[Solved] Help with Focal BP374 Subwoofer schematics - Page 1
Forum section: electronics / repair

author: rfengg
author_rank: Regular Contributor
timestamp: April 17, 2022, 04:45:44 pm
content: Hi All,Need your help with the schematics for my Focal BP374 BASH Subwoofer ......have attached pics of the same.The amp blows its fuse when plugged in and sure enuf, I found a shorted IRF730 in the power supply.However, I have a broken resistor as well on the board.....the board is covered by this terrible conductive gluethat they have on the board (brown color) and the resistor is hidden behind it. The glue has allmost eaten away the resistor color code and all I can see is the third band which is orange.Does anyone have a schematic or the service manual of these amps made by Indigo so that I can put in a new resistor as well?

author: rfengg
author_rank: Regular Contributor
timestamp: April 18, 2022, 01:35:38 pm
content: Did some circuit tracing and the resistor in question seems to be directly across one of the 220uF, 200V filter caps (the biggest 2 blacks caps on the attached photo).Is there a value I should start off with like say 100K as I cannot get hold of the schematics?Is the purpose of the resistor only to bleed the filter caps when power is switched off?

author: SeanB
author_rank: Super Contributor
timestamp: April 18, 2022, 05:49:09 pm
content: Will be one of the same value across the other capacitor, and often serves double use as both a bleed resistor and start up resistor for the power supply. Clean off all that glue, and also replace any small electrolytic on that main controller board by the power devices, that likely is the cause of blowing the power switches, plus all those sitting between the regulators on the secondary side, they will be nicely toasted by now.

author: rfengg
author_rank: Regular Contributor
timestamp: April 18, 2022, 07:00:33 pm
content: Easier said than done Sean.....the glue is hard as a rock.....using a knife is extremely time consuming to take all the glue off......do you have any easy methods to remove the glue?Freezing it or any solvents?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 18, 2022, 07:24:15 pm
content: Quote from: rfengg on April 18, 2022, 07:00:33 pmThis stuff works well.https://uk.farnell.com/servisol/100019000/flux-remover-160-200ml/dp/3821470?st=flux%20removerIt is good at removing the hard black corrosive conductive stuff, but is not much use for removing the browner not yet a problem stuff.

author: rfengg
author_rank: Regular Contributor
timestamp: April 18, 2022, 07:41:35 pm
content: Quote from: Audiorepair on April 18, 2022, 07:24:15 pmI metered the "browner" areas of the glue with my Fluke 289 and it shows a resistance of around 40Megs, while the yellow regions of the glue do not show any conductivity.Are the browner areas not a problem yet?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 18, 2022, 08:02:18 pm
content: Quote from: rfengg on April 18, 2022, 07:41:35 pmYes, sorry, that was quick from memory.The Yellow stuff is unaffected yet, and isnt yet a problem.The brown stuff does indeed start metering megohms, and this is now indicating a definite problem.The black stuff is the brown stuff gone corrosive and extremely dangerous, as it will now eat the PCB and component leads.The servisol solvent I posted a link to is very good at removing the dangerous corrosive black stuff, but you will need to replace eaten components at this stage.It is less good at removing the brown conductive stuff, but it is still quite helpful in this process, it will make things much easier.Trying to remove the yellow stuff is a total nightmare, forget it.

author: CaptDon
author_rank: Super Contributor
timestamp: April 18, 2022, 10:13:28 pm
content: Electronics manufacturers have known the shortcomings of that ply-o-bond type rubberized glue being a total disaster around heat and electricity since the mid 1980's. Kenwood had to eat thousands of repairs on TS-440 radios as well as others. Behringer used the stuff in their SMPS power supplies with equal disaster. I don't know what year that subwoofer was made but that smacks of another cheesy chinese company producing crap as fast as they can with book smart but real world stupid greenhorn engineers. It just happens over and over again, and yet the failure mode of that glue is well known and well documented. Hope you can fix it. Look at all the components around that blown fet to make sure it doesn't simply blow up again the instant you power it up. Trying to bring up SMPS supplies on a variac is often known to blow up a good supply.

author: rfengg
author_rank: Regular Contributor
timestamp: April 19, 2022, 12:01:58 am
content: Quote from: CaptDon on April 18, 2022, 10:13:28 pmThanks CaptDon.The Focal subwoofer was made in 2014 in China but being a BASH amp, the design is Canadian afaik.This is first time I am opening up the amp since it was bought and I cannot believe what I see inside.....real piss poor workmanship all around in the electronics......and to think I paid close to $1000 for this piece of junk.The driver itself is solid though and that is the only saving grace.

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 19, 2022, 02:01:00 am
content: Hi!I have to admit that the only thing you definitely should do with any Chinese manufactured piece of electronic equipment is draw it out whilst it's still working or at least write up a full components list of what's in it – I do this with anything I buy new, then when it eventually goes "piff–paff–puff" as my amiable brother–in–law calls it, you've got a record of what was in there to begin with!A case in point being the sub–woofer assemblies in Logitech Z906 speaker systems, there's countless tales on this Forum of these things having either conked out, gone bang or failed in some other way, resulting in cries for help, but nobody seems to have taken the bull by the horns and drawn up a schematic or parts list yet – I won one on eBay with the intention of doing just that, but the seller refused to honour the sale and sent me a nasty message, and so far even blown up or faulty ones are being listed with silly bloody prices that are more than I want to pay!Chris Williams

author: rfengg
author_rank: Regular Contributor
timestamp: May 01, 2022, 04:08:51 pm
content: Glad to report that after a lot of elbow grease to remove the majority the dreaded yellow glue on the Focal BP374 main board, in the process of which I inadvertently damaged a few of the traces as well, and after the replacement of both the IRF730's which I procured from Mouser for couple of dollars each, the sub is back to its normal operation.Regarding the damaged resistor, I found a similar power supply section from another sub and the value seems to be 47kohms, so I replaced that as well with two 100k resistors in parallel.Thanks to everyone on this forum who provided advice to my queries

author: johnparker99
author_rank: Newbie
timestamp: May 05, 2023, 02:31:37 pm
content: I found this thread after my BP 374 blew its fuse, thanks. Looks like I have the identical problem. 47k resistor is shorted. The smaller one next door has a resistance, and my example is undamaged enough to see it is not 47k. I can see Brown Something Yellow. When I measured the resistance with my Fluke, it was 3M but then dropped down as I left the probes on. I'll try cleaning to see if I can get the middle band

author: rfengg
author_rank: Regular Contributor
timestamp: May 12, 2023, 08:28:24 pm
content: Are you sure the 47K is shorted? The usual failure mechanism is to go open circuit or have a higher resistance......I dont have the exact schematics but have something quite similar which helped me to fix mine.......

author: johnparker99
author_rank: Newbie
timestamp: May 16, 2023, 05:46:10 pm
content: I pulled the resistor and was not shorted, thanks for the response. I now see the 2 IRF730s mentioned are on the transverse heat sink, so I'll examine them in detail as they look like they might have slightly cooked the board

author: rfengg
author_rank: Regular Contributor
timestamp: May 16, 2023, 07:34:54 pm
content: On my amp, one of the IRF730s was finito.....but they should be available across the pond, for less than 2 pounds per piece.