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I'm repairing a speaker power supply (KEF LSX), and I'm trying to find a replacement for component L802 (see arrow), this is either an inductor or ferrite bead, but I'm not fully sure.

Picture of rear of KEF LSX power supply

The primary and secondary speaker fortunately share the same power supply. I replaced most of the faulty parts, but I don't know how to replace this inductor. When I switch the inductor from the working to the faulty power supply, the faulty power supply comes to live again, so this is definitely the issue.

I measured the following on my LCR bridge (which only goes up to 200kHz):

Frequency Ls Rs Q
100Hz 20uH 85 mOhm 0.15
1kHz 20uH 130 mOhm 1
10kHz 19.1uH 807mOhm 1.5
50kHz 15.5uH 4.4 Ohm 1.1
100kHz 13.0uH 7.1 Ohm 1.14
200kHz 10.7uH 9.5 Ohm 1.4

But I genuinely don't know how to interpret these values. Does anyone know what more I need to measure to find a suitable replacement? I have access to function generators that go to higher frequency ranges, if the need arises. I could also take measurements in between if that's helpful.

I'm not sure if this is a ferrite bead (and how I can best replace it) or perhaps a power inductor. It is a 1806 SMD component, and I can basically only find ferrite beads with this form factor.

Picture of HAMEG HG8118 LCR bridge

As requested by a lot of commenters/replies (thanks for taking the time!) here is a small reverse engineered schematic of the surrounding area: reverse engineered schematic around inductor

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    \$\begingroup\$ Are you able to reverse engineer a schematic of the part of the circuit that L802 is connected to? That may allow determination of potential values for the inductor / ferrite bead. \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ The inductance is suspiciously high, but the low Q is consistent with a ferrite bead, yes. Measurements at higher frequencies would be more illuminating, as well as reference values for comparison (open, short, resistor, etc.). \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ @ChesterGillon I've reverse engineered a small part of the schematic around the inductor. Is this helpful, or should I reverse engineer a bit further (if so, what direction?) \$\endgroup\$ Commented yesterday

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I genuinely don't know how to interpret these values.

As test frequency rises, parasitic capacitance within L802 affects the measured inductance value hence, the value appears to droop as test frequency increases.

So, I would make the reasonable assumption that a replacement of 20 μH or 22 μH (a standard value) should do the job just fine. L802 is a recognized failure point for this type of speaker too so, that matches your swap with a working unit.

So it boils down to how much current it is handling. This could be determined by measuring the current in the working unit by putt an ammeter in series but, to be honest, I'd just locate one that is approximately the correct value of inductance and about the right size. If it pops again then you might need to choose one with a higher current rating but, by the looks of it, there's plenty of room around it to try a slightly bigger version.

I would also say that if you find an inductor with no-more than a specified series resistance of 0.1 Ω this will help track down a suitable replacement.

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  • \$\begingroup\$ FYI to readers; the impedance explanation is incorrect. \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ You are entitled to your wrong opinion of course. Merry xmas Tim. It all depends on whether the meter has changed-over to display effective parallel inductance on the higher frequency ranges as some meters (including mine) do. Note the auto setting on the meter. It will switch to parallel at some point during testing. \$\endgroup\$ Commented yesterday
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Consider the purpose of an inductor in a DC power supply.

Almost certainly, it serves one of two purposes:

  • It is part of a resonant circuit for a switch-mode buck voltage regulator. (It seems more likely a power supply for a specific application would use the slightly simpler and more efficient buck circuit than boost.)
  • It serves to block high-frequency components of the chopped DC from getting into the audio amplifier.

In either case, its exact value should not be critical, but it must pass enough current for that amplifier. Certainly, a 22 μH inductor would work, but try to estimate the wire size on the existing coil, note the supply's current rating (or fuse), and/or use a table to calculate the current rating for the replacement. For example, 20 AWG (0.5mm2) wire is rated for ~9A for chassis wiring (though it might be less for use in a coil).

Replace with a coil that has at least the same current rating as calculated, lest the core saturate. Use a coil withe leads or surface mount, as long as you can get it to fit.

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  • \$\begingroup\$ I would not be so certain. It could be a ferrite bead on a snubber for example. There is simply insufficient circuit information to reach such a conclusion. \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ @TimWilliams, so what practical difference would it make to use a coil? Would the amplifier not work??? \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ If it's on a snubber for instance, using an inductor versus a ferrite bead might cause excessive peak voltage leading to destruction. The AC resistance and saturation characteristics might matter a great deal. \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ @TimWilliams I've attached a small part of the schematic that I reverse engineered just now. \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ No topology I would call a buck uses a resonant circuit. Resonant topologies are a good deal more complex to design and control than a simple filtered chopper. \$\endgroup\$ Commented yesterday

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