What Capacitors to replace on this power supply? - diyAudio
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Old 28th July 2013, 10:37 PM   #1
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Join Date: Jul 2013
Default What Capacitors to replace on this power supply?

Hello, i'm trying to repair my velodyne servo subwoofer amp. Its showing all the signs of bad power supply caps. At low volumes it sounds good but as soon as you crank it up just a little, there is distortion during heavy bass notes. Sofar I replaced these caps:
C6, C7 = 220uF 16V
C18, C19 = 4700uF 50V
C11 = 4.7uF 50V
C15 =470uF 16V
the caps listed above were replaced with nichicon brand audio quality caps.
but now there seems to be static present at any volume level so I'm guessing i missed some caps. Also I replaced the big 4700uf caps with nichicon KW(M) and the original ones were VX(M). I dont know if thats the cause for the static present now.

I've included the schematic, any help would be greatly appreciated!

You may need to save and open these so you can zoom in a bit more.

Click the image to open in full size.
Click the image to open in full size.

Last edited by funfordds; 28th July 2013 at 11:34 PM. Reason: need to mention to save schematics to see it.
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Old 29th July 2013, 02:14 PM   #2
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Location: BC, Canada
The schematic you posted is fuzzy. Can't really read the parts designators or the values without guessing. Posting a pic of the unit is also helpful. High resolution shots of the component and solder side will show problems points and the quality of the components used.

Velodyne of the 80s use motion feedback technology, from the schematic it appears they still do. This design requires a stiff power supply and operation within the correct line voltage or the unit will exhibit motorboating/low frequency oscillation.

Back to your current problem, the static you are hearing describe it in more detail. Does it exist even without anything hooked up to the unit? Short the input and observe if the static still exist at different volume settings or appears only when the volume is rotated? The static could be from bad solder joints and depending on your eyesight, a magnifying glass might be helpful to spot the cracks/rings. It could also be from a faulty component and a dose of can of cold spray might bring the culprit out in the open.

BTW, the Nichicon KW series are rated for 85C, I bet the amp for the sub is inside the unit with minimal ventilation hence the premature failure. You are better off using low ESR, 105C and 4,000H-12,000H rated capacitors. Digikey/Mouser sells these caps with minimal price difference than "audio-grade" capacitors. Stay away from Mouser's house brand Xicon and stick with Japanese brands (UCC, Panasonic, Nichicon and Rubycon).

If you have any coupling aluminum electrolytic capacitor directly in the audio path that are 10uf or less, replace them with stacked metal films. Panasonic, Kemet, etc. have small footprint parts that have almost zero DC leakage, a desirable parameter when you don't want your volume controls to become scratchy. The input stages of the sub amp are DC-coupled, any DC that originates at the input going though the AC-coupling cap will propagate throughout the rest of stages, but first make sure the opamp outputs have stable ~0x.x mV DC with the input shorted.
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Old 31st July 2013, 03:42 AM   #3
dangus is offline dangus  Canada
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Location: Vancouver Island
Have you checked the physical condition of the woofer? It's old enough to suffer from foam rot, and sometimes a surround cracks near the frame where it is difficult to see.
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Old 31st July 2013, 04:13 AM   #4
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Thanks for the replies guys. Yes I've checked the surround and its in perfect shape. So heres the story, I have two of these powered woofers that each had the same symptoms. Both fluttered and sounded like they were bottoming out during heavy bass notes. I called velodyne and they dont service these subs any more, but told me it sounded like I needed to replace the caps. I replaced some of the caps that looked like they were in the power supply circuit but I may have missed some. The ones I replaced are listed in the my previous post.

I replaced the same caps in each sub and the problem now presents differently in each. One sub sounds great at low volumes but motor boats at louder volumes
heres the video of it in action. I think I need to replace the rest of the caps on the board. http://youtu.be/VSlZ3X8vUcI

the other sub ended up having a bad solder joint and i had to go back and repair it, it actually got lifted up from the track on the PCB so I had to use jumpers to fix it. Its shows signs of static and motor boating at any volume levels not just higher ones. So do you think I could have cooked something on the board? such as the full-wave bridge rectifier?
heres the video of it in action: http://youtu.be/GhQ5_bIrbqE

heres the PCB Click the image to open in full size.
20130730_232028 by funfordds, on Flickr

Click the image to open in full size.
20130730_232049 by funfordds, on Flickr

Last edited by funfordds; 31st July 2013 at 04:29 AM.
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Old 6th August 2013, 09:05 PM   #5
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Join Date: Jan 2007
Location: BC, Canada
Can you post a better schematic?

There is is at least 1 electrolytic capacitor(100uF) that is used for DC bypass for the STK4040 differential input stage positive supply (pin 15), if that is dried up it will cause motorboating. It is perfectly safe to double the value. Velodyne didn't provide DC bypassing of the differential input stage negative supply which is interesting.

The circuit board are inside the bass driver enclosure. I bet the original parts used are rated for 1,000H-2,000H 85C only are mostly dried up. If it was me I would just replace all of them with low-ESR, 105C and long life (Panasoinic FM/FJ/FC, UCC KY/LXY/LXZ, Nichicon HE, etc.).

Avoid the notion of using audio-grade electrolytic capacitors (common pitfall for DIYers), most if not all of them are rated for 85C operation and a couple of thousand hours operation only.

Ask yourself this, when thing was new , reviewed by audio magazines and raved by audiophiles, did it came with boutique capacitors straight from the factory? Answer will be a big NO so why bother. Replace the 4u7 coupling cap at the input with a stacked metal film capacitor type to exclude any external cause.

Check also the connectors and cabling especially the ones related to the accelerometer. Replace all the electrolytic capacitors on the vertical daughter boards.
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Old 17th August 2013, 05:03 AM   #6
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Thanks for the reply. I replaced all the electolytic caps on the main board but not the vertical boards. The motor boating is much better now but still present at louder volumes. 2 new issues have presented. 1.) When turning off the sub, there is a loud power off pop. 2.) there is now a low level ripple sound present at any volume without any input signal present. A scratching sound can also be heard every few seconds as well. 3.) the existing motor boating is present still but only at louder volumes.
any suggestions to fix this issue? Is it possible that some of the replacement caps got cooked when I was soldering.
which other parts would you recommend replacing other than the electolytic caps?
I can post a better schematic if needed.
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