Logitech Z906 Subwoofer Power Supply Board Burnt (New Revision – Blue PCB) - Page 1
Forum section: electronics / repair

author: playboygamer1
author_rank: Newbie
timestamp: May 10, 2025, 10:20:06 am
content: Hi everyone,I’m trying to repair my Logitech Z906 subwoofer, which suddenly stopped working after switching it on.There was a strong burnt plastic smell, and two of the input LEDs briefly lit up – after that, the whole system went completely dead.Upon opening the subwoofer, I discovered a severely burnt SMD capacitor on the power supply board, located next to a ferrite transformer labeled ER40.The damage is significant – the burn mark goes through the PCB to the other side, maybe not through, but is on the other side too, as the picture shows.🔍 Unit details:Model: Logitech Z906Revision: latest version with blue PCBLikely manufactured in 2022Integrated power supply board, hard-mounted inside the subwooferI did not clean the board using ethanol (spiritus) to uncover the burnt site, but I plan toI have comparison photos from another identical working board, so I could locate the capacitor, but not identify its exact value/type⚙️ Tools available:Soldering iron + hot air rework stationMultimeter (just a simple one)Solder suckerUV-curing solder maskThin copper trace wireCleaning supplies: spiritus, acetone, cotton swabs, fine brushes❓ My Questions:Can anyone measure or identify this capacitor on their own Z906 board?Is there any schematic or part list available for the power supply board? I couldn't find anything in Logitech’s service docs or at that nomenclature PDF file (pdf-p-classtruncatedtext-module-lineclamped-85ulhh-style-max-lines5logitech-z906-power-supply-nomenclature-schematic-p_compress).How can I safely test other components, like MOSFETs or diodes, to make sure there’s no further unseen damage?Would it be better to bring this to a Hi-Fi repair shop, and what would they typically do to trace the cause and confirm it's repairable?I'd really like to save this unit—it’s considered Logitech’s flagship speaker system, available since 2011 and having undergone several revisions.Given that this is the latest version with the blue PCB, I expected this to be a mature, long-lasting product – not something that would suffer a catastrophic failure this early on.If anyone can help with photos, component IDs, measurements or advice, I’d really appreciate it. 🙏Thanks in advance!

author: playboygamer1
author_rank: Newbie
timestamp: May 10, 2025, 10:20:51 am
content: https://prnt.sc/Ql5AAIITUfDshttps://prnt.sc/gykwKEZF84hQhttps://prnt.sc/52OQY_rs4FLqhttps://prnt.sc/x8k2O_IXnbmO

author: MathWizard
author_rank: Super Contributor
timestamp: May 11, 2025, 05:11:58 am
content: Some wide shots of the whole PCB would be nice too, but I'm guessing the large SMD cap blew up, and that it's between a rectifier diode, on one of the windings on the output side of T1, and the local, output side ground. The 5R11 and caps on the other side of the PCB, are probably part of an RC snubber circuit, also on the output of T1. I wonder what triggered all this, like did the cap or something near it just fail on it's own, or was the voltage driven too high on power-up for some reason.Do you know what voltage rail this was part of, it might have been the standby voltage rail, and then you could really have a problem getting it to turn on again.If I owned it, I would make my own schematic, and probably remove a bunch of stuff, thats a lot of extra work tho, and not a quick fix. With it unpowered, I would check lots of parts with a DMM, just looking for shorts, or things that don't seem right. You may have also shorted layers inside the PCB.Do you have a 100W incandescent light bulb ? You can use 1 of those, in series with the subwoofer, and if for any reason it draws too much current, then the light bulb just starts lighting up as normal, and using most of the available voltage and power. It might flicker a bit on startup too, as huge caps get charged up. So then your device under test won't get fried right away again, if there's still a big short. I wired up an old light bulb socket, and AC wall switch and outlet, onto a piece of 2x4. That works great, and can really save you in repairs like this.Once I had the damaged area's cleaned up, and checked for damaged parts, then I'd use the 100W light bulb in series, and plug it in and see if there's any low voltage Vcc to the controller IC's on the primary side, and if they are running or not. Maybe only 1 output rail was damaged, and the rest still work.There's a lot of things I would worry about 1st, before what size caps need replacing. But I guess some people would just replace the big one with some 10uF, and try it out, without checking a ton of other stuff. In a snubber circuit, they might have a few nF to a few hundreds of nF. But I'd want to see some of the circuit mapped out 1st.

author: playboygamer1
author_rank: Newbie
timestamp: May 13, 2025, 07:00:48 pm
content: Hi again @MathWizard,Thanks again for your detailed advice and support earlier!🔧 Quick update:I just ordered the exact same Z906 system again (brand new), so in theory, I could try to measure components on that working board if needed – though I’d have to open it up, which I’d only do carefully.The thing is, I’m just a hobbyist (not a pro), and I’m not even sure if my multimeter can measure capacitance. I don’t recognize the correct symbol for it, and I don’t fully understand how to check for AC or DC properly. Some friends told me I’d need to get a Farad value reading – but I’d appreciate any help in understanding how to measure the right way.📌 Some more background:When the failure happened, I turned on the system as usual (it had been always plugged into the wall). A brief knack or pop sound was heard, and only 2 or 3 LEDs briefly lit up on the control console. Then it went totally dead.I found a heavily burned SMD capacitor (C86) near D29 and RT1, with burn marks also on the reverse side of the board.Another user (PedroStarr88) had the same failure and shared photos. His friend fixed it by replacing the cap and also resoldering one of the amplifier ICs (it had bad joints). After that, the system went into test mode and worked again.Pedro’s board also had a blue film capacitor added between D29 and RT1. I’ve messaged him asking for more details, but so far, his friend (who did the repair) hasn’t confirmed the exact capacitor values – he just said that the root issue was a broken leg on an encapsulated diode that caused a power failure to the amplifier stage.Here are the pictures again from Pedro’s fix:🔗https://prohardver.hu/tema/logitech_z906/hsz_5201-5300.html#msg5201🛠️ What I’d like to ask you:If one of the amplifier ICs could be loose (like Pedro's case), how can I test that safely? I heard something about checking with continuity mode (OL), but I’m not sure where to probe exactly or what to expect.If I wanted to test the affected zone, how could I check for broken traces, open joints, or separation due to the burn? Would a diode mode or resistance check help?Do you think the added blue capacitor between D29 and RT1 was acting as a snubber or suppressor? Could that be a helpful mod?Someone suggested using a surge protector + inrush current limiter combo in a power strip – do you think that’s a safe solution to avoid this kind of failure in the new unit? Or would the limiter interfere with the normal startup?For reference, I live in the EU (Germany), where voltage should be stable (230V), and I was surprised this kind of damage happened from a normal wall outlet.Thanks so much for your time – I know this is already a deep rabbit hole, but I’d really like to understand the cause and avoid it with the replacement unit too.

author: MathWizard
author_rank: Super Contributor
timestamp: May 14, 2025, 01:01:23 pm
content: I'm not sure how or why that big cap cooked, how old was the system ? It sits right under the heatsink, and shouldn't be directly touching it, so it probably gets pretty hot, after running for a while tho.I'm guessing your new Z906 is under warranty and you don't want to mess with it.But you could probably add a small fan somewhere, a typical 2 wire 12V computer fan should run off 5V and only use about the same power as 2-3 of LED's. But I'd want to see the PSU and test it to find a safe place to connect it.I'm not sure what RT1 connected to, or what size cap that blue one is, but it reminds me of high voltage, low capacitance caps, probably many 1,000's or more times smaller than that big SMD cap.A typical continuity tester might beep when there's less than 50 ohms between two points, which is still a lot of resistance. A good solder joint and short wire or trace on a PCB, should be under 0.1 ohms. If you have a magnifying glass or good camera, you might see a solder joint on that chip that looks "funny" compared to the rest. But to test it w/ a DMM, you just measure resistance from the top of the pins coming out of the IC, to the far side of the solder joint, or even better, to where the trace/wire goes to. But on those types of PCB's, it can be hard to find where stuff connects. Continuity mode is fast for narrowing down where stuff connects to.To really test the PCB around the burnt areas, I'd probably remove a bunch of stuff, including the big transformer, and use solder-wick, to clean away solder too. And then I'd be looking for places that I would know, should or should not be connected. It might only be a 2-sided PCB, with no copper layers inside. That would be way easier to check over.I'd definitely buy some soldering flux for SMD use, and solder-wick / copper braid, and some Isopropyl alcohol for cleaning with cotton swabs.And some electronics fluxes, are not really safe to use on these PCBs, and beginners doing repairs, as I found out the hard way, removing stuff too many times. It can get under traces, and the PCB coating gets scratched and cooked away, and some fluxes can leave a conductive reside.Just cheap waxy type rosin flux is pretty safe, you don't have to spend a fortune, buying 10mL needles of BGA chip flux.

author: playboygamer1
author_rank: Newbie
timestamp: May 29, 2025, 03:49:03 pm
content: Hey @MathWizard,Thanks a lot for your helpful insights and suggestions!The defective Z906 unit is from 2022, so it’s not old. I don’t exactly know why it failed.I recently bought another used Z906 system, which is over 10 years old, and still works perfectly. That one was mainly intended for a sibling, but I might use it for measurements or comparison to help diagnose the newer, defective one.Since my new unit is still under warranty, I don’t want to open it up or risk losing that protection. But I did make a few improvements:• I added a 300×20×140 mm heatsink, cut to 229×160 mm, and mounted it at the back of the case for better passive cooling. (https://prnt.sc/0ByGH0CS7kDu)• I’m now using a surge-protected power strip and plugged the Z906 into an SCL 3680 inrush current limiter to reduce startup stress on the PSU.I’m also considering going to a repair café or a local electronics repair shop that offers a €30 estimate, which is credited toward the repair, and they offer a 6-month warranty.Regarding your suggestions:• Adding a small fan sounds great, but some others said it would cause negative pressure, so I went with the heatsink solution.• I appreciate the reminder about flux types. I got some solder wick and flux from China. I think they should do it.Best regards