KEF Coda S - burnt driver or what?

sirlukas

Active Member
Hi guys, about two months ago I purchased second or third hand speakers via AVForums classifieds, and now the sub has gone faulty.
I think that the driver is gone as it hums in low frequency constantly as soon as power goes to it and there is smoke too!
Below you will find a recording of that phenomenon, so please advice if it is the driver or the circuitry problem as one of the two big capacitors is slightly bloated, but only slightly.

Video of the problem, see the smoke at the end when I kill the power:
YouTube - KEF Coda S - burnt, smoking driver?

Images showing bloated capacitor, the one on the right:

Image 1 (~ 1MB)

Image 2 (~ 1MB)

What do I do, buy just new driver, replace capacitor or buy a new sub from BK, using the group purchase in the other thread?
 

sirlukas

Active Member
Anyone, please?
Is the driver gone or something else?
Is there any sense in fixing such old sub, or should I buy a new one and sell this one as faulty?
 

Frank1471

Member
Anyone, please?
Is the driver gone or something else?
Is there any sense in fixing such old sub, or should I buy a new one and sell this one as faulty?

The sub-woofer's amplifier is faulty.
It is outputting a high level of DC which is burning out the loudspeaker.

The loudspeaker may already be damaged by the faulty amplifier.

What type of sub is it?

Frank
 

sirlukas

Active Member
Frank, thank you for your answer, much appreciated.
I hope the speaker is still OK, as it still produces sound.
I will check the voltage on the output from the amp to the speaker as you suggest and report back.

What could cause such fault, could it be something I have done, like connected both input RCAs to single RCA from the amp?

The sub is an old school Kef Coda S (still made in Canada, so hopefully good).

The rest is as per my sig below.

PS. I can provide with better pictures if needed, but I could not see any of the components to by dodgy, apart from that slight bump on the capacitor mentioned earlier.
 
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sirlukas

Active Member
Do you think it could be one of the amp sections (transistors)?
It happens as soon as I switch on the power, does not have to receive signal at all from the amp.
I will check the DC output in those two wires that go to the speaker and report back in the evening.
 

Frank1471

Member
Do you think it could be one of the amp sections (transistors)?
It happens as soon as I switch on the power, does not have to receive signal at all from the amp.
I will check the DC output in those two wires that go to the speaker and report back in the evening.
My gut instinct is that it will not be one of the 4x large output transistors but one of the smaller ones but it is impossible to say without doing some measurements.

Have a look at this with regard to testing your loudspeaker.
Personally, I would use a 1.5v battery.
YouTube - Testing a speaker with a 9 volt battery

Frank
 

sirlukas

Active Member
So the polarity on the woofer matters as well, good to know because both wires disconnected themselves from the driver and I did not know which one was where and connected them back by guessing and without testing at all.
The "good" news is that I have done it after the sub has failed, so it was not me who did the damage :)

I will have to take the driver out and test if it is faulty or not now.
Thank you.
 

sirlukas

Active Member
OK, I have borrowed a multimeter and an adjustable PSU.
If I set it to 1,5V, what sort of current should I set it to to test the driver?
 

Frank1471

Member
OK, I have borrowed a multimeter and an adjustable PSU.
If I set it to 1,5V, what sort of current should I set it to to test the driver?
Disconnect the wires from the loudspeaker and measure the voltage on them with the amp powered - Don't short them together or hold them.
There will probably be about 50v DC.

Use a 1.5v battery to test the loudspeaker - safest way. (any type AA or larger)

The loudspeaker polarity does matter but will be marked on the loudspeaker terminal with either a "+" symbol or a red dot.

Frank
 

sirlukas

Active Member
I've measured the voltage on the wires and it is 47.7V
which is not great as you say Frank.

The speaker is probably faulty as well,
I think as it only bounces once and does not stay up and drains all the current from my PSU as shown in the video below:

YouTube - Kef Coda S faulty driver

Unfortunately I have left the sub powered to the mains now,
without the driver and I think the damage is even greater as the PCB is burnt now.
I will attach an image in few moments :(

Is there any point in repairing it, please be honest.
 

Frank1471

Member
I've measured the voltage on the wires and it is 47.7V
which is not great as you say Frank.

The speaker is probably faulty as well,
I think as it only bounces once and does not stay up and drains all the current from my PSU as shown in the video below:
Unfortunately I have left the sub powered to the mains now,
without the driver and I think the damage is even greater as the PCB is burnt now.
I will attach an image in few moments :(

Is there any point in repairing it, please be honest.
You are doing something wrong with the bench power supply.
It's going into current limit.

Turn the output volts down to 2v or so and the current limit to max.
The PSU output voltage shouldn't drop when you connect the loudspeaker.

Or you could just use a 1.5v cell like I suggested earlier.

Whilst you still have the meter, measure the loudspeaker's resistance (Ohms range on the meter).

If the loudspeaker is OK then it's still worth repairing. It is possible that the excessive fault current has damaged the output transistors.
Frank
 
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sirlukas

Active Member
You are correct again, I had the current limiter on set to minimum, when raised to max the speaker seems to be in working order and it draws around 0,5A and the resistance is 03.3 Ohm.
 

sirlukas

Active Member
Firstly, I would like to thank to Frank for repairing my faulty amp.
I must admit you have done an incredible job, very tidy, clean and impressive, pure professionalism.
All the kudos to you! :thumbsup:

Unfortunately, it turned out that the speaker itself did not take the failure very well (possibly due to my prolonged "diagnosis"),
and when connected and trying to play it sounds like an (excuse my French) old farting noise.

My question is now, do I continue and try to source the speaker, being original or a replacement,
or do I give up, take a bullet and buy proper BK sub :confused:

Caution - Large photos!

1. Speaker on the outside
2. Speaker on the inside - I think it says R10AMI on it, but unfortunately I did not find any references on the internet, apart to some dodgy websites when searched for it.
 

Frank1471

Member
Unfortunately, it turned out that the speaker itself did not take the failure very well (possibly due to my prolonged "diagnosis"),
and when connected and trying to play it sounds like an (excuse my French) old farting noise.

My question is now, do I continue and try to source the speaker, being original or a replacement,
or do I give up, take a bullet and buy proper BK sub :confused:
Sorry to hear that the speaker is duff, not surprising as once the "magic smoke" escapes that's normally it.
I am assuming its a 10" speaker?
A suitable replacement might be:
Peerless 10 inch hifi bass speaker woofer SKP254 NEW on eBay (end time 30-Mar-11 15:58:17 BST)

I have used these to replace the 10" loudspeakers in the Kef PSW2010.
They work fine but are about 2mm larger diameter than the original.

It's a bit of a risk but cheaper than a new sub. If it's no good, you could probably sell it on again.

Frank
 

sirlukas

Active Member
Frank, thank you for all your help and hints!
That Peerless speaker is surprisingly good for its price and materials used.

I am now another happy customer, and can not recommend you highly enough!
 

sirlukas

Active Member
Decided to sell that sub, so if anyone is interested you can find it on the most popular auctioning service :)
 

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