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?