Need advice on repairing some blown MOSFETs on a Headphone circuit board - Page 1
Forum section: electronics / repair

author: Fulmir
author_rank: Contributor
timestamp: June 10, 2024, 11:07:25 pm
content: TLDR: I'm looking for help identifying what I need to swap on this board, I know it'll be at least one MOSFET, probably two, and possibly other parts, as well as identifying suitable replacements for them, ideally ones that won't suffer a similar fate if this was a case of under-spec'd parts from pandemic supply chain nonsense...The board in question belongs to a pair of headphones I quite like, and I would rather not pay full price to replace. I'll spare folks the background on the headphones and how they broke, but the short-short version is they still *mostly* work, but won't connect via one of their connection modes. I found a similar post pointing to a dead part on the circuit board, so I opened mine up to see if anything similar jumped out, and it certainly did!Thanks to a previous RMA, not for a similar issue, just a bad spring switch, I have a "destroyed" pair of the same headphones, but from an earlier production batch that I can use for reference. I'm wary of scavenging parts off them though, because swapping the boards is a decent last ditch option, and I'm not positive that they haven't tweaked the design slightly between versions. There's definitely evidence of them doing this with other headphone products.In case the forum eats the image quality here's an Imgur album:https://imgur.com/a/jucQwsyYou can see on the right side of the "New" image where things have gone wrong. It looks to me like one MOSFET slightly exploded, dislodged and damaged another, and possibly took a chunk out of what I *think* is a zero ohm resistor?Between the remains of these components and the other parts on the board I have the following guesses for parts:One MOSFET marked A19T, on the old board the equivalent is marked P05 with a sideways POne MOSFET that's too exploded to read anything off of on the New board but is a 702 with a :C with the colon downwards on the new board. It may be the same on the old but with a different odd character after the 702, as there's at least one other 702 MOSFET on the New boardOne blank black box with a chunk taken off the silver strip at one end. Measures no resistance or capacitance on the Meter. Some poking around suggests it may be a 0 Ohm resistor?I think the 702 MOSFET is possibly an 2N7002.The A19T is one of several components: HM3401C, HX3401A, or KI2301TThe P05 is possibly this: SO2907So, what I'd like help on here...Can someone confirm my component IDs here? I can take Multi-meter measurements if that would help, I just haven't had to ID an SMD MOSFET like this before.Is there anything else that looks damaged and would need replacing? This is a simple circuit board that's I think just two layers or so, and it the failure doesn't seem to have damaged the actual board, but I'm no expert on SMD components so I'm not sure if any of those tiny caps may be dead or something.Suggestions on better, eg less likely to fail, versions of the components. Since I don't know the mechanics of the circuit this MOSFET is hooked into I'm worried about accidentally making it non-functional in attempting to up the specs of a replacement component. Also frankly I'm not 100% sure why this would have failed in the first place given that this shouldn't be a super high voltage setup or anything.Any and all help appreciated, thank you all for your time!

author: JFerris321
author_rank: Contributor
timestamp: June 10, 2024, 11:38:36 pm
content: Hi, im afraid im not expert on part identifacation so i can not help you there, but what i will say is in my expereance transitors dont often just blow apart for no reason although it could be that one failed short and blow the other, before replacing the parts i would check that the diode J107 is working correctly and not shorted.Also the component you believe to be a zero ohm resitor is most likly an inductor, to confirm look at the marking next to it and see what letter it starts with, C for capacitor, R for resistor and L for inductor.

author: fzabkar
author_rank: Super Contributor
timestamp: June 10, 2024, 11:48:53 pm
content: A19T = AO3401, 30V 4A P-Channel MOSFEThttps://www.aosmd.com/res/data_sheets/AO3401.pdf(datasheet)https://ae01.alicdn.com/kf/H866a91a651c1499b96d72c13455948e0p/50PCS-AO3401-SOT23-A19T-SOT-23-SOT23-3-SMD-IC.jpg(photo)P05 = LP2305LT1G, 30V 4A P-channel MOSFEThttps://doc.platan.ru/pdf/datasheets/lrc/LP2305LT1G.pdf(datasheet)https://ae01.alicdn.com/kf/S7a37469aa9314162af0e13ae6cb050c9l/10pcs-lp2305lt1g-sot23-p05-mosfet.png(photo)702 is a "jelly bean" MOSFET (2N7002):https://he.aliexpress.com/w/wholesale-702-mosfet.html

author: Fulmir
author_rank: Contributor
timestamp: June 11, 2024, 03:53:56 am
content: Quote from: JFerris321 on June 10, 2024, 11:38:36 pmYou are correct! It's an Inductor! Thanks!Quote from: JFerris321 on June 10, 2024, 11:38:36 pmThe J107 however is a capacitor (also has a C on the board next to it), not a diode. It doesn't seem to be dead, shorted, or otherwise malfunctioning, and comparing it to the same component on the other board shows identical readings. Though there's a bunch of other J107's on the other side of the "New" board and those show different cap readings with the Multi-meter soooo IDK.Also I've included the other side of the board with its myriad capacitors, none of which *seem* to have blown?Anything else I should check here? I'd rather not map out the entire board, but if it comes to it there are worse ways to go about this... lolQuote from: fzabkar on June 10, 2024, 11:48:53 pmFirst, thanks so so much! Super helpful!Also I hadn't actually run across the term "Jelly Bean" to refer to generic stock components, so thanks for introducing me to it! (I know, I know, it comes from EEV Blog, but I haven't watched nearly every video yet)

author: JFerris321
author_rank: Contributor
timestamp: June 11, 2024, 06:57:22 pm
content: Hi, sorry that was my mistake a very quick google search came up with diode i can see now its a tandaum, its worth noteing that these can also fail short so still worth checking. That said i would also check the board for diodes in that area, will be marked with a D and just check none show a dead short. Other components on the board can make it hard to test diodes in circuit but if its a dead short its safe to say something is probably wrong with the diode its self.

author: Fulmir
author_rank: Contributor
timestamp: June 11, 2024, 08:08:11 pm
content: I still need to check components, and will do so, but this may be relevant...Some additional info on a likely failure mode is that these headphones have two connection options, one to a "base station" and another via Bluetooth, and the two can be active at the same time, as well as being able to adjust volume between the two. The specific failure is that the headphones won't connect to the base station at all, won't stay powered on unless plugged in or connected via bluetooth, and won't actually let me turn on bluetooth unless I set them into searching for connection for the base station while plugged in and then turn on bluetooth. None of this is expected, everything should turn on from any mode.My original supposition was that the problem may have to do with some kind of current or voltage spike when turning on one or both modes, though as noted I'll definitely be checking for dead components.If I don't find any I'd appreciate recommendations on possible upgrade paths for the MOSFETs. I'm familiar with them failing, but generally in higher power applications where I know things like the activation voltage. Here I'm rather wary of mucking things up since I don't know the expected properties, and don't have a great way of measuring.Oh, also, is there any way to ID that Inductor? It is missing a small chunk, probably from 'shrapnel' from that one MOSFET going, and I'd rather not find out it's the cause of all my woes here when something else goes "pop"...

author: fzabkar
author_rank: Super Contributor
timestamp: June 12, 2024, 02:09:42 am
content: Can you identify the markings on the smaller square IC near APA2172? I suspect that we could get a better idea of what is going on in this area of the PCB by examining the application diagram in the datasheet.Datasheet for APA2172:https://crossic.com/wp-content/uploads/2022/02/APA2172.pdfhttps://www.anpec.com.tw/en/product-download/0/APA2172-1/On the other side of the PCB (opposite the damaged MOSFETs) are what look like the output inductors and capacitors of a class D stereo amplifiier. I don't understand why there would be MOSFETs at this point, assuming that the small square IC is the amplifier.