SpeakerTalk Forum Index SpeakerTalk
This forum has been set up to facilitate discussion of 1970s KEF speakers and drive units. The owner of the Forum has no connection with KEF Audio.
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

KEF 104aB tweeter volume very low

 
Post new topic   Reply to topic    SpeakerTalk Forum Index -> KEF Speakers from the 1970s
View previous topic :: View next topic  
Author Message
bepaald
Introductory Contributor


Joined: 25 Aug 2013
Posts: 5

PostPosted: Mon Aug 26, 2013 8:07 am    Post subject: KEF 104aB tweeter volume very low Reply with quote

Hi! I recently got my hands on a pair of KEF Reference 104aB's. They were pretty cheap I believe, 80 euros for the pair, but it was a bit of a gamble as I was unable to give them a listen before buying.

When I hooked them up yesterday, I immediately noticed one of the tweeters was not working at all. I had already read about how difficult (or at least expensive) it could be to replace these so I was not too happy. But I decided to just unscrew it to take a look at it anyway, and to my surprise, it seemed the tweeter was just not connected (http://imageshack.us/photo/my-images/96/h6mv.jpg/)!

So, after checking how the black wire was attached underneath that sheath, I did the same to the blue wire (it was just twisted together). Now the tweeter works, but its volume is very low. It is difficult to hear with all the other noise coming from the cabinet, but with the balance all to one side, the treble all the way up and my ear close to the tweeter, I think it sounds right (no crackling or distortion), it's just very quiet.

So, the questions:
What could be causing the low volume, did I just not connect it properly? Is the tweeter ok, is it salvageable? When I saw the disconnected wire I really hoped that was it. Should I switch the tweeters to locate the problem or is the problem clear already (I'd rather not mess with the working speaker if it's not necessary)? What else could be wrong and are those things I can fix (or get fixed for a reasonable price)?

Reading up on these things, I saw many threads where people rewire, recap and replace many parts of these speakers. I had an incredibly hard time just twisting those wires together. The other part of the blue wire was too short to come out of the hole, and the hole too small to fit more than one hand. How do people get to the innards of these cabinets? They seem built like a safe...

I should note that I have virtually no experience with electronics, twisting those wires together is pretty much all I have confidence in doing (though I'm not scared to try new things and wouldn't mind learning), so feel free to talk to me like I'm stupid.

Thanks,
Bas Timmer
Back to top
View user's profile Send private message
audiolabtower
VIP Contributor 500+


Joined: 06 Jan 2009
Posts: 544

PostPosted: Mon Aug 26, 2013 11:17 am    Post subject: Reply with quote

Hi, from memory the only way into these cabinets is through the drive unit holes. Unscrew the crossover (switch plate) and maybe it is easier to put the tweeter back and connect the wire to the crossover pulled out the front of the cabinet? The aB crossover is longer than it's hole, so you have to pull it out at 90 degrees to clear the printed circuit board. In any case it would be easier to put a slightly longer wire in that way.

The wire really needs to be soldered and insulated. If the twist is a good connection I would select phono on the amp and turn the volume up with no input so there is a lot of hiss. Put the speakers next to each other, and put your ear close to each tweeter to judge if the volume/type of hiss is the same. If the same everything is ok. If there is a volume difference perhaps some crossover caps need replacing?

ps make sure things are airtight with the gaskets good when you put it back together.
Back to top
View user's profile Send private message
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Mon Aug 26, 2013 11:55 am    Post subject: Reply with quote

... before you solder the wire, measure the d.c. resistance of the tweeter. It should be somewhere around 6.5 Ohms. The voice coil may have been overheated in the past and now have some shorted turns. If the resistance is there or thereabouts, then swap over with the other system to confirm that there is a crossover fault.

p.s. the only way to work on these systems is through the drive unit holes; much easier if you remove the woofer and some of the foam.
Back to top
View user's profile Send private message
bepaald
Introductory Contributor


Joined: 25 Aug 2013
Posts: 5

PostPosted: Mon Aug 26, 2013 3:05 pm    Post subject: I think I may have found the problem... Reply with quote

Thanks for the replies guys.

I had already whipped out my multimeter and set out to switch the tweeters, but first I took out the crossovers. And look:

The working speaker
The faulty speaker

I guess that's what a blown capacitor looks like. Unless you guys are going to tell me this cannot be the cause of the problems, I think I won't switch the tweeters (the working one is nicely soldered, and I'd rather not do that myself as I'm bad at it).

Now just from googling around, I understand to replace this I need a cap which is non-polarized, has the same capacitance (3.3 uF) and is rated for at least 50V right? Any other hard requirements? And I understand polypropylene would be better than electrolytic. Would this do? And would it be wise/recommended/necessary to replace the same cap on the other crossover? Or all of them in one go?

There is a shop not far from my house, by the way, which though not specialized in audio, is specialized in just soldering circuits for people, so I'll have them do it and properly solder (replace) the wire as well...

thanks again,
Bas
Back to top
View user's profile Send private message
audiolabtower
VIP Contributor 500+


Joined: 06 Jan 2009
Posts: 544

PostPosted: Mon Aug 26, 2013 8:25 pm    Post subject: Reply with quote

Ouch! I don't think much audio is getting through that one Smile

The best place for the correct type is here

http://www.falconacoustics.co.uk/replacement-capacitor-sets/replacement-capacitor-kit-kef/kef-104ab-replacement-capacitor-set.html

I would replace the lot if your drive units are fine, ~30 years on they will all have drifted to some extent. if not ask them to quote you just for a pair of tweeter sets, they can match them. If you replace series alcaps with polyprops or polyesters you need to add an ohm or so of series power resistor to match the original design spec.
Back to top
View user's profile Send private message
bepaald
Introductory Contributor


Joined: 25 Aug 2013
Posts: 5

PostPosted: Mon Aug 26, 2013 9:25 pm    Post subject: Reply with quote

Hey, I just found that site and came here to edit my post and ask about those replacement sets, but you beat me to it. So indeed, it seems like a good idea to just get the replacement set, after all it is not terribly expensive.

But since I've still never heard this speaker actually work, and I don't know if anything else (less repairable) is wrong with it, so I would like to just get a compatible capacitor locally to try it out. If it works, it can just do its job there for a few weeks, while I wait for the proper replacements to arrive. That way at least I know the speaker can make a sound before I pay 30+ pounds for shipping the replacement set over. Also, if the speaker stays dead I might try to return it and I wouldn't want to wait too long (shipping will take a while) to do that.

So, locally I found this. I think the specs are okay, 3.3uF, rated to 100V and electrolytic. Am I correct in thinking that bi-polar and nonpolar are the same in the case of these caps? And maybe there are other important properties the cap must have that I don't know about? If I click on the 'data sheet' link on the site I linked earlier, I see a tolerance of 20% which seems high, could that cause problems?

Obviously I don't expect it to sound perfect at all, but will it at least 1) not destroy what's left of the crossover, and 2) tell me if the tweeter works? Any thoughts?

Thanks again!

*edit*
I found some other places where I can quickly and cheaply get compatible caps (also axial and with better(?) tolerance levels), like here and here. Still any input before I do something stupid is greatly appreciated.
Back to top
View user's profile Send private message
audiolabtower
VIP Contributor 500+


Joined: 06 Jan 2009
Posts: 544

PostPosted: Mon Aug 26, 2013 10:38 pm    Post subject: Reply with quote

No, perfectly good plan, bi-polar and non polar are the same, will get you going to see if everything else is ok.

Kef originally selected the 20% and 10% capacitors into 1 or 2% bands, which then matched up with the computer measured drivers and wound to spec inductors to give the target flat response. Random 20% caps will give greater errors between channels and might upset the flat frequency response and/or solidity of stereo image. This is why Falcon offer the matched sets.
Back to top
View user's profile Send private message
bepaald
Introductory Contributor


Joined: 25 Aug 2013
Posts: 5

PostPosted: Tue Aug 27, 2013 6:13 pm    Post subject: Success! Reply with quote

So, this morning I went to the local electronics place and picked up a proper cap. Then after work, I got one of my colleagues to help me out and solder it in place...

It turned out like this. (The red one on the left obviously)

And... success! I'm so happy none of the drivers are faulty. To me, the thing already sounds awesome! Even with a random unmatched cap replacement. But then maybe I was just used to the blown capacitor sound...

Anyway, I want to thank everyone participating in this thread with helping me out. I'll order the replacement kit soon, and use the shipping period to practice my soldering skills (I think it will be very satisfying to fix up the speakers myself). I'll update here when it's all done, but I think it'll take a while.

Thanks again!
Back to top
View user's profile Send private message
speakerguru
VIP Contributor 750+


Joined: 18 Nov 2005
Posts: 958
Location: Green Hut, Tovil

PostPosted: Tue Aug 27, 2013 6:18 pm    Post subject: Reply with quote

Well done. I do love a happy ending. Very Happy
Back to top
View user's profile Send private message
SaSi
Senior Contributor 200+


Joined: 24 Aug 2008
Posts: 223

PostPosted: Sat Oct 12, 2013 11:28 am    Post subject: Reply with quote

That was probably it.

I have a pair of Calindas that had not a blown capacitor in the tweeter network but a badly drifted one. The highs were just non existent on this set. The faulty capacitor, a 3.3uF as well, measured about 5uF.

After replacing that single capacitor, the sound of the highs returned back and the speakers sounded good. After replacing all of the capacitors, the life returned to the speakers and the human voice became alive, the harmonics just fine and the high bass frequencies had a life in them.

30+ years old capacitors just ain't good anymore. I can't say the same about the drivers - they must have lost something in the years but they do have a lot of life in them still.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    SpeakerTalk Forum Index -> KEF Speakers from the 1970s All times are GMT + 1 Hour
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group