Symptoms of a blown Fourteen B?

[Lucky Pierre]

So, my wife yesterday was listening in the kitchen to my KLH Twenty Seven with the Fourteen Bs.  She had her phone connected via the aux jack.  I have a feeling she had her phone volume low and the receiver  high.  When she finished her show, she flipped the selector to FM without lowering the volume.  I heard the most horrible noise from outside, with the windows closed.  She immediately shut everything down.  When I powered it back up, the left speaker was all sorts of scratchy.  I verified that it is the speaker and not the receiver.

Can you tell me what likely happened?  Fried voice coil?

 

Thanks!

[JKent]

The good news is, the drivers are pretty easy to find. Check each one individually to see if they are both fried.

-Kent

[Analogman]

It might be a good idea to check for DC on your taps BEFORE you hook any speakers back up to that receiver

Check with receiver at idle as you normally would for general amp health and then at any source selected except PHONO during mode transition (switching)

I've had more than a few solid state amps dump large quantities of DC on the taps on shutdown as well as transient pops due to said same while changing modes

Craig 

[Lucky Pierre]

Thanks!

Kent, what am I checking on the drivers themselves?  Impedance?

Analogman, can you describe the DC check for me?  I am pretty good with restoring speakers, but I am just learning how to test and work on amps.

[JKent]

6 hours ago, Lucky Pierre said:

Kent, what am I checking on the drivers themselves? 

I would open the back as shown in the photo above then run wires directly from your amp to one of the 2 drivers and see how it sounds. Then do the other one. They are the same drivers used in the KLH 21 radios, also used without any kind of filter in the KLH 21 extension speaker. You can run a signal to them safely as long as you keep the volume low.

[Lucky Pierre]

Ah, got it!  The simple test.

Are there any tests with a multimeter that will show anything wrong with the voice coil?

[Analogman]

On 1/24/2018 at 11:22 AM, Lucky Pierre said:

Ah, got it!  The simple test.

Are there any tests with a multimeter that will show anything wrong with the voice coil?

Disconnect at least one wire from the suspect driver(s), INDIVIDUALLY, NOT THE WHOLE SYSTEM and check continuity across the voice coil (that would be with your "ohm" setting)

You should see a nominal value, close to what the driver is marked or should be - if the reading is infinite (meaning "zero") them you have an open coil or broken or burned wire to the tinsel lead

It won't always correspond exactly to the markings on the driver: example, a driver marked "8 ohms" can and will often read around 5 to 6 ohms - this is normal - the makings on a driver are it's NOMINAL value

On the other hand, a driver marked "8 ohms" that reads 1 or 2 ohms would indicate a partial short in the voice coil - that being a problem too as much as being "open"

Also check from speaker terminal lug to frame for any possible short to frame (rare, but it does occur) - that check SHOULD read infinite - with the meter still hooked up in this way, gently move the cone inward, if you can - not always possible with tweeters - you should not observe continuity  

For DC on your amp's taps - SOLID STATE ONLY - disconnect your speaker wires at the amp - turn the amp on and let it warm/stabilize for 10 or 15 minutes - set all controls to ZERO or defeat depending on what you have - volume control at ZERO and BALANCE at center - and set at any mode EXCEPT PHONO with NO input source (just pull your interconnects)

Set your meter to the lowest DC setting it has - preferably millivolts (mV) whatever your meter's LOWEST range is for DC - and take readings ACROSS the SPEAKER TAPS, same as if your meter was a speaker

What you'd like to see is 0-15mV which is ideal, but in all likelihood if your amp is a vintage model that's never been adjusted you'll probably see a little more

16mV to 50mV is acceptable and could be improved on but it's not enough to damage anything

50mV to around 85mV definitely means the amp is out of tune and you're not getting the best it could be, but still not enough to cause damage in the sort term

IF you see a 100mV or more, then your amp DEFINITELY needs service - something is tired, really out of adjustment or just plain worn out - this is enough to cause your protection circuit to kick in on some models and if run long enough to DEFINITELY damage speakers  

Now, after you have established this baseline as a reference, leave your meter attached to the speaker taps and power down the amp while observing your meter and see what it reads after you power down - you may see a small quantity of DC for an instant or so but not too much as described above

Obviously, do all of this on BOTH channels, starting with the one that initially is having the issue (left)

It might not be a bad idea, after you have done the above, to leave the meter on the taps WHILE MAKING MODE CHANGES all the while paying close attention to what the display is telling you as well as observe the DC on the tapes from a cold POWER UP just to be thorough

On cold start up it is normal to see a SMALL amount of DC on the taps (see above for normal, "OK" ranges) but it should fairly quickly start ramping down towards ideal once the amp is on and begins to warm up - and don't get hung up on tiny deflections of the readings - they may move up and down a point or two versus rock solid steady, this is normal - just healthy normal ranges- as different amps have different circuits and ways to handle DC

This will help you narrow down the problem area IF the basic amp health checked out OK

I don't know what amp or receiver you have, or if it's ever been adjusted or re-capped, but many are adjustable and many are not - you MAY be able to remedy this problem (if in fact you have one) with a DC off set and bias adjust, WITH THE AID OF A GOOD SERVICE MANUAL

I emphasis this (SERVICE MANUAL) because if you start turning adjustment pots, and turn the wrong one the wrong way, you will not only have a broken speaker but a broken amp as well

It's also good practice, even on a 100% healthy amp to set your speaker selector switch to "off" FOR POWER UP and allow for a few minutes of warm up - even on Solid State

Every little bit helps

Good luck

Craig

[Lucky Pierre]

Thanks Craig!  Those are excellent instructions.

I will do this one evening this week.

Peter