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Thread: Non working SMPS

  1. #1
    Join Date: May 2008

    Location: Surrey

    Posts: 5,708
    I'm Rob.

    Default Non working SMPS

    My son bought a dead Klipsch Subwoofer recently with SMPS. I did some simple testing and found that there was no positive supply reaching the amp board. There is a 7815 and 7915. I assumed the 7815 was dead and replaced it (although it looked ok) still no POS supply, so there is something gone on the SMPS board. Having Googled it seems that this is not uncommon. The board is made by BASH who make plate amps and some have bought one of these simply to replace the SMPS which is identical. However these of course have failed too.

    It seems that the required voltages are +24v/-24v and 3.3v. I have thought about making a linear supply with appropriate transformer. What puzzles me is why there appears to be so much circuitry on the SMPS just to provide a couple of voltages.Are these for current limiting/short circuit protection or such like.

    Would I be wasting my time? Trying to fix the SMPS would be beyond my capabilities as I have no test equipment.
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  2. #2
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 617
    I'm Andrew.

    Default

    Quote Originally Posted by Puffin View Post
    What puzzles me is why there appears to be so much circuitry on the SMPS just to provide a couple of voltages.
    Switch mode power supplies are very complicated compared to linear power supplies - at least, I think so. They're operating at very high frequencies - at least a few tens of kilohertz - and there has to be an oscillator for that. Of course, there's the actual switching transistors, and there needs to be feedback which monitors the output etc. And yes, there's current limiting. The whole thing is complex and I'm sure trying to isolate a fault down to a single component would be a nightmare. Fortunately they're made a vast numbers so the price is cheap for something so complex. It's usually cheaper/easier to just replace the power supply rather than trying to fix it.

  3. #3
    Join Date: Sep 2012

    Location: Dubai (was Kent,UK)

    Posts: 12
    I'm Nick.

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    Agreed - SMPS supplies are tricky to fix unless you're an EE and as mentioned, it's often cheaper and easier to replace it.

    Determining the failure mode could be worthwhile - look for obvious component overheating on the PCB or components - you also need to replace like-for-like, i.e. a replacement SMPS needs to be as good or higher spec than the original.

    There is a world of difference in quality & noise between a cheap and a good SMPS - don't get the cheapest - get the best quality you can afford for your job - "budget" can be good if you're lucky and positively dangerous if not.

    I designed a few low-noise SMPS supplies and it's not for the faint-hearted - to debug them you need experience and decent test kit - they also tend to have rectified mains AC inputs, so you can have about 320V DC lurking on the board (that's why there are 400V caps there) - a 320V zap from an electrolytic cap can really sting. Just be careful.
    Last edited by nickds1; 24-07-2017 at 14:41.
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  4. #4
    Join Date: May 2008

    Location: Surrey

    Posts: 5,708
    I'm Rob.

    Default

    Thanks for the replies. Yes, I measured about 340v in some places and thought "what the F" is this all about! As I said in the original post, there are new supplies available as part of the BASH plate amps, but are prone to failure. I would like to try a Linear PS but it is difficult to check the exact voltages needed when only half the existing PS is working. The schematic shows the con block from the SMPS to the AMP board, but only shows some voltages, the PWM voltage and bulk monitoring voltages are not shown.
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