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We had a thunderstorm a few days ago and our Asus DSL-N14U modem router stopped working. I opened it up and as far as I can see everything on the PCB is in good shape except two ceramic capacitors which have burst and on further inspection it also seems as if the inductor pictured in the second image also got damaged. I have looked for a datasheet or some piece of information as to what specification capacitors I should replace the damaged ones with but I come up with nothing. What type of replacement capacitors should I get? enter image description here

enter image description here

Apache
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    They look like MOVs ([Metal Oxide Varistor](https://en.wikipedia.org/wiki/Varistor)) to me. Their role in life is to protect the rest of the circuit from things like lightning strikes. It seems like they may have fulfilled that role. – Majenko Nov 18 '16 at 11:40
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    @Majenko they look like MOVs but the circuit reference is C354 / C355 with a symbol for a capacitor. Apache we need a close-up of the printing on the capacitor. – Steve G Nov 18 '16 at 11:48
  • @Majenko At first this is what I thought, then I saw the PCB was cut unerneath the one on the first photo, like it is sometimes between between isolation barriers. Moreover the component next to it (H20202DL) is an ethernet transformer. It wouldn't make much sense to have a MOV around there (and, as Steve said, the symbol is very clear). – dim Nov 18 '16 at 11:50
  • @Majenko I'm pretty sure any element which was torn in half can't possibly fulfill its role. – Dmitry Grigoryev Nov 18 '16 at 12:06
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    I meant "for the last time". – Majenko Nov 18 '16 at 12:08
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    Certainly on the FXS input I would expect some form of TVS like MOVs. Ethernet really needs them too, or something to protect it. And MOVs can happily explode with lightning. Anything where long runs of cable are involved basically... I notice there are TVS devices missing on the second image - unpopulated pads. – Majenko Nov 18 '16 at 12:13
  • @Majenko yep. Consumer hardware :) Probably the same board is sold in a variant for commercial use, twice the price, addition a few TVS diodes, more voltage-stabilizing caps, less annoying firmware. – Marcus Müller Nov 18 '16 at 12:15
  • @SteveG unfortunately there is no printing on the components just the blue casing. So it is safe to say I can just butcher this PCB for parts? – Apache Nov 18 '16 at 12:31
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    @Majenko. Well, I have never seen ethernet signals protected by MOVs on any consumer product yet (although with TVS diodes, sometimes - but seldom). And I'm not sure having one leg of the MOV on each side of an isolation barrier makes sense. Wouldn't that defeat the purpose of isolation? – dim Nov 18 '16 at 13:00
  • @dim who can say what is happening without a schematic? All I know is they look very much like a MOV. – Majenko Nov 18 '16 at 13:58
  • If you want a last-ditch attempt, snip the caps out and try it. It might work without them. If it does not work toss it, if it does work then source and replace the parts before reinstalling it. – Spehro Pefhany Nov 18 '16 at 14:36
  • so after removing them I saw typed on the mystery components "68 2KV" so that means they are capacitors or should I say were? @SpehroPefhany the power LED does still come on when I power it up – Apache Nov 18 '16 at 14:59
  • 68pF 2kV So a Murata DEA1X3D680JA2B less than $1 for 2 at Digikey plus shipping. But it's a long shot. Possibly traces are blown off the board, for example. – Spehro Pefhany Nov 18 '16 at 15:12
  • @SpehroPefhany will let you know what happens. Busy trying to set up a WiFi connection between two points 500m apart from each other with dense foliage between them using directional antennas. – Apache Nov 19 '16 at 05:31
  • @dim: the requirements on isolation and protection (by a surge arrestor) are indeed in mutual contrast, though not entirely exclusive. To arrest a particular current surge = Coulombs (never mind voltage in a lightning strike), arrestors rated at lower voltage will have a higher Joules capacity. It's a design decision, which of the two you value more: isolation, or surge protection. Also, some standard burst mode EMC (ingress) tests require a capacitor across the isolation gap, otherwise the device cannot pass. – frr Nov 20 '16 at 20:21

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What type of replacement capacitors should I get?

None. This thing is broken, busted, kaput. Unless you need a door held open, toss it and move on.

If a lightning strike caused this much visible damage, it almost certainly caused much more invisible damage. The few parts with obvious damage are just the tip of the iceberg.

High voltage got into this circuit someplace. The parts weren't rated to withstand that, so even parts that would normally block current can't be counted on to have done that. This thing was seriously abused.

Again, ditch it and don't look back.

Dave Tweed
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Olin Lathrop
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    Generally, I'm all for repairing stuff, but I have to agree. Admittedly, it's one of the better consumer routers around (not that this says much) for around 45€ retail, but hunting down the parts that were *not* damaged will be harder than just washing dishes for the same amount of money. By an order of magnitude. – Marcus Müller Nov 18 '16 at 12:14
  • Yup, this box is probably gone for good. But, given that this is a DSL router, I would say that the DSL line lacks proper surge protection. I happen to have some pretty recent experience with SHDSL modems (up to 15 Mbps) and surge-protecting such lines. Forget about transils and MOVs, they (alone) either have too much parasitic capacity to pass the signal, or are too feeble to arrest "a lightning". I recommend that the OP protect his end of the DSL circuit by a simple arrestor, containing a spark chamber to protective earth for each wire (two of them for a pair). – frr Nov 20 '16 at 20:28
  • This is a very short video of a DSL arrestor sustaining a test pulse, 2.5 kA @ 10/350 us: http://support.fccps.cz/download/bleskojistky/snap.avi The pulse really happens within a single frame of the vid. It's the audio track that adds a lot to the overall impression :-) – frr Nov 20 '16 at 20:32