Subwoofer Thumping

[SouthernTransplant]

Hey all,

I have a 7 year old (pre-Mustachian) Harmon Kardon 100W subwoofer in my home theater setup that has worked perfectly up until a few weeks ago, but unfortunately I'm having some major issues now.

For a few days, while watching movies, I would get an intermittent "thump" sound - this might happen a few times in a minute, then would go away for an hour.  This started getting worse, and is currently at the point that it constantly makes the (very loud) sound every second or so - even if there is no cable connected to the receiver.

I'm going to open it up tonight to see if there are any loose solder points, put I'm thinking that this might be an amp or a protective relay issue.  Has anyone had experience with getting a subwoofer repaired?  I would prefer not to have to buy one, but I don't want to pay for repairs and have it break again in a few months.

Alternatively, any other suggestions for what I could try on my own to fix it?  Thanks in advance!

[VCaddy]

You might find a new plate amp that will work at Parts Express.  http://www.parts-express.com/cat/subwoofer-plate-amplifiers/332  Or a whole new DIY kit.  They have reasonable prices, and the few subs I've built with their parts performed very well. 

[guitar_stitch]

Bad capacitor is very likely.

[HipGnosis]

Sorry I can't help, but I'm curious...
Is this a powered subwoofer?

[SouthernTransplant]

Sorry I can't help, but I'm curious...
Is this a powered subwoofer?

Yes, powered subwoofer. 

Bad capacitor is very likely.

I hadn't thought about this - good suggestion!  I'll see what I can figure out once I manage to open it up.

[zolotiyeruki]

7 years old?  There's a decent chance of a bad cap.  It sounds like the "soft-on" circuitry isn't working, so it's suddenly switching itself on or off, with an accompanying thump.

[SouthernTransplant]

So - after more difficulty than I would have guessed, I got the subwoofer open.  The amp board was glued to the shell, so it took a little effort to get it out.

I didn't immediately see any burn marks, but there does appear to be some brown residue leaking on the digital amp board, between two capacitors and something that looks like a coil?  One of the capacitors is a little "bowed" at the top vent side, which I'm guessing isn't a good sign.

The part number on the amp board is 104-DS15000V-E.  I'm guessing it would be easier to replace the entire board, but I can't seem to find it online.  Any suggestions for where else to look?

I have attached a picture of the board, in case anyone was curious.  Also, here is a Youtube video from someone else, but that has my exact model and issue.  https://www.youtube.com/watch?v=MVgklyp6jUg

Any thoughts would be appreciated - thanks for all the responses so far!

[zolotiyeruki]

Yup, bad caps, and they'll give that exact kind of symptom.  Lots of things from about 8-10 years ago had bad caps.  The way I heard the story, a name-brand capacitor maker leaked a fake recipe for a key ingredient, which all the chinese suppliers then copied.  And a couple years later, everyone had failing computer monitors, motherboards, power supplies, TVs, you name it.  For a while, I had a side business repairing computer monitors with blown caps.

The goop all over them is probably just glue to hold things in place and avoid metal fatigue from the vibration.  Replacing those caps is actually quite a simple repair--10 minutes with a soldering iron and $0.50 worth of parts.  With all that available board space, you could even upgrade that power supply to get a smoother output.  Heck, pay for shipping and I'll fix it for you, if you want.  PM if interested.

I'd suggest you look around the board to see if any other capacitors are bulging or leaking, or are of the same brand--you'll want to replace any suspect parts while you're in there.

[SouthernTransplant]

Thanks Zolo - I'm going to give it a shot myself, though I'll reserve the right to accept your offer later :)

Two of the capacitors looked bowed, both were 10uF 100V NP 105deg.  Radioshack only had 50V varieties, and I'm having trouble finding and radial versions online - only axial.  The axials might be a tight fit - any idea where else I could look online?  Ebay seems to have some, but they have to ship from Taiwan/China?

Thanks again.

[zolotiyeruki]

mouser.com and digikey.com are the go-to places for electronic components of all types.  When you get new ones, you need to meet or exceed the voltage rating.  For the capacitance, in this application, you want to meet or exceed the value.  I tend to either match the rating, or go up one standard size (10uF -> 22uF, or 22uF -> 47uF, or 47uF -> 100uF).  The only real downside when it's a smoothing cap on the power supply is a slight increase in cost, but it comes with a host of benefits (lower ESR, longer life, smoother output power, etc).

[Reddleman]

Lots of subwoofer amplifiesr of about that era had similar issues- I've had a Polk and some other consumer- level sub that did the same thing. 

I love the idea of replacing the caps- have to get back to soldering!  BTW- anyone have a link for a good intro. to basic circuit design?  I'd love to dive into more of this myself, but am not always sure on troubleshooting steps.

[zolotiyeruki]

I got a BS in EE, so that was my "tutorial", but troubleshooting bad caps doesn't require any expertise--just look for bulging or leaking caps.  Are you interested more in circuit design, or in the troubleshooting?

[2ndTimer]

Wow, Zolo, thanks.  I had no idea that the cap issues might be visible to the eye.  Will open my speakers and take a look.  I have a mid that is cutting in and out at low volumes and I have suspected a failing cap.

[SouthernTransplant]

Hi all,

Thought I would offer an update.  I replaced the two capacitors that were leaking, and tried firing everything back up.  The immediate thumping has stopped, though anytime I start watching anything it comes back.  Further inspection shows some scorching on the board that I missed before, which may have affected things.

I'm disappointed it wasn't a complete fix, but it was worth the $5 to try it, and I got to use my old soldering equipment again, so it's hard to be too upset!

The next question is - is it worth it to replace the entire  plate amplifier, or would I be better off buying a new subwoofer?  Is there a quick way to find out what amplifier boards would fit/work with my sub?

As always, thanks for all the help and advice!

[zolotiyeruki]

Can you post a new pic of the board?  If it's still thumping at high volume, that suggests to me that you need either A) higher-value capacitors, or B) you need to use lower-ESR capacitors.  Also, are there any other caps that you might have missed?  Caps can also fail without necessarily leaking or bulging.  I can't tell the brand of the large capacitor in the background of the pic you posted, but there are a number of brands that are known for this issue.