drivers yet.

This is what I'm starting from: https://imgur.com/a/p0hMzW6

This is where I'm at presently: https://imgur.com/y45Vmi2

The storage place is near my place of work, but that's a long way from home (fifty mile drive each way). I've had to spend this weekend sorting out my car's mechanical woes, so I haven't had a chance to do anything with the KM1's yet. I hope to start in the coming week, but I'm realistic that this is going to be a lengthy project. I'll update here as things go.

I am desperate for in-depth technical information on the drivers.
I believe that 'speakerguru' here was involved in their development. There may be other users as well with knowledge.

Speakerguru - if you have the time and interest in this, I have many, many questions I would like to ask of you. Please let me know if you're willing to help.

My hope is to get these loudspeakers back to 100% health, both functionally and cosmetically. My chances are heavily dependent on the knowledge and help of probably only a handful of people.

[Lee in Montreal]

From the pictures, they are not as bad as described

The tweeter's dome can be "sucked out"

Carefully cut all domes from the B110 mid drivers and replace them with new one.

If the B300s have no coil rub, then you only need to glue one dome. But don't forget to rotate each woofer by 120°. B300s are notorious for their sagging Butyl surround.

As for the electronics, you have a whole bunch of QUAD 405s in there. They re simple power amps to work on.
_________________
Current line up
2 sets basic Kef 104/2
2 sets bi-wire Kef 104/2
Plenty of KEF components for a few fun builds

[audiolabtower]

I've never heard of a B300 having sag because the surround is PVC not butyl. My 105s from 1979 have the drive units as good as new and the sound as good as too. That B300 was one of the best drive units Kef ever made because of the BBC cone design plus Kef's high power coil tech of the time.

The Quad 405s have the advantage of not needing any bias setup, if the four feedforward bridge components are wthin their 5% tolerances the distortion and other specs will be met - all other components can be changed for new without any special setup required.

[Lee in Montreal]

[Paul Marchant]

@ the OP - The T52 tweeters don't have Ferro Fluid, so that's one less job to perform. And you can probably push the dome after removing the face plate.

I've read in multiple places (which could all be wrong) that the unique version of the T52 in the KM1 is a ferrofluid tweeter unlike any other T52s.

On a positive note, the T52 domes aren't visibly damaged. It's a mystery whey someone would poke in the mids but not destroy the tweeters but I'm not complaining.

[audiolabtower]

[audiolabtower]

The KM1 tweeter did use ferrofluid.

[Lee in Montreal]

[audiolabtower]

It may be a very sensible recommendation but stll does not warrant the original description applied to the B300.
That would be far more appropriate for the Spendor BC1 driver where the heavy bextrene cone and flexible spider did lead to cone sag in most samples after 10 years or so. The result of the BBC research on that cone to achieve that exemplary mid range colouraton performance led to the cone damping layers and spider the required bass performance. There are samples (mine included) that are still going strong after nearly 50 years but all actually do require rotation to maintain performance.

[speakerguru]

Sorry I don't look at this forum regularly. It has been very quiet lately.

Ask away at any questions re KM1 and I'll try to remember. I was one of the main engineers on the project. As your KM!s are beaten up black ones I would guess they started life in BBC Maida Vale studios 4 and 5. Production ones were Walnut and I think there was also just one light Ash pair made for the BBC Golders Green Theatre. No others AFAIK.

The drivers were stock moving parts apart from the B300 and B110 voice coils which ended up around 5 Ohms in order to maximise the power that could be extracted from a Quad 405 module.

Other variations were:
1. larger magnet and ferrofluid for the T52
2. drilled and tapped B110 backplate for heatsink attachment
3. Stiffer B300 rear suspension to reduce magneto-mechanical rectification on overdrive

[Paul Marchant]

[speakerguru]

1) and 2)KEF at that time only used one type of ferrofluid. It was the same in the T52 and B110
I'm pretty sure the tweeter diaphragm assembly is stuck to the magnet top plate with a thin 3M yellow double sided adhesive tape. 1) and 2)KEF at that time only used one type of ferrofluid. It was the same in the T52 and B110
I'm pretty sure the tweeter diaphragm assembly is stuck to the magnet top plate with a thin 3M yellow double sided adhesive tape. KEF had rolls of die-cut blanks. You'll have to cut your own.

2) have you read the "last resort" sticky?
http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1751

MEK works on the brown bostik like adhesive on the rear suspension. It needs repeated applications - the fumers are bad - best done outdoors or with lots of ventilation. It was a std KM part.

The front surround was vacuum formed, in-house, onto the cone and a card ring. You can't buy it from KM. The card ring is stuck to the painted metal chassis with a water based (pva?) type adhesive. Now long cured, it will not soften with water. However the card ring will delaminate and you can re-stick it with any paper adhesive.
Early LS3/5 B110s had a rubber surround
Ferrofluid quantity for B110: I don't remember!
Don't overfill it will make a mess. Half fill, leaving a gap, put the coil in, see if it wets all round. If not, add a bit more.repeat until the coil wets all round.

Past my bedtime. That's all for now.

[Paul Marchant]

That's encouraging news about the ferrofluid.

With the T52, are there alignment dowels or the like to ensure the voice coil is centred on reassembly? Or is it a 'feel your way' sort of thing when (re)assembling it?

I hadn't seen the 'last resort' sticky prior to just now. I really like the smell of MEK... that's probably not a good thing. If damage occurs to the rear suspension in the process - was it the same as any other B110's or are they unique to the KM1 drivers?

So it seems possible, in the hands of a skilled person (which isn't me, but hey that's what Wembley Loudspeaker Co. is known for) to non-destructively disassemble the drivers. I presume it's a foregone conclusion that the ferrofluid has at least partially evaporated?

[Lee in Montreal]

[Paul Marchant]

They're solid aluminium bars, which bolt to the finned heatsink on the back of the mid / tweeter housing. There's some detail and pictures at the bottom right of this: https://drive.google.com/file/d/1WPjG7Fzr7IiDS2G8-2UYdZQYV1et6y_5/view