this voice coil burnt?

Hi friends,

Recently I tried an untested Krell amp to drive my 4311B, causing damages to one of the woofers (2213H).
I found out later the Krell (Class A) had bad transistors with leaked DC.
I heard nothing from this troubled woofer when I hooked it up to a different amp, except for the mid-range/high range.
Fortunately the other woofer wasn't damaged.
Now, please take a look at the attached pics of the damaged woofer.
Is there any way you can notice something wrong there? Looks like the two wires from the input binding posts are okay.
Then, do you think the inside voice coils have been fried or burnt away?
Thanks for your advice as always.

The fried voice coils I have had - purchases of cores and friends' gear, I have never had it happen to my equipment yet - smelled burned. Not science, but a free and instant procedure. Transformers burned out smell largely the same way.

Measure the resistance of the coil by placing a multimeter set to measure resistance across the terminals. If you get a few ohms, rather than nothing, then it's not burnt into.
Press on the cone. If it moves freely without rubbing, then the varnish on the voice coil hasn't bubbled up from overheating enough to rub in the gap.
Not sure what else you can do without taking a look.

Quote:

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Originally Posted by JeffW

Measure the resistance of the coil by placing a multimeter set to measure resistance across the terminals. If you get a few ohms, rather than nothing, then it's not burnt into.
Press on the cone. If it moves freely without rubbing, then the varnish on the voice coil hasn't bubbled up from overheating enough to rub in the gap.
Not sure what else you can do without taking a look.

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I checked it with the multimeter, with the following outcome:
6.5 ohm on the undamaged woofer
0.9 ohm on the damaged woofer
(attached pic)

So, this one has its voice coil burnt, right?
What do you think?

By the way, I forgot to tell you this:
The moment I turned on the Krell switch,
I heard very loud hum from both speakers for several seconds,
and then smelt strongly something burning from inside the left speaker.

Not burnt but probably very weak. May be corrosion on terminals. Test with meter on solder posts to rule out terminals are bad. Use alligator clips and attach to solder post or tinsel wires.

Quote:

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Originally Posted by script56

Not burnt but probably very weak. May be corrosion on terminals. Test with meter on solder posts to rule out terminals are bad. Use alligator clips and attach to solder post or tinsel wires.

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Yes, you're right. Listening for the woofer, I hear some faint sound from it, sourced from the mid-and high-range cones.
I'm waiting for a genuine replacement for this damaged one. Thanks.

Quote:

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Originally Posted by script56

Not burnt but probably very weak. May be corrosion on terminals. Test with meter on solder posts to rule out terminals are bad. Use alligator clips and attach to solder post or tinsel wires.

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IMHO, the damaged woofer, having a LOWER DCR value than normal, indicates most of the voice coil is shorted out, and certainly way out of OEM spec. Pretty much needs a recone, or a new voice coil installed by Edgewound, or replacement by another driver.

Hope whomever supplied the Krell is paying for the replacement driver.

Quote:

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Originally Posted by pyonc

Yes, you're right. Listening for the woofer, I hear some faint sound from it, sourced from the mid-and high-range cones.
I'm waiting for a genuine replacement for this damaged one. Thanks.

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Whoa. Has anyone noticed the tinsel lead is touching the basket?

Always check for DC on the taps of any solid state equipment of unknown condition, no exceptions no matter what the original owner or a tech may have said was ok
I realize that hindsight isn't very helpful now, but this should be standard operating procedure no matter how highly respected a piece of gear may be, NO EXCEPTIONS EVER

Now, I even do it with new equipment right out of the box only takes about two minutes

I destroyed a beautiful old 175 diaphragm learning this lesson the hard way with an old Pioneer receiver that had been given a clean bill of health by a so called pro :(

Sorry about your nice woofer

Quote:

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Originally Posted by script56

Not burnt but probably very weak. May be corrosion on terminals. Test with meter on solder posts to rule out terminals are bad. Use alligator clips and attach to solder post or tinsel wires.

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Might affect output level but I doubt it and definitely not the meter's reading as shown in the poster's photo,the terminals are insulated from the frame
They'd both have to be shorted to the frame, something metal or to each other at the same time to cause burn damage
Years ago, the return side of the signal and the caps were often tied to the frame on old radio and systems using first order networks
Works fine as long as the speaker frame is mounted to something non-conductive like plastic or wood

Quote:

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Originally Posted by Joseph Smith Jr

Always check for DC on the taps of any solid state equipment of unknown condition, no exceptions no matter what the original owner or a tech may have said was ok
I realize that hindsight isn't very helpful now, but this should standard operating procedure no matter how highly respected a piece of gear may be

Now, I even do it with new equipment right out of the box only takes about two minutes

I destroyed a beautiful old 175 diaphragm learning this lesson the hard way with an old Pioneer receiver that had been given a clean bill of health by a so called pro :(

Sorry about your nice woofer

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Thanks for your kind reminder.:) A good lesson I learned this time.

Maybe a circuit of this kind could be a future benefit. It could also be mounted inside the Krell amp if its not to tight :)
http://www.ebay.com/itm/Assembled-Sp...TukkxV0RxiEM5A

The only failure we ever suffered to any of our Cogent field coil compression drivers occurred when a customer tried some lame-azz, supposedly great trendy solid state amps with our bass horn drivers and fried one of the voice coils with several volts of DC offset. In this condition the voice coil is forced partly out of the gap and the portion rendered out of the gap cannot transfer heat and turns funny colors and melts. Lovely!