Can this amp be salvaged? (Revel B12 / Infinity Kappa) - Page 1
Forum section: electronics / repair

author: vmallet
author_rank: Regular Contributor
timestamp: October 01, 2019, 10:12:19 pm
content: A friend of mine brought me his powered sub-woofer that wouldn't turn on anymore. I took a look and there's some nice charred drama on the amplifier board!There seems to have been a little fire in there. Scary. Looking at the board from the top, I can easily see four caps with nice domes so these would have to be changed. But looking at the bottom side of the board it looks like there was some real damage caused by whatever happened.Questions:1) What can cause this type of fiery death? Can a cap going bad be enough to produce something like that? Can someone theorize on what happened here?2) Could this be salvaged / repaired? (Note: the important part here for me would be the learning experience, not the cost/time ratio)3) The RTV-Silicone-looking thing all around these caps & inductors is new to me; I saw in another thread about repairing a car audio amp that it can be used for mechanical properties for vibrations; could it be the case here too? For the low-frequency vibrations in the sub?4) I haven't found the schematic for the amp yet. Could this be resurrected without a schematic?5) This stuff smells really nasty (even though my friend says this thing has been dead for months). Is it toxic to breath or it's just stinky no worries?Note: this board is mounted vertically in the enclosure with the two giant caps located towards the top; this explains the fire mark going "up" along the boardJust in case someone knows about where to find the schematic: the sub is a Revel Concerta B12, and the amp board inside says: "Infinity - 400 Watts Amplifier - Kappa-Rev-01"Top view; notice the four blown caps:Bottom view; curious to see if this can be saved:

author: TheMG
author_rank: Frequent Contributor
timestamp: October 02, 2019, 12:39:50 am
content: Looks like a class D amplifier. My guess is the bulged capacitors failed gradually over time due to heat and high ripple current (although class D amps are quite efficient, the output inductors do usually run quite warm). Once the caps ESR went high enough, it probably caused the amplifier to go into a destructive oscillation which cooked some components (probably snubber network?).Without a schematic due to the level of damage it would be difficult to tell what the burned up components need to be replaced with.The damage to the board, if it is only a double-sided board, could definitely be repaired.The good news is, even if the amplifier ends up being beyond practical/economical repair, there are several manufacturers that make all sort of "plate amps" in a wide variety of sizes and power levels, some very reasonably priced, so the subwoofer should be salvageable with a whole brand new amplifier module if all else fails.As for the smell... yeah, it smells pretty strong, but once the smoke clears I don't think there's any concern, it's just tiny traces of chemicals at that point.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 02, 2019, 01:37:42 am
content: Looking at the layout it seems the bulged electrolytics were part of the output LC LPF (class D), if that's the case, it's a bad idea to start off with to use electrolytics there. Then once they've spewed enough juice on the PCB, with enough time to soak in and enough voltage the conductive PCB sizzles away.Sure it could be repaired, a lot of things can be if you want to go to the trouble. On this one I'd be looking for a slightly better designed class D board to slap in place of the original crap.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 02, 2019, 04:04:58 am
content: Grab an old toothbrush and a bottle of isopropyl alcohol and start scrubbing the board. Until you've removed all the soot and crap you won't know how badly the board is damaged. It could be nothing more than a few surface mount resistors that burned. If the board is cooked or corroded through, it's probably not worth the effort to repair.The way they poured in that silicone is crazy. They might as well have wrapped the capacitors in a nice cozy blanket. It practically guaranteed that they'd die.Ed

author: don.r
author_rank: Frequent Contributor
timestamp: October 02, 2019, 04:55:01 am
content: You will have to remove all the RTV as those caps buried in need to be replaced. My take would be figure the input pinout and buy an off the shelf class D sub amp and retrofit it. Looks like they lifted this straight from one of Infinity's car amps.

author: vmallet
author_rank: Regular Contributor
timestamp: October 04, 2019, 05:21:10 pm
content: Thanks for the input. I haven't had too much time to dedicate to this so far but I found the service manual for a sub with an amp that looks really close to this one, the Infinity Alpha 1200s. The layout of the amp PCB is very similar, and a lot of the component locations and designators are identical. It might help understand some of what should be there...https://www.manualslib.com/manual/922762/Infinity-Alpha-1200s.htmlI did email Revel and Infinity to try to get the official service manual but they told me to send the amp to an authorized service center because it's proprietary information.The board looks cooked through, I tried to take a photo with my cheap usb microscope but it's not very good:It looks a bit beyond my skills but it will be really fun to try and see how far I can get with this, not giving up just yet! Next step will be to remove the crazy silicone and start inspecting the board from the top.To be continued...

author: Yansi
author_rank: Super Contributor
timestamp: October 04, 2019, 06:02:40 pm
content: ROFL ROFL ROFL!!Do I really see bipolar electrolytics used within the output lowpass filters?

author: vmallet
author_rank: Regular Contributor
timestamp: October 04, 2019, 06:26:54 pm
content: Quote from: Yansi on October 04, 2019, 06:02:40 pmshakalnokturn had mentioned this being a bad idea too earlier; would you mind explaining why it is so bad for those like me who are just starting to learn this stuff?Thanks!

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 04, 2019, 08:38:26 pm
content: It's a bad idea if you want the amp to last long, not a bad idea if you want your (foolish enough to stay dedicated to the name) customers to buy new products regularly.In the output LPF the capacitors are submitted to high frequency and highish current, if the capacitor's ESR isn't very low this means heat to dissipate. Electrolytic capacitors don't do well with heat, as they degrade the ESR rises further...The only thing is that a electrolytic is smaller and cheaper than what should have been used.

author: Yansi
author_rank: Super Contributor
timestamp: October 04, 2019, 08:53:39 pm
content: I am sorry, I have skimmed through and haven't noticed it was already mentioned.I have never ever seen bipolar caps in any class D amps ever. This is a kind of first. Even the cheapest of them all from Aliexpress I got hold of have used at least ceramic caps (not really suitable, but miles better then electrolytics) or foil ones (quite suitable).Problem with those electrolytic caps is obviously the longevity (.. or lack there of). Caps in the filters are subject to rather high current ripple (up to couple of amps!) in high power class D amps. Due to inherently higher ESR of the electrolytics they are subjected to elevated heat, which leads to fast deterioration. Cheap and substandard caps can even blow like the ones in your amplifier.In a proper higher power class D amplifier, the output caps shall be low loss high current capable foil types, such as polypropylene (best) or at least polyester (MKT, good enough). And this costs money. A good MKT foil cap for the output filter can cost about $0.5 in volume, MKP type even more so. Therefore it is obvious these components are subject to (brain-dead) cost reduction, as they are quite expensive, compared to the rest small-signal parts of the amplifier.Also the output chokes must be wound on a high quality core materials for the amplifier to retain good efficiency (low core heating due to high current ripple) and must be very linear*, as the output filters of most class D amplifiers are outside of the feedback loop, so their distortion is not compensated for.*by linear inductor, one means mostly that its inductance is not dependent on the passing current. Most high-permeability cores (iron, ferrite, iron powder,...) have quite substantial amount of non-linearity and care must be taken when designing the magnetics. Typically, only ferrite cores with an air gap or low-permeability iron-powder types (mix #2 for example) are suitable for the output filters.

author: andy2000
author_rank: Frequent Contributor
timestamp: October 04, 2019, 09:24:06 pm
content: It probably can be repaired, but it might not be worth the time. You'll need to clean the area and cut away all of the blacked circuit board. Then, you will have to install jumper wires to replace the missing traces. There's also the possibility of hidden damage such as blown outputs.

author: vmallet
author_rank: Regular Contributor
timestamp: October 12, 2019, 09:46:30 pm
content: Yansi & shakalnokturn, thanks a lot for your helpful explanations. I read up some more on LPF filters for audio amps and indeed, what were they thinking! Now I understand the laughsQuestion: if I were to replace these caps hoping to revive this amp, could I (should I) replace them with MKP/MKT caps instead of the NP electrolytics? Can I just pick MKP caps with the same capacitance/voltage/temperature ratings and find a way to fit them in there? Or would there be more to the picture than that and I should stick to the same type and be back at it in another 10 years?Yansi, re: inductors: how would I know if there are using high quality core materials? Can I measure them somehow and know? I pulled them out of the board for now so I could put them in a little test circuit I guess but I don't think I would know how to go about it at the moment. Any input there would be appreciated.Thanks!

author: vmallet
author_rank: Regular Contributor
timestamp: October 12, 2019, 10:01:49 pm
content: Quote from: andy2000 on October 04, 2019, 09:24:06 pmThanks Andy. I'm in it to learn right now so it makes it worth the time (if anything to learn what's not worth spending time on...)When you say "blown outputs", do you mean the output transistors being blown? These guys were on the other side of the fire and they look pretty normal from a visual point of view, but who knows...

author: vmallet
author_rank: Regular Contributor
timestamp: October 12, 2019, 10:31:04 pm
content: I removed a lot of that silicone business from the board and pulled out the components around the "fire" out to expose the damage. From what I can tell one of the LPF caps was really damaged and the hole in the board is pretty much under it, so that might have been the cause.So I would have to cut out a good chunk of the board but it seems it's mostly around the few LPF components I pulled from the top of the board, and a few SMT passives underneath. This section of the amp appears identical to the schematics I found so I might be able to know what's missing underneath.The question is whether I can reuse the inductors or not. How would I know? I tested them with an LCR meter, the two big ones read 70.3uH and 71.9uH @10kHz (schematic calls for 70uH) and the small ones read 31.8uH and 0.7uH @10kHz (for 30uH in the schematic). I'm guessing the 2nd small one is shorted. Is that fixable?Also, what is this last one? It's the component on the left in the picture above, It looks like an inductor with four leads, and two wires inter-coiled. It's L12 as in:Thanks!

author: Yansi
author_rank: Super Contributor
timestamp: October 12, 2019, 10:36:34 pm
content: Google for a "common mode choke". That is used as electromagnetic interference filter typically.PS: I wouldn't bother repairing this board.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 12, 2019, 11:22:40 pm
content: That's a lot of damage. Once you cut away all the charred PCB material you'll be left with a pretty big hole in the PCB. How do you intend to support the replacement components and traces?If you somehow proceed with this, I would use film capacitors instead of the electrolytics. Wrong choice of capacitors is likely what caused this failure in the first place. I'll bet one of the caps leaked electrolyte due to excessive heat and ripple current, the leaked electrolyte bridged some power traces, and the rest is history.

author: Yansi
author_rank: Super Contributor
timestamp: October 13, 2019, 12:11:26 pm
content: ...and with film caps, you will run out of the available space.

author: Fraser
author_rank: Super Contributor
timestamp: October 13, 2019, 01:00:13 pm
content: This thread is quite an education in how nasty some modern amplifier designs can be !Thinking about the potential for repair and what the OP is trying to achieve..... I would say “go for it” and try to repair the amplifier if it is not too expensive for the components. It is a learning exercise after all and, thanks to the comments in this thread, the OP now knows a lot more about why the unit failed.Just for the experience and fun of the challenge I would repair this boardFirst the PCB damage....... I would cut out a rectangle of PCB that encompassed the badly damaged area. I would then either insert an etched replacement PCB section or a piece of prototyping FR4 board that has the through plated holes in it. The replacement section needs to be securely mounted to the original PCB. I would use a combination of epoxy resin adhesive and brass strips soldered to 0V copper plain. The exact details depend upon the PCB topology. Once the OCB hole is filled the replacement components can be refitted.Regarding the replacement components and considering that this is a bit of a learning experiment..... I would reuse the inductors and Common Mode Choke. I would rest the discrete components like transistors, diodes etc. If they test OK, you have the choice to reuse or replace as desired. Replacement is advisable if the components are suspected of having been stressed in the failure event. Now the controversial comment..... for the sake of this experiment and to discover whether this amplifier has any life left in it, I would purchase high quality very low ESR electrolytic capacitors, such a some nice Panasonic types and fit these into this amplifier. I know that this just repeats the OEM error but it will work to test the amplifier and is affordable. If the amplifier works then the OP can discuss potential capacitor upgrade paths and costs. There is no point in buying high quality expensive capacitors only to find the amp has issues that are not worth repairing.Now if a prototyping PCB has been used in the repair, new PCB tracks will need to be created using suitably dimensioned copper wire. I would strip such copper wire from common 2 core and earth mains cable that is available in various sizes. It solders well and is more than suitable for the task at hand. The new copper tracks are soldered to the component leads on the underside of the PCB to recreate the required layout. As with a proper etched PCB installation (if used) there will be a need to connect the new PCB to the surrounding original with with copper wire. What you will end up with is a replication if any missing or damaged PCB traced using round cross section copper wire. It may not look pretty but consider it ‘artistic’ !I have had to repair irreplaceable PCB’s in this manner to maintain mission critical systems or to maintain operation until a new PCB can be made/procured. It works well if done properly and with some thought. An example is high value spectrum analyzer that was dropped. The CRT PCB flexed so much due to the weight of the LOPT that the PCB broke in half ! I field repaired that CRT board after a visit to the local model and DIY shops. I repaired that PCB using brass strip, brass box section and copper wire from 2 core and earth cable. It actually looked quite pretty when finished and is still working to this day, 10 years later ! I shall have to see if I still have a picture of it.So let us get back to the OP’s PCB. I see it as eminently repairable and a very useful PCB repair exercise. Provided the OP and owner of the amplifier understand why it failed, that it will likely fail again if electrolytic capacitors remain in the failed section, I see no harm in proceeding. It will be relatively cheap to repair in terms of getting it running anyway. PCB repair is a skill worth learning and this is a perfect opportunity for the OP as it is a simple PCB design. I was trained to be a component level repair tech but that also included repairing the actual PCB material in cases such as this. There are many techniques available for such work including careful layer reveals in conical PCB ‘excavations’ and epoxy filler repairs with new PCB traces adhered to the repaired sections..... it is an art and patience is essential. Well worth playing with a simple PCB to learn the basics though. Knowledge and experience can be of far greater value than the item being worked onAt the end of the day, if the amp is still dead after a repair attempt, the OP has learnt how to repair a holed PCB and so there is value in that.There is quite a lot of information on holed PCB repair on the Internet and those considering undertaking such a task may benefit from some reading first as there are some great techniques worth learning.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 13, 2019, 01:16:15 pm
content: In case anyone wonders how best to remove the burnt section of this PCB, there are various options.1. Sharp scalpel using many repeated cuts until full penetration is achieved. Hard work !2. Chain drilling and filing of edges to create smooth edges to the aperture.3. Dremel or similar rotary tool equipped with cutting wheels and burs.4. Suitable milling machine. I have two types of Proxxon micro milling machine for work requiring precision.I use the Dremel method most often.When using any tool that created lots of dust, wear eye and breathing protection. Afterwards clean the PCB with IPA to remove any conductive particles.Fraser

author: Yansi
author_rank: Super Contributor
timestamp: October 13, 2019, 03:27:04 pm
content: With this much damage to the pcb, I can bet you can not remove the black, unless removing the whole pcb. This is not just a surface arc flash. This is a deep fried chicken.

author: Fraser
author_rank: Super Contributor
timestamp: October 13, 2019, 03:38:30 pm
content: There is also the potential for further damage in the power supply traces and/or circuitry. But this is a good learning exercise for the OP.This is not what I would call high performance equipment so there is little at stake apart from the cost of a few cheap components and the OP’s time. In the case of a severely burnt PCB, I cut out the burnt areas, as detailed and bypass any power traces that are damaged within the PCB.It is true that some PCB’s cannot be saved. Classic examples are multi layer PCB’s that have suffered severe battery electrolyte leakage over a period of time. The electrolyte gets deep into the PCB and it is rarely recoverable, even with chemical baths and leeching agents to remove the electrolyte.Fraser

author: vmallet
author_rank: Regular Contributor
timestamp: October 13, 2019, 05:21:35 pm
content: This is very exciting, thanks Fraser. The idea of trying to see where I can get with cheap components for now and upgrade later if it ever works sounds good.The thing with reusing the inductors is that one of the 30uH seems shot (measures 0.7uH with my meter) and I'm not sure how to find a replacement that matches the other one yet (a quick look on Mouser left me with more questions than answers).Also re: low-ESR electrolytics, the ones to replace are non-polarized and that reduces the selection quite a bit. I'll go through the datasheets of what's available and pick the lowest ESR versions available. We'll see!ThanksVince.

author: Yansi
author_rank: Super Contributor
timestamp: October 13, 2019, 06:05:38 pm
content: I think that way better learning experience for the OP might be to design a decent class D amplifier from scratch, rather then spending tens of hours on trying to repair this piece of crap.Get a couple of IRS2092 chips and let's make it work. Quite a decent class D driver chip actually.

author: Fraser
author_rank: Super Contributor
timestamp: October 13, 2019, 06:42:18 pm
content: Yansi,How does designing a new amplifier teach the OP anything about PCB repair ?This is a relatively simple place to start with such learning. Designing is a completely different experience.Fraser

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 13, 2019, 07:23:48 pm
content: Either way there are things to learn.In terms of hours spent, usually designing from scratch takes longer (at least for me) in this case I'm not certain!It is a piece of crap, there's no way under five times the price of the active speaker's price new I would have even tried to repair that PCB, sometimes you just need to prove it can be done though.

author: Fraser
author_rank: Super Contributor
timestamp: October 13, 2019, 07:48:17 pm
content: Totally agree. It is the OP’s desire to learn from this investigation or repair that prompted my comments to help him. As a commercial repair the unit is definitely scrap as it is beyond economic repair in man hours alone, let alone any modifications required to improve reliability. As others have said, a replacement, better designed power amp module or even a complete new sub woofer unit would likely be a better course to follow if the OP was not keen to learn from this units repair. That said, I have taken on repairs for friends in the past and bitterly regretted it after spending many hours sorting out a real sticker of a design that belonged in the bin ! I learnt from those experiences thoughI think the worse was a friends Trio Kenwood TS9xx (TS940?) base transceiver...... lots of failed capacitors and more dry joints than I have ever seen in a unit previously. If you moved any PCB you were likely to cause more failed solder joints. I found out that the particular model was infamous for such failures and was basically a rabbit hole you did not want to enter ! It took me two months of evenings working on it when I had the time and motivation before it was repaired and relatively reliable. I honestly lost count of the solder joints I desoldered, cleaned and resoldered. It was in the hundreds of joints for sureI gave it back to the friend and told him that I never wanted to see it again and he should sell it while it still worked ! Why did I not give up on the unit as beyond repair ? Well I should have done but he was a good friend and I hate to be beaten. If I had my time again, I would have stopped work on the unit after a week of chasing multiple dry joints and failed capacitors on many PCB’s. The unit is just not worth the effort, even when repaired ! A good lesson learnt though. Did I charge for the work.... nope, I used to repair equipment for free as I liked the challenge and learned a lot along the way.

author: EHT
author_rank: Frequent Contributor
timestamp: October 15, 2019, 08:39:23 pm
content: I know its not as exciting or educational, but I recommend you throw the burnt amp away and just use a standard Class ABexternalamp which you could pick up cheaply. You couldn't really get a worse environment than being boxed into a subwoofer being shaken to bits with no airflow.

author: vmallet
author_rank: Regular Contributor
timestamp: October 15, 2019, 08:45:04 pm
content: Update: I cut out most of the black on the PCB after studying what I would have to replace and reconnect. It feels a bit daunting!I also found that the service manual is lying in placesWhat's drawn in the schematic doesn't quite match the board layout a few pages away. Makes it harder to proceed.I pulled the two big 15,000uF caps to test their time constant in a small RC circuit and they still look good, I'll keep these.I still need to figure out what to do with the dead inductor. I might try to re-wind it after applying some plasti dip to the areas of the wire where the insulation melted but I'm fearing the result where I would be missing a couple of turns because it wouldn't be wound as tightly as the original.. To be continued

author: negativ3
author_rank: Regular Contributor
timestamp: October 17, 2019, 02:00:40 pm
content: Christ, I think I can see the event horizon through that black holeKeep up the good work, great challenge and learning!

author: edpalmer42
author_rank: Super Contributor
timestamp: October 17, 2019, 03:43:18 pm
content: I have a piece of equipment where there was an extensive design change. Rather than replace the circuit board or add a bunch of jumpers, they abandoned the section of the board where the change was. A new board was installed above the existing one with stiff wires connecting the two. Works fine. This isn't some cheap retail device, it's a cesium frequency standard!If the inputs and outputs of the damaged section of this amplifier can be identified, the same idea might work well for this repair. Mount the new board with insulating hardware and drill mounting holes where convenient.Ed

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 18, 2019, 12:25:04 am
content: Quote from: edpalmer42 on October 17, 2019, 03:43:18 pmInfinity also made Cesium frequency standards? I bet they take forever to clock...

author: XSdB
author_rank: Newbie
timestamp: November 10, 2022, 02:47:28 am
content: Here is a blog I did about repairing this exact issue from 2011:http://www.cryo-sonic.com/This article was featured on Hackaday also:https://hackaday.com/2014/11/13/extreme-repair-of-a-burnt-pcb/There is also a mod that needs to be done that eliminates the chirp at power on:change C109 and C132 both to 47uF / 16V / 105 Cand remove R263 from preamp boardThere are 3 muting mechanisms in this amp. Line level mute (which part of it serves a double purpose as the limiter), power mute (powers down power amp) and the speaker relay. Initially, all 3 would engage/disengage at different times when going in and out of standby.We found that the line level mute did more harm than good for turn on/ turn off transients when everything else is functioning properly. Removing R263 disables the line level mute from engaging during regular use. However, the limiter function is still able to work properly with R263 removed. Also, the line level mute will still function if the sub goes into standby because of a protection issue.