The No.1 Website for Pro Audio
Dynaudio BM14S sub - Fuse exploding when power on :(
Old 3 weeks ago
  #1
Here for the gear
Dynaudio BM14S sub - Fuse exploding when power on :(

Hey, I came back to my workshop after the weekend to discover my Dynaudio BM14S sub powered down, fuse blown with black marking inside. I replaced with same S2A250V fuse, powered on and poof same black blown fuse. Moved sub to different power supply, disconnected everything, tried one more fuse which literally exploded with glass all over the place.

I read through this thread which suggested:

Quote:
You might have to adjust the bias setting.
It happened to me once.
Here is how:
You will need to open the power module.
There is one blue quiescent current potmeter for eact output(tweeter and woofer).

Turn the appropiate potmeter counter clock wise. Power on the module (it is a good thing to use a pair of 100 Ohm resistors soldered across the blown fuses, this way it will not destroy the newly replaced FET´s if there is a problem.)

Power on the module and slowly turn it clock wise while measuring the voltage across the Drain/Source resistors. When you reach approx 6 - 8mv after more than 5 minutes of power on time and it stays there you are done. Replace the resistors with real fuses and test it

Is this avenue worth pursuing or should I contact a service centre?

EDIT: I pulled the back panel off, no sign of any blue quiescent current potmeter or any damage.
Old 3 weeks ago
  #2
Lives for gear
 
Radardoug's Avatar
 

More likely to be a blown diode or power transformer.
Old 3 weeks ago
  #3
Here for the gear
Reading on several audio forums, the Dynaudio power transformers seem to be a common fault resulting in blown fuses. Anything in here worth poking with a multimeter?
https://i.imgur.com/sj6a8E3.jpg
Old 3 weeks ago
  #4
Lives for gear
Quote:
Originally Posted by Edwardhyde View Post
Reading on several audio forums, the Dynaudio power transformers seem to be a common fault resulting in blown fuses. Anything in here worth poking with a multimeter?
https://i.imgur.com/sj6a8E3.jpg
Not intended in any way to be a slight but based on your first post you should probably hand this to someone with more experience. There are dangers and gotchas in testing power supplies, getting accurate readings from diode bridges can be tricky for example.

In older power supplies rectifier diodes and caps are more likely to fail than a transformer.
Old 3 weeks ago
  #5
Here for the gear
I'm from a smallish area so unfortunately after calling around town there doesn't seem to be anyone qualified to check it out.

I'll remove and check the MOSFETs, that's something I know how to do myself.
Old 3 weeks ago
  #6
Lives for gear
 
Radardoug's Avatar
 

Diodes are easy to check with a multimeter. Check them first.
To check the power transformer:
Disconnect all secondary wires.
Wire a 100 watt light bulb in series with your power input to the device. Switch on. If the lamp comes on glowing brightly and stays that way, the transformer is toast. If the lamp flashes briefly and then goes dim, transformer is OK.

Just had this happen to a Lexicon 480L. However the original power transformer manufacturer is still in business and did me a new transformer! Phew! pricy though.
Old 3 weeks ago
  #7
Here for the gear
I disconnected the two primary MOSFETs outlined in red on the below image and tested, they appear to be fine. If I'm not mistaken removing them should disconnect the transformer. So powering up now still blows fuses, I assume it's a diode somewhere on the right of the below image...

https://i.imgur.com/udEZWNK.jpg
Attached Thumbnails
Dynaudio BM14S sub - Fuse exploding when power on :(-imgp4628.jpg  
Old 3 weeks ago
  #8
Lives for gear
 
brianroth's Avatar
 

OK, that last pic tells me it's a switching supply verses the older-school linear supplies.

Switchers are a PITA to troubleshoot. That one looks like it could have been an off-the-shelf OEM module. If so, just swap out the module and then SHRUG?? lol

Bri
Old 3 weeks ago
  #9
Here for the gear
On Radardoug's advice above I started checking the diodes and capacitors, the top right CD293 SERIES capacitor appears to not be responding when checking with MM in capacitance mode. All the other same model capacitors on the board are showing ~488-490 which I believe is correct. Could this be the fault?
Old 3 weeks ago
  #10
Quote:
Originally Posted by Edwardhyde View Post
On Radardoug's advice above I started checking the diodes and capacitors, the top right CD293 SERIES capacitor appears to not be responding when checking with MM in capacitance mode. All the other same model capacitors on the board are showing ~488-490 which I believe is correct. Could this be the fault?
Are you testing the caps in or out of circuit?
Old 3 weeks ago
  #11
Here for the gear
Quote:
Originally Posted by mrfantastic View Post
Are you testing the caps in or out of circuit?
Testing in circuit, I realise it's not perfect but I don't have an ESR meter. The capacitor in question has a ~1.5mm bulge on the top and has some odd markings around the edges leading me to suspect it's at fault.

I'll pull it off today and test properly.

I purchased a replacement today from here. THe measurements are slightly different but it'll fit.

EDIT: Pulled it off the board and getting a reading of ~0.047 — 0.052nF instead of the rated 470uF. I'll have to wait until the replacement arrives before progressing further.
Old 2 weeks ago
  #12
@ Edwardhyde testing in circuit can often give false measurements as you probably know but judging by the bulged top and the measurements out of circuit i'd guess that's that culprit.
Old 2 weeks ago
  #13
Here for the gear
Quote:
Originally Posted by mrfantastic View Post
@ Edwardhyde testing in circuit can often give false measurements as you probably know but judging by the bulged top and the measurements out of circuit i'd guess that's that culprit.
My concern is that the faulty capacitor may be the result of another problem. I'll make sure to update once the replacement arrives in a few days. Thanks for the help.

BTW I investigated taking the sub to the nearest "pro audio" support centre. They want a $200 deposit to diagnose the problem although I suspect that'd ramp up to at least double after the fix. I'm sure it's valuable work for most, but it's a great experience of doing this myself (with pointers from here). Besides, I have a spare BM14S so I'm not short on bass in the meantime.
Old 2 weeks ago
  #14
@ Edwardhyde yes could be caused by another fault but caps can also be the sole cause so definitely worth trying with the repair to the cap just to see if it's the problem. Standard electronic repair method for tricky faults is changing the cheapest components first.
Old 2 weeks ago
  #15
Here for the gear
capacitor arrived, fitted and powered up. Unfortunately I got a spark and a puff of magic blue smoke from the area marked in red. Any ideas?

https://i.imgur.com/lJo58Nr.jpg
Old 2 weeks ago
  #16
Lives for gear
 
Radardoug's Avatar
 

What, fuse didn't blow? That black thing looks like the diode bridge, did you ever check that? With switchmodes, you check and check and check again. High voltage on all those bits. Is it getting above your pay grade?
Old 2 weeks ago
  #17
^^ Yes - i'd suspect the diode in that case. Maybe worth taking out of circuit and testing.
Old 1 week ago
  #18
Here for the gear
Quote:
Originally Posted by Radardoug View Post
What, fuse didn't blow? That black thing looks like the diode bridge, did you ever check that? With switchmodes, you check and check and check again. High voltage on all those bits. Is it getting above your pay grade?
Hah, it was above my pay grade as soon as I changed the fuse. Unfortunately after further research, the parts & labour from a professional will exceed the replacement cost (I live in an expensive country and not anywhere near a qualified technician). I'm quite into electrical engineering theory though, so this is a great practical project and I appreciate the guidance.

The fuse blew however I was testing with a slightly higher rated fuse, so that may've been the culprit.

The diode bridge rectifier is showing a higher than expected reading on one of the diodes at ~1.0V instead of ~0.57V for the others. I'll pull it out of circuit to test properly.

Yeah, out of circuit it's reading 0.5V, 0.5V & 0.9V. I'll source a replacement

https://i.imgur.com/TDXJ2DF.png
Old 1 week ago
  #19
Lives for gear
 
Radardoug's Avatar
 

Might I suggest, wire a 100 watt lightbulb in series with the power supply, in the phase line. Then if you have a problem, the bulb will light up brightly and stay lit. If the supply has no shorts, the lightbulb should flash briefly and then go dim. The supply should be able to start and provide voltage with the lamp in series. Once its going OK, take the light bulb out of circuit.
Topic:
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Forum Jump
Forum Jump