KEF LSX Teardown and Repair

[wouter]

It only took me a couple of months but my power supplies are back to working condition! It's been too long of a ride with a couple of unexpected issues (accidentally slipping with a set of tweezers and blowing up the working PSU, oh well!) to start-to-finish document everything that went wrong, but it was a fun project to work on!

If anyone is ever taking on a broken KEF PSU, definitely get yourself a cheap Optocoupler tester! The TO2 optocoupler tester helped me identify some of the issues I was running into, and a thermal camera ended up being a super helpful tool too. I ended up identifying into a couple of broken parts on the bottom of the PSU near the TI LM339 IC, such as a broken IC and a shorted capacitor.

I also noticed last week the gate driver on the front side (the side with all the capacitors) quickly heated up to 150C, causing the mosfet on the back to heat up as well. I replaced the gate driver and a diode besides it, since that one appeared to be shorted too. Which brings me to the question: does anyone happen to know what a good replacement for Z803 and U806 are? That's the missing pieces I need to permanently fix the PSU, since I borrowed them from the working PSU to test this one.

I think it's marked as

J3 4
   3

But I'm not entirely sure.

Okay, figured that part out. The J3 43 part is a 12V to 5V LDO, so I'll be replacing that with a TI LDO. It doesn't seem to use the most common pinout, but the TI LM3480IM3X-5.0 should be a good drop-in replacement.
My last struggle is finding a good replacement for the inductor (or ferrite bead?) on the back of the power supply:

It's a 1806-sized part, and I can basically only find ferrite beads in that size.. It measured as a 20uH inductor on my LCR bridge, with ~125-130mOhm series resistance:

This is definitely beyond the limits of my electrical engineering knowledge, so if anyone has good suggestions for a replacement part I'd love to hear it! I've tried emailing KEF and a local service center, to see if they might be willing to share some of their knowledge, but they either weren't willing or didn't have access to the service manual unfortunately. How did the other people on this thread get access to service manual snippets?

 

[wouter]

After some back and forth with KEF they shared that L802 is a Murata BLM41PG102SN1L.

 

[FraPia-62]

After some back and forth with KEF they shared that L802 is a Murata BLM41PG102SN1L.

Sorry for the late reply...
I guess that in the mean time you found the replacement, since the Murata BLM41PG102SN1L is rather easy to get (e.g. Mouser has it). Anyway if for some reason you need to procure an equivalent one, it is a ferrite choke, with an impedance = 1kOhm at 100MHz and max current=1.5A. The exact impedance is not particularly critical, just use the closest you can find.

 

[wouter]

Sorry for the late reply...
I guess that in the mean time you found the replacement, since the Murata BLM41PG102SN1L is rather easy to get (e.g. Mouser has it). Anyway if for some reason you need to procure an equivalent one, it is a ferrite choke, with an impedance = 1kOhm at 100MHz and max current=1.5A. The exact impedance is not particularly critical, just use the closest you can find.

All good! Just wanted to share the info I got from KEF in case it helps anyone else my replacement BLM41PG102SN1L (along with some other parts) is on it's way, so the power supply should be back to working order soon!