B&W ASW 2500 / Indigo 640021 SMPS - Page 1
Forum section: electronics / repair

author: cs.dk
author_rank: Unknown
timestamp: January 05, 2019, 11:12:40 pm
content: I'm faced with this sub-woofer from a friend of mine.It seems like the SMPS is dead - The bridge rectifier is on a separate PCB. It works, and delivers ~340 VDC to the input terminals. How ever, there is no output from it.Nothing seems burned, and it does not have that smell of failed electronics. I'm a newb fixing things, but usually there is a swollen cap, a burned component, or similar.Any clues where to start? Attached is a picture of the board, it is single sided, so no pictures from the bottom as i think they won't help.I can take more photos if needed.EDIT: Looked for a servicemanaul, but no luck.All i got was this (rather useless thread)https://www.canuckaudiomart.com/forum/viewtopic.php?t=50088Checked some diodes, resistors, caps, etc. seems to be fine.The small caps on both sides of the transformers shows ~199 nF, and 3 ohm ESR.. Since they both do it, I guess it's fine, or something is obscuring the measurement.

author: Armadillo
author_rank: Super Contributor
timestamp: January 06, 2019, 11:56:25 am
content: Hi;Firstly, 340VAC is a lethal voltage. You need to be aware of it and read up safety precaution and how to discharge the capacitors before you even touch the board. With that out of the way.;This is apparently a self-oscillation circuit. Just only a few components of the primary side.I would firstly check why it didn't oscillate.edit: per each side1.Check the 4 zener diodes out of the circuit with a power supply in series with Resistor, say 1KOhm.2. then check the 2 nfet out of the circuit with the component tester.3. then check that the primary coil of the transformer is continuous.4. With above component out of the circuit, measure the ohms of the gate pad of each FET wrt ground.

author: cs.dk
author_rank: Unknown
timestamp: January 06, 2019, 12:24:24 pm
content: Thanks,The big electrolytics are quickly discharged through some resistors, i've measured them, just to be sure not to get a nasty surprise. But thanks for the warningThe zeners you refer to are these four? (The blurring hides the last two)I don't have a componenttester.. Multimeter, scope and LCR meter are in my toolbox.

author: Armadillo
author_rank: Super Contributor
timestamp: January 06, 2019, 12:34:54 pm
content: The set of zeners are connected back to back, so I suspect those spark oscillations.In this case, check all zeners on the board.But come to think of it, since you have 2 transformers, how it both failed together? ehm....ehm....?Please better describe the symptoms.....Also provide bottom side view in case some components hiding below.

author: Jaymx
author_rank: Newbie
timestamp: August 15, 2020, 02:35:47 am
content: I know this is an old thread, but it’s legitimately the only internet mention I can find on the topic. I too have a 640021 amp with a bad PS. Were you able to fix yours? It sounds like your symptoms are exactly the same as mine. Any insights would be appreciated.Thanks!

author: cs.dk
author_rank: Unknown
timestamp: August 16, 2020, 07:59:51 am
content: I forgot this one... Still have the board lying around. Never looked more at it.Maybe we should see if we could get these going.

author: Jaymx
author_rank: Newbie
timestamp: August 16, 2020, 01:33:25 pm
content: I have done some repair work, but I’m not great at diagnosing something like this. If you do dig in, please let me know what you find. Any pointers or suggestions for a troubleshooting process would be awesome.