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Switching power supply voltage drops under load

Discussion in 'Troubleshooting and Repair' started by calmsea, Dec 13, 2014.

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  1. calmsea

    calmsea

    12
    0
    Jun 3, 2014
    Hi,

    I have an Allen & Heath mixer with a faulty power supply I'm hoping to fix.
    The power supply puts out +/- ~14 volts with no load (plus a 48v for the phantom power), and I think it's supposed to run at 12v, but connected to the mixer it drops to a little over 5v. Even with a little 5W lightbulb on it it dips to less than 7v, and it's supposed to supply up to 45W.
    It's a switch mode supply based on a UC3842BN controller. I've been learning a bit about SMPSs from repairfaq, but I haven't seen anything that describes this problem.
    On visual inspection, nothing looks burnt or deformed, and solder joints all look good.
    Any suggestions as to what I should be looking for?

    Many thanks,
    Galen
     
  2. davenn

    davenn Moderator

    13,866
    1,958
    Sep 5, 2009
    hi Galen
    welcome to EP

    2 possible causes come to mind

    1) the mixer circuit has a fault and is loading down the PSU more than it should
    2) the PSU itself is faulty .... look at replacing any electrolytic capacitors in the output stages
    you may find some of them are bulging

    show us pics

    Dave
     
    Last edited: Dec 18, 2014
  3. calmsea

    calmsea

    12
    0
    Jun 3, 2014
    Hi Dave,

    Thanks for your reply.
    I tested the power supply with a small light bulb. It drew ~200 mA and the voltage fell to 6.4v. The power supply is rated at 45w so I figure that means it must be the power supply that's at fault. Is that reasonable?
    Looking at the power supply, the only thing that I noticed is that a couple of the output caps are very slightly domed on top.
    Testing them with the cap function on my meter they are in range though. Is it possible that they are still bad? Is there anything else I should look at?

    thanks

    allenheath_ps3b.jpg
    allenheath_ps3b.jpg photo 4.JPG photo 5.JPG
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Hi there. Yes, that could very well be the problem. Bulging indicates overheating and venting has occurred, and this reduces the capacitance and increases the ESR of the capacitor, which increases the heating, and so on. So their days have been numbered for a while now.

    When some electrolytics are showing signs of venting, I replace all electrolytics of that brand, using low-ESR parts from a good-quality Japanese brand (Nichicon, Rubycon, Panasonic, UCC).

    It looks like the faulty ones are "Samxon" brand - Chinese, I expect. If you post the complete markings on all Samxon electrolytics, we may be able to find data sheets for them and recommend suitable replacements.

    Where are you located? And do you have a preferred component supplier?
     
  5. calmsea

    calmsea

    12
    0
    Jun 3, 2014
    I had a couple of new 1000uF/35v electrolytics lying around so a swapped them for the two suspect caps and sure enough, that fixed it.
    I'll pick up some good 470uF/25v to finish the job (that was the original value). Or should I try a slightly higher value for the replacements? Should I replace all the caps in the output section?
    I'm in Vancouver, Canada, btw. I usually pick up components from a local supplier, Main Electronics, or order from digikey or mouser.
     
  6. davenn

    davenn Moderator

    13,866
    1,958
    Sep 5, 2009
    excellent well done :)
    its a very common PSU fault
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Standard electrolytics aren't suitable and will fail. You need low-ESR components.
    As I said, I would replace all electrolytics in the output section that are the same brand as the ones that failed.
    OK, Digi-Key will definitely have suitable replacements. Can you make a list of the "Samxon" capacitors in the output stage, and I'll make a list of Digi-Key part numbers for you to order. For each Samxon capacitor type, can you give
    • All markings (not just capacitance and rated voltage)
    • Diameter
    • Lead spacing
     
  8. calmsea

    calmsea

    12
    0
    Jun 3, 2014
    Wow, thanks!
    There are six Samxon caps in all.
    5 of them are 470uF, 25v
    There is a GF, which I believe is the Samxon series, an M (for +/-20% tolerance?)
    YDA, which looks like an EIA tolerance code, but the D in the middle doesn't seem right.
    The last line says 105°C8.
    It's 10mm across, with 5mm spacing.
    I found many candidates on Digi-Key from Panasonic and Nichicon, but most of them have a blank ESR field. Not sure how to choose. The one with the longest lifetime rating? The mixer the power supply is installed in will be on essentially 24/7 until it dies.

    The other Samxon cap is 120uF, 63v, GT(M), Z1A, 105°C8, 7mm and 4mm.
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Right, I Googled Samxon GF series and the first result was the PDF data sheet! You have the 10 mm diameter, 12.5 mm height version. The specifications are 80 mΩ maximum impedance (at 100 kHz) and 865 mA maximum ripple current.

    It looks like the five large capacitors are not the same - three are green and two are black. Are they all Samxon GF?

    Given that Samxon is a no-name Chinese electrolytic manufacturer, these numbers could have been pulled out of thin air (or somewhere darker), or from another manufacturer's data sheet, so they may have been operating within their claimed specifications.

    You should replace them with the same value or slightly higher, the same impedance or slightly lower, and the same ripple current or higher. It's actually the impedance you need to match, not the ESR, although these capacitors are called low-ESR!

    Here's the selection filter I used: http://www.digikey.com/product-search/en?FV=fff40002,fff80009,fffc000a,fffc01ed,fffc0235,fffc04a5,3400b7,b8367d,b8387a,b8388a,b8388f,f00339,f0033c,f00340,f00343,f00344,f00345,f00354,f00360,f00367,f00381,f00388,f00393,f003a4,f003a8,f003b2,f003bd,f003c0,f003c1,f003c3,f003c5,f003c7,f003d4,f003f1,f003fb,fc01ea,1140050,2dc0000,2dc0051,2dc1e56,7f00004,b540002,b540004,b540005,b540007,b54000c,b54000d,b54000e,b54000f,b540011,b540014,b540016,b54001d,b540037,b540042,b540046,b540047,b540048,b54004e,b54004f,b540057,b540060,b54006f,14940090,1494010c,14940275,149403c5,149403ec,14940442,1494044b,149404b5,14940567,149405d5,149406b7,149407bd,149408b9,14940c2a&ColumnSort=1000007&page=1&stock=1&quantity=1

    This is the one I recommend: http://www.digikey.com/product-detail/en/EKY-250ELL471MJ16S/565-1549-ND/756065. It's a United Chemi-Con (UCC) KY series part. 470 µF, 25V, 60 mΩ impedance, 1.21A ripple current, 16 mm tall, 7000 hours at 105 °C. USD 0.51.

    The GT one is specified for 264 mΩ impedance and 362 mA ripple current. The UCC KZN part (http://www.digikey.com/product-detail/en/EKZN630ELL121MH20D/565-4087-ND/4843897) looks pretty good. Its impedance is much lower, at 77 mΩ, and this will increase the ripple current, but its ripple current specification is 1.2A!

    Actually the highest impedance you can get from Digi-Key for that value is 85 mΩ. Darn them for only stocking good quality manufacturers! I doubt there will be a problem with that one, but if there is (e.g. power supply not stable - makes a sizzling or whining noise), you can always add a small resistor (e.g. 0.15Ω) in series with it.
     
  10. calmsea

    calmsea

    12
    0
    Jun 3, 2014
    Cool, thanks!
    The two darker ones are the same. The colour difference seems to be related to their failure.
    I found this pretty good document while looking up ripple current and capacitor impedance, in case any one is interested:
    http://www.cde.com/resources/catalogs/AEappGUIDE.pdf
     
  11. Meredith Tompkins

    Meredith Tompkins

    12
    0
    Dec 23, 2014
    Look to the Caps first. They can full you.
     
  12. Meredith Tompkins

    Meredith Tompkins

    12
    0
    Dec 23, 2014
    New here. Did not look down. LOL! Sorry.
     
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