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Depth Subwoofer Issue

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TJGlowacki

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I think I drove this sub too hard. Now when it's turned on, all controls down, and no input, all three drivers oscillate at around 60 Hz. It starts quiet, then gets much louder until it maxes out on the volume. I'll get it to the shop this week but unless its a fixable issue it looks like it's fried. I know it's impossible to diagnose something like this with the info I provided but I'm open to ideas. I opened it up and looked at the board. No apparent blown parts or burnt traces. Any info would be appreciated.
 

khenegar1

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I think I drove this sub too hard. Now when it's turned on, all controls down, and no input, all three drivers oscillate at around 60 Hz. It starts quiet, then gets much louder until it maxes out on the volume. I'll get it to the shop this week but unless its a fixable issue it looks like it's fried. I know it's impossible to diagnose something like this with the info I provided but I'm open to ideas. I opened it up and looked at the board. No apparent blown parts or burnt traces. Any info would be appreciated.
sounds like the servo system has gone bad! good luck did u contact ml?
 

TJGlowacki

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Thanks for the replies. Everything is unplugged except the power. Might be the servo. I’ll keep trying.
 

gadgetguy11

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Troubleshooting suggestions:

1- Unplug Subwoofer & unplug the receiver or control preamplifier driving it from Mains (120VAC wall receptacle).
2- Unplug input cable at the subwoofer input that sends the signal to the Subwoofer from the control preamp.
3- With the Subwoofer input signal open, plug in the Subwoofer to Mains (120 VAQC)
4- Turn Subwoofer ON & slowly increase Gain (input level) from 0 to full.
5- If hum is GONE, you proved the Subwoofer is fine & your issue is a ground loop.
6- If hum is present, there is one additional test to prove that is true.
7- Connect the Subwoofer input to ground as follows: Insert a "grounding plug", in other words, the inner conductor of an RCA jack needs to touch the outer ring (ground). This is an INPUT & it simply pulls the input to 0 VAC. This assures absolute 0 input to the subwoofer amplifier. Now if there is hum, you have assured without a doubt that you have an issue with the Subwoofer Amplifier / Servo System. On the contrary, if you have no hum, you proved it is fine.
8- A simple way to address a ground loop issue, if you find you have this issue, is to "float" the ground by plugging the Subwoofer plug into a "Cheater Plug":
  • Grounding adapter standard grade 2-wire to 3-wire, $0.88 at Lowe's, Item #409865Model #5501.
Try both directions in the wall receptacle for minimum hum.

Please private message me with any additional questions you may have. I have repaired amplifiers to component level since 1977; these are the basic steps to qualify the integrity of the amplifier upon receipt from the customer.
 

18000rpm

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I also suggest trying a cheater plug to see if the problem goes away. If it does, you can then try to isolate which equipment is causing the ground loop.
 

TJGlowacki

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Thank you so much I will try this but I could have been a little clearer with the issue. Imagine that all the cones start going in and out rather fast and create a low frequency sound. Not really a hum but a single tone like a low frequency sine wave. I can measure the frequency.
 

gadgetguy11

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Thank you so much I will try this but I could have been a little clearer with the issue. Imagine that all the cones start going in and out rather fast and create a low frequency sound. Not really a hum but a single tone like a low frequency sine wave. I can measure the frequency.
You need to eliminate the variables first so you can zero in on the root cause. I HIGHLY RECOMMEND you eliminate the ground loop possibility FIRST, as explained earlier - at least use a cheater plug to determine whether there is a ground loop issue. In 5 minutes you eliminate one of the most common causes of a constant low frequency hum. There is nothing to lose.
 

ttocs

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Thank you so much I will try this but I could have been a little clearer with the issue. Imagine that all the cones start going in and out rather fast and create a low frequency sound. Not really a hum but a single tone like a low frequency sine wave. I can measure the frequency.
Actually, measuring the frequency would help a lot. If it's 60Hz or 120Hz in the USA, or 50/100Hz elsewhere, then it's electrically induced hum. A cheater plug might help, but also might not be definitive if there's a fault inside the cabinet. But the frequency might help point the checking in the right direction.

It might be of interest to know that the new ML subs have two conductor plugs, no ground.
 
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