Monitor Audio BXW10

[MattM]

I have a BXW10 subwoofer. It powers up and will sit happily in standby in auto mode (where it is sensing from the AV receiver) however as soon as a load is applied the relay clicks and it blows its fuse - the IEC socket is fused.

I have looked at the boards and there is nothing obvious although I know the caps in the RSW12 version were an issue.

Has anyone had any experience of this or ideas on where to start please?

Thank you

[momaka]

Re: Monitor Audio BXW10

If it's blowing a fuse, definitely have to check the output stage and power supply for short circuit.

Start with opening the woofer, removing the amplifier board, and taking good pictures. Then post them here. Service manual may be helpful too, if you got it.

[MattM]

Re: Monitor Audio BXW10

Thanks very much for the reply. I have uploaded some photos. The two boards are connected together fairly extensively so I have taken them as best I can without desoldering wires. Thanks again

[MattM]

Re: Monitor Audio BXW10

Photos now attached

[momaka]

Re: Monitor Audio BXW10

Do you have a multimeter? At this stage it would be needed, along with a soldering iron.

According to the user manual, this subwoofer has a Class-D amplifier with an SMPS (switch-mode power supply), which your pictures confirm. In that case, I suspect it is most likely the power supply that's bad and I suggest to start with that.

But first, let's get the easy things out of the way: use your multimeter to check the resistance of the woofer and post it here. If the woofer is connected to the amplifier board with terminal connections, remove those first. We want to see if the woofer driver is shorted. Generally, that's rarely the case. And even if it was, the power supply shouldn't blow a fuse. Nevertheless, we have to make sure we don't overlook anything, so first I always check the easy/simple things.

Next, the power supply (SMPS): we will need some good straight-down pictures of both sides of the power supply in good light so that we can see components more clearly. I see that the two boards on your amplifier are connected with connectors, so you shouldn't have to desolder anything, I think. Separate the power supply board by itself so it's easier for you to work on it. Once we have some better pictures, we can guide you which components to check.

Also, if you or anyone else is able to find the service manual, please feel free to post it here. I tried looking for it, but no luck so far.

[MattM]

Re: Monitor Audio BXW10

Hi thanks so much for this. The speaker is measuring 3.05 ohms on the terminals. The two boards are mostly connectors but bizarrely there are two wires that aren’t that come off the 240v / 110v switch. I have attached some more photos which I hope are better. I will see if I can get the service manual. Thank you again much appreciated

[MattM]

Re: Monitor Audio BXW10

Photos attached

[budm]

Re: Monitor Audio BXW10

https://www.badcaps.net/forum/attach...8&d=1545254488
I would check the resistance between the two inner legs and between the two outer legs of the main bridge rectifier, the check the resistance between S & D pinsof the power MOSFET's mounted on the hot side heatsink. See if any of them shows very low low resistance or not.

[momaka]

Re: Monitor Audio BXW10

I concur with what budm says.

Attached below is a partial schematic I was able to draw from your picture as a help guide. Basically, what budm was referring to check is the resistance between the "~" legs of the component labeled "BRIDGE RECT." and also between the "+" and "-" legs. Report what you get back.

Next, check resistance between the three pins on the two switching transistors (boxed with red on bottom of the picture). Again report what you get. Refer to the leftmost pin of each of these transistors as pin 1 and the rightmost as pin 3. That way, we should be able to get an idea if there is anything shorted and between what terminals. Hopefully that should also tell us if these "switching transistors" are MOSFETs or BJTs.

Lastly, can you read us a part number off of U209? This is the main power supply controller. I omitted making any markings on the standby (STB) power supply, as that appears to be working on your amp.

[kaboom]

Re: Monitor Audio BXW10

I've attached an edited image with the gate drive components marked.

At least two components in each group have been damaged. These are small SMT diodes and transistors. They're arranged in such a way as to allow rapid turn off via local discharging of the gate capacitance. This must be addressed in order to properly drive the MOSFETs.

Looks like Q909, by the gate drive transformer, has been scraped against something. Damaged?

[MattM]

Re: Monitor Audio BXW10

I concur with what budm says.

Attached below is a partial schematic I was able to draw from your picture as a help guide. Basically, what budm was referring to check is the resistance between the "~" legs of the component labeled "BRIDGE RECT." and also between the "+" and "-" legs. Report what you get back.

Next, check resistance between the three pins on the two switching transistors (boxed with red on bottom of the picture). Again report what you get. Refer to the leftmost pin of each of these transistors as pin 1 and the rightmost as pin 3. That way, we should be able to get an idea if there is anything shorted and between what terminals. Hopefully that should also tell us if these "switching transistors" are MOSFETs or BJTs.

Lastly, can you read us a part number off of U209? This is the main power supply controller. I omitted making any markings on the standby (STB) power supply, as that appears to be working on your amp.

~ ~ = approx. 5 M ohm
- + = 134 K ohm
Leftmost switching transistor:
pin 1 + pin 2 = 2.0 M ohm
pin 2 + pin 3 = 400 k ohm
pin 1 + pin 3 = 2.0 M ohm

Right switching transistor:
pin 1 + pin 2 = 02.5 ohm
pin 2 + pin 3 = 0.005 ohm
pin 1 + pin 3 = 02.45 ohm

I am struggling to locate U209 please can you give me a pointer.

Thanks once again

[MattM]

Re: Monitor Audio BXW10

I've attached an edited image with the gate drive components marked.

At least two components in each group have been damaged. These are small SMT diodes and transistors. They're arranged in such a way as to allow rapid turn off via local discharging of the gate capacitance. This must be addressed in order to properly drive the MOSFETs.

Looks like Q909, by the gate drive transformer, has been scraped against something. Damaged?

Thanks for this also. It hasn't been apart so the only way would be component failure of when I was disassembling but I think that's probably unlikely. I have taken a close up which I hope might help

[kaboom]

Re: Monitor Audio BXW10

Thanks for this also. It hasn't been apart so the only way would be component failure of when I was disassembling but I think that's probably unlikely. I have taken a close up which I hope might help

Your resistance readings indicate the right transistor is shorted.

Can you take macro pix of those small SMT gate drive groups?
When the MOSFETs short, they backfeed raw DC into the gate drive components, which short/burn/disintegrate.

I've seen a very similar topology in a Line 6 SMPS-powered guitar amplifier. It originally had two blown MOSFETs (there was a bulletin on these), which were replaced. Still blew fuses- gate drive components were blown and never replaced. When it got to me, I subbed known good ones and rebuilt the gate drives with thru-hole parts, as it was just for testing. Sort of had drive now, but waveforms not symmetrical. Tried subbing an SMT transistor on the drive IC side of the GDT that was suspect, but still wasn't right. Also noticed excessive loading of the aux supply when powering main converter. After a few tries, something shorted and burned some SMT resistors. Ultimately junked it.

Owner should've sent it back- would probably have got updated/corrected boards, likely tougher MOSFETs too.

Actually, it was condemned shortly after I first looked at it; the subbing and playing with the gate drive/controller IC was "just for fun" before I threw the boards out.

You'll need to replace:

Both switching MOSFETs
Completely rebuild the gate drive circuits, after determining what they are
Possibly troubleshoot/repair the drive circuits, on the IC side of the GDT

[MattM]

Re: Monitor Audio BXW10

Thanks very much for the advice, I do really appreciate all the replies.

I had hoped we were looking at a few blown caps or the like but I just don't have the test kit, or indeed the bench kit, to do the level of repairs that look necessary.

I might try to see if I can see if there is someone who can otherwise Monitor Audio sell the PSU as a part but it isn't cheap.

Thanks again for everyone's help. Happy holidays

[MattM]

Re: Monitor Audio BXW10

Is there any forum member in the UK who would be interested in taking this project on - remunerated of course

[kndm17]

Re: Monitor Audio BXW10

Hi All,

I have The same issue of monitor audio bxw-10. I replaced the SHORTED Q904 PI8N50c and also the Burnt Varistor 5D-11 but still the same. when I tried to turn off the power. There is a oscillation and 5V StandBy voltage. but when I switch the toggle for PHASE, The FUSE got Busted and the Q904 got Shorted again.

I also Tried To checked all the SMD components near the Q904 but it seems they're Okay.

Please help me with this. Thanks. ^_^

[momaka]

Re: Monitor Audio BXW10

I also Tried To checked all the SMD components near the Q904 but it seems they're Okay.

Unlikely, if the switching transistors blew.
Those smaller driving transistors may have become open-circuited from the failure and thus the reason you may think they are "OK".

U209 should be protected from failures, as it appears it drives the switching transistors through a transformer. But it is still possible for a major failure to backfeed through the TX and damage it. Speaking of which, what is the part number of U209 on your board?
.
.
.
By the way, with all these failures, I wonder if it may be easier to just gut the whole main section of the power supply and add a new one - new, as in self-built and *not* a replacement board. If we know what voltages are needed for the main board, maybe that won't be too hard to do. Just throwing some ideas out there for anyone else who happens to stumble on this thread.

[kndm17]

Re: Monitor Audio BXW10

Hi, I replaced the Filter capacitor 330uf 200v. And it's working now. There is oscillation but there is another problem. No output sound. I'm doing signal tracing in the input to the output of the speaker. But need schematic to the main board of this unit. Hope you can help me. Thanks btw

[kndm17]

Re: Monitor Audio BXW10

FYI here's what I found and Replace to my Deadset BXW-10.

Varistor - 5D-11
Filter Capacitor -330V
Q904- PIN8N50C
Fuse - 3Amp.

Now The power is stable but there is another problem in the Audio Signal Circuit.

Hope It can help the others. Thanks.

[momaka]

Re: Monitor Audio BXW10

I still haven't found schematics for this unit (though I'm not the best when it comes to searching for things like that.)

So instead, I suggest you find a datasheet for the main class-D amplifier chip and go from there. If the class-D amp has any analog inputs, you could always try inserting a signal through those to see if anything gets through to the speakers (that is, after checking that the class-D amp is getting all of its necessary supply voltages.) Speaking of the class-D amp chip - what is it? Could you tell us the part number and/or attach its datasheet here? I think that would be the easiest way to work into this unit without schematics.