KEF LSX Teardown and Repair

[AnalogSteph]

Honestly active monitors do not seem that failure-prone overall. Granted, there's those cases like the gen 2 KRK Rokits with the classic "killer glue" (those into their vintage hi-fi may recognize the issue), or my old Tascam VL-X5s with their relay issues which were "helpfully" glued shut, but...

If a device dies from underspec'd ceramic caps (which are a bit of a weak spot of modern consumer electronics anyway, ideally we'd be going automotive), then that's an engineering blunder that's clearly on the manufacturer.

 

[Andy1978]

Finally i fixed my lsx. My creazy idea was right. I do it like on pics and it works fine few hour in closed case

 

[demuffin]

You should not replace components at random before finding the root cause of the failure. The risk is that the root cause gets hidden, or, worse, you add a new problem that will need to be found.
The loud pop suggests that now there may be an issue (also) with the power amplifier. Did the loudspeaker pop at power off also before you replaced the capacitors? Is there DC on the loudspeaker's voice coil when the amplifier is powered up? If yes it could be that there is indeed a problem with the amplifier, and maybe the PSU was just reacting by opening the relay thus cutting off the main power supply. Who knows!

Yes, the loudspeaker did pop before I replaced the capacitors.

So far, the power supply is working again but I've noticed that the DC voltage output from the bridge rectifier is very imblanced. The + output side is measured across the 220uF 250V capacitor at 120Vdc and - output side is also measured across the capacitor at 200Vdc (same measuring location on my previous reply). I compared to the other power supply that I have and they should be balanced at 160Vdc on both sides.

Does anyone know or suspect what might be the causing the imbalance?

Thank you!

 

[FraPia-62]

OK, the fact that the loudspeaker did pop also before the replacement of the capacitors suggests that likely there is a problem with the power amplifier. You may try the following:
- Disconnect both the woofer and the tweeter from the power amplifier, i.e. remove all loads from the amplifier.
- Then turn on the system: does the relay remain closed or will it open immediately after turn on?
- Measure the voltage at the output of the power amplifier: if you find any voltage higher than a few mV then the power amplifier has a problem.
In that case it could be either the amplifier chip or some external component that is bad.

Measuring the voltage at the bridge rectifier when the relay is open makes no sense. The relay, when it opens, cuts the power to the rectifier and all you measure is the voltage remaining on the capacitors when the power is removed. If the relay stays closed when you remove all loads from the power amplifier you will most likely measure the correct 160V on both capacitors.

In any case don't force the relay to close! If the relay opens it is because it is protecting the rest of the circuit, if you force it to close you will most likely make further damage to the KEF loudspeaker.

 

[Mal]

Richer Sounds (UK) offer a six year guarantee on the lsx, KEF direct offer 5 years on the drivers and 2 years on the electronics... Anyone have experience of getting an lsx repaired through Richer Sounds?

 

[bezromval]

I’ve got the same problem. And I find out the amp on the other side of pcb it’s TPA3245

 

[FraPia-62]

Thank you for reporting the part number (TPA3245) of the power amp chip.
Unfortunately C433 is bad, likely an entire batch of bad capacitors was used by KEF given how common this failure is. Just replace it and L236 and your LSX should hopefully work again.

 

[bezromval]

I’m leaving it here for the manufacturer. If he would like to know part number or manufacture date or something

 

[capslock]

Interesting, TPA3245 seems to be a lower voltage version of TPA3251/55. Can one do post filter feedback and have KEF implemented this?

 

[somebodyelse]

Interesting, TPA3245 seems to be a lower voltage version of TPA3251/55. Can one do post filter feedback and have KEF implemented this?

See TI's app note SLAA788A 'TPA324x and TPA325x Post-Filter Feedback'

 

[bezromval]

So I was able to replace L238, C433, C434 & C435 and amazingly the speakers are working like a charm! A big thanks again for the great thread and the detailed instructions on page one!

For documentation and to help others, I changed
- L238 with Laird MI0805J102R-10 (0805, 1kOhm@100MHz)
- C433 with Würth 885012209073 (1210, 10uF, 50V)
- C434 with Kemet C0603X104K5RACAUTO (0603, 100nF, 50V)
- C435 with Würth 885382207006 (0805, 10nF, 50V)

Due to minimum order quantities I have quite a few components left (especially L238) so if anyone in or near Germany needs some, hit me up!

I think C433 should be 1206 not 1210

 

[FraPia-62]

I think C433 should be 1206 not 1210

C433 should indeed be 1206. There is however enough space to mount also a 1210 capacitor rather easily if you only found that size.

 

[FraPia-62]

See TI's app note SLAA788A 'TPA324x and TPA325x Post-Filter Feedback'

You may get even better performance if you implement a regulator (PID or similar) with a separate opamp in front of the TPA32xx rather than using the simple feedback around the TPA32xx amplifier as suggested in the TI app note. Amplifiers such as the 3e Audio A5 or A7 are implemented that way as far as I know and have excellent performance.

 

[bezromval]

Everything works fine after replacing all those components above in 12V circuit. I’ve replaced 3 capacitors and 1 ferrite. Thanks a lot. Not as perfect as I wish to but for the first time smd soldering with just an iron I think it’s good.

 

[Cmqz]

Supergreat thread!

I get the scary flashing red LED and sound mutes after 20-30 mins, as many already described here. After cooling down the pattern repeats.

Given that there seems to be concensus on thermal issues as at least one root cause (and seemingly also less than perfect component choices) I decided on a brute force route - namely COOLING!

Also a deciding factor was that my soldering skills are limited to through-hole solutions (I’m old‍).
Fastened a small (diam. 40 mm) AMD CPU cooling fan on the bass reflex tube pointing at the component area mentioned in the thread and now running the 12 volt fan on 8 volts (almost silent and even the barely audible noise heard is much better than a having a door stop blinking red!)
Not elegant but has now played (with box closed) for 4+ hours every day with flawless sound for over a week.
In my case it didn’t help that the white damping material was pushed tight against the board — great for increasing risk of hot spots i would guess.

 

[Waldosukbat]

Wonderful teardown and great job fixing. I got a pair of these for free. Connection issues the app sucks the previous owner said. Being an IT-guy I figured I could get them working but no dice. I hate that they have no true analogue functions for a jackstick without the need for the app. Left them standing in my basement thinking to fix them one day or replace the indsides with something more straightforward. Just got gifted two sets of white DTK30active speakers 4" driver. They might be underpowered compared to the Kef LSX 1. I have the green ones and my lady likes the design. The DTK30active does actually have nice loud sound.

1. I could move the control board from the 1 set of DTK30active to Kef LSX.

2. I could maybe fit the 4.5" drivers from the Kef in the DTK.

3. I could just repaint the other old set of DTK30active's in a colour the lady likes.

What do people think about this?

 

[Tomswaugh]

Hey all,

Been putting up with this issue for a long time, finally gave in and opened up the LSX master.

Found C441 had popped…. Anyone able to advise the info for that capacitor from their good unit?

The KEF document on page 2 of this forum only goes up to C440

 

[Tomswaugh]

Got an email back from KEF today.

“We're sorry for the inconvenience caused.
Unfortunately, KEF has stopped supplying LSX's main board so that we would suggest you contact our local supplier for possible options.”

But they did share the snippet from the service manual, covering C441 capacitor, see attached.

10uF 50V. 1206 size.

 

[stingalleman]

Got an email back from KEF today.

“We're sorry for the inconvenience caused.
Unfortunately, KEF has stopped supplying LSX's main board so that we would suggest you contact our local supplier for possible options.”

But they did share the snippet from the service manual, covering C441 capacitor, see attached.

10uF 50V. 1206 size.
View attachment 435219

Thank you for sharing, I opened up mine today and discovered this. Going to order some caps I guess.

 

[Salt]

Not caps only, I would guess