Definitive Technology Subwoofer repair

[robross0606]

I have a Definitive Technology compact subwoofer that went belly-up and started buzzing constantly instead of producing any sound. After research, I decide to try replacing the primary 6800uf 80v capacitors . After replacing the caps, the behavior has changed, but not really for the better. Instead of getting a constant buzz, I get a periodic buzz/pop/burp sound. Hard to describe, so I posted a video clip:

**broken link removed**

Unless I'm crazy, the sounds starts out slow and picks up repetitive speed as it remains powered. If anyone has any suggestions for what might be wrong and whether it is reasonably fixable, please let me know.

 

[Nigel Goodwin]

Randomly replacing unlikely components rarely helps - a loudly buzzing amplifier is a VERY obvious sign that the amplifier output stages have blown.

 

[robross0606]

Randomly replacing unlikely components rarely helps - a loudly buzzing amplifier is a VERY obvious sign that the amplifier output stages have blown.

I don't recall saying "Randomly" anywhere. In fact, I said " After research" which is the opposite of random. Maybe the research ended up being incorrect, but what I chose to replace was no more random than you saying it is an "obvious" sign of whatever. So let's please tone down the condescension and keep this non-toxic if possible. That being said, "the amplifier output stages have blown" is not exactly useful information to me. Further steps to diagnose? Steps to fix? Any useful suggestions?

 

[robross0606]

Randomly replacing unlikely components rarely helps - a loudly buzzing amplifier is a VERY obvious sign that the amplifier output stages have blown.

I should also point out the first thing I did was check all fuses, as well as for any obvious signs of something burnt out or loose (cold solder, etc.). Everything looks to be intact, so please explain what you mean by something having "blown".

 

[Nigel Goodwin]

I don't recall saying "Randomly" anywhere. In fact, I said " After research" which is the opposite of random. Maybe the research ended up being incorrect, but what I chose to replace was no more random than you saying it is an "obvious" sign of whatever. So let's please tone down the condescension and keep this non-toxic if possible. That being said, "the amplifier output stages have blown" is not exactly useful information to me. Further steps to diagnose? Steps to fix? Any useful suggestions?

I would consider replacing the reservoirs a fairly random decision - and I fail to see where 'research' would suggest such a thing?.

If you don't understand "the amplifier output stages have blown", perhaps you shouldn't be attempting this repair?. DC coupled amplifiers are like a chain of dominoes, if one device fails it's very likely many more will have failed as well - and it's very easy to cause more damage.

Do you have a schematic you could post?, without that it's difficult to make suggestions - for a start is it an IC amplifier, or a discrete one?.

 

[Nigel Goodwin]

I should also point out the first thing I did was check all fuses, as well as for any obvious signs of something burnt out or loose (cold solder, etc.). Everything looks to be intact, so please explain what you mean by something having "blown".

The output devices (IC or transistors) have probably failed short circuit.

 

[robross0606]

I would consider replacing the reservoirs a fairly random decision - and I fail to see where 'research' would suggest such a thing?.

If you don't understand "the amplifier output stages have blown", perhaps you shouldn't be attempting this repair?. DC coupled amplifiers are like a chain of dominoes, if one device fails it's very likely many more will have failed as well - and it's very easy to cause more damage.

Do you have a schematic you could post?, without that it's difficult to make suggestions - for a start is it an IC amplifier, or a discrete one?.

The unit was already 100% not working, so me attempting to fix it was a no-brainer. The next step was to go in the trash. Maybe I should clarify that, when I say "loud buzzing" I don't mean "in addition to sound". I mean "instead of sound".

I'm not an electronics expert, or I wouldn't be on this forum. The research came from testing the main filter caps on the subwoofer, and this:

buzzing subwoofer bad capacitor - Google Search

If this is the type of responses I'm going to receive, I'd actually prefer to not be on this forum. I'm getting real help on badcaps.com, so I don't feel the need to either justify myself or continue a clearly toxic conversation.

 

[unclejed613]

i worked on deftech equipment for several years... what model sub is it?

 

[robross0606]

i worked on deftech equipment for several years... what model sub is it?

Pro Sub 800. You can see it in the video clip.

 

[crutschow]

If you can't fix it, you might consider just replacing the whole amp, such as with one of these.

 

[unclejed613]

just replacing the whole amp, such as with one of these.

the problem with that is deftech recesses the amp into the back of the cabinet instead of mounting it with a flange on top of the wood. one common problem i do remember, is when people use monster cable RCA connectors, they must be twisted off the RCA jack when unplugging them, if they are pulled straight off, the springs in the ground shell fingers can pull the ground shell out of the RCA jack, and when another cable is plugged in, there's no ground connection, so all you get is buzzing and no actual audio. so, check the RCA jack and make sure the metal ground shell is there around the outside of the RCA jack. other than a power supply problem or a broken ground inside the amp, those are the only things that can cause the problem you describe. somebody above suggested blown output transistors in the amp, but that would blow the power supply fuse, as would a shorted rectifier in the power supply.

 

[crutschow]

the problem with that is deftech recesses the amp into the back of the cabinet instead of mounting it with a flange on top of the wood.

A piece of plywood over the hole should work if the new amp doesn't fit the old hole.