KLH-20 Tweeter Dissection

[Pete B]

Kent (JKent) sent me a KLH-20 tweeter for analysis and I quickly took a look

at it this weekend. I didn't expect this to be easily repaired and expected it to

only be useful for analysis. There is a perforated metal screen covering the

paper cone and I was able to soften the glue with the heat from a soldering

iron. A heat gun would have been better, but I didn't have one handy.

It became easy to get a screwdriver under the screen and pry it up, moving

around the perimeter to remove it. The glue seems to be construction

adhesive. I did not worry too much about bending the screen as I didn't

think this tweeter could be repaired.

I was surprised to find that the voice coil lead in wires come out from under

the dust cap, are simply suspended in the air over the cone, and are

glued down to the frame with a dab of what looks like silicone glue. The

break was right at the dab of glue. There was enough lead remaining to

solder it to a fine copper wire and run it to the input terminal. I glued it

down with a dab of hot glue as was done originally.

The tweeter measures 3.20 ohms, and plays sounding just fine with a quick

listen.

The cone is 1.75" across and the dust cap is about .8" or 13/16 of an inch in

diameter. This tweeter looks a lot like what is called a phenolic ring tweeter.

Kent do you want this tweeter for a spare? Please let me know.

The screen could probably be reused but you'll have to figure out how to

remove the glue; I can think of a few solvents, and then you'd have to flatten

it again.

I'm not going to complete the Dissection since this tweeter is working again,

perhaps with another. I'll try to measure the T&S parameters when I get

a chance.

[JKent]

Pete,

Congratulations! Looking back on my failed attempt to repair the hair-like leads on an Allison Four tweeter, I'm sure I would not have been able to do that repair!

You've been more than generous, giving me a replacement tweeter (sorry I have not had a chance to install it yet, but I tested it just now and it works. Currently working on Rectilinears and the first pair of 20s). If you have any use for this tweet by all means keep it. A spare is always a good thing!

regards

Kent

[Pete B]

Pete,

Congratulations! Looking back on my failed attempt to repair the hair-like leads on an Allison Four tweeter, I'm sure I would not have been able to do that repair!

You've been more than generous, giving me a replacement tweeter (sorry I have not had a chance to install it yet, but I tested it just now and it works. Currently working on Rectilinears and the first pair of 20s). If you have any use for this tweet by all means keep it. A spare is always a good thing!

regards

Kent

Ok just let me know if you need it in the future.

[Pete B]

Any suggestions for a solvent to dissolve the face plate glue would be appreciated.

[Pete B]

One member opened his KLH17 tweeter and did not find a broken wire, he asks what

I would do next:

One question would be does it read open circuit? I'll assume yes.

There is a good chance that the tweeter cannot be repaired but it is worth continuing

to see how it is constructed.

The VC wires are attached to the phenolic ring, so if we pull the cone with the

ring there should be no problem breaking them. The outer suspension does look

to be fragile so I'm not sure how this will go.

I would desolder and remove the main input wires to get them out of the way.

I would then try to release the glue holding the phenolic ring in place.

Lets assume that the spider is held in place with a similar glue to the AR-4x

tweeter, and next put a few eye droppers worth of alcohol under the cone so

that it soaks the spider glue at the outer lip of the spider.

You'll probably have to do this 2 or 3 times, then while the 3rd application is still

good and wet pull on the cone. Pull hard and hope it releases. Worst thing is you'll

have tweeter parts for measurement if it rips coming apart.

Tell us how you do, and we can go from there.

You can have this tweeter that I just repaired if you'd like, just let me know.

[Pete B]

One thing to keep in mind is that a wire break is sometimes

not so obvious since the wire stays almost touching the other

piece. If you clip an ohm meter on the leads, and then move

the wires sometimes you'll see continuity. Also give the wires

a very gentle tug away from the dust cap, and from the dab

of glue on the ring. It will usually let go VERY easily if it is

already broken. I'd do this step first before proceeding.

A simple tug with a toothpick under the wires should show

the break if it is there.

Our member tried the above and found that the wire was

actually broken right at the dustcap with nothing left to solder

to.

There are two choices at this point, remove the dustcap and see

if there is a bit of wire that can be soldered to, or proceed to

try to remove the entire cone assembly as detailed above.

I'd probably try to loosen the glue by dabing some alcohol

on the glue joint where the dust cap meets the cone. A

toothpick or a non-magnetic tool to try to pry up the cap.

I'm expecting that you'll find about 1/8" of lead wire under

the cap.

Otherwise try to pull out the cone and if it comes out without

destroying the driver I can try to guide you to attatch new

lead out wires to the voice coil.

[Pete B]

Some have asked for tips on how to solder: http://www.classicspeakerpages.net/IP.Boar...?showtopic=4261

[Pete B]

Measured this same KLH-20 tweeter:

KLH 20 Tweeter PLB#1 8/8/08

UNIT MARKINGS: N/A

UNIT DATE:

Impedance file: KL20T1Z1.ZF2

T&S Impedance file: KL20TTS1.ZF2

Rvc = 3.24 ohms

Fs = 1560 Hz

Re (LAUD) = 3.78 ohms

Qt of 1.82

Qm = 2.03

Qe = 16.9

Zmax = 4.32 ohms

Zmin = 3.94 ohms

Z10K = 5.27 ohms

Lvc10K = 45.1 uH

I would suggest this as a modern 1" dome replacement:

http://www.madisound.com/catalog/product_i...roducts_id=8312

Fs is lower, one might try reducing the series tweeter cap to compensate.

Perhaps 20% as a starting point.

Don't know if the sensitivity matches, one could install a 4 ohm Lpad in

order to match against a working unit, then install a fixed pad. This assumes

that the Vifa is more efficient, not certain about this.

[Pete B]

Another 1" 4 ohm dome alternative:

http://www.madisound.com/catalog/product_i...products_id=374

[Analogman]

What/how is the compliance handled for this driver?

Resin treated paper or cloth?

Is there a "surround" that would benefit from being treated as with old cloth woofer surrounds that have lost their material?

Thanks,

Craig

[Analogman]

What/how is the compliance handled for this driver?

Resin treated paper or cloth?

Is there a "surround" that would benefit from being treated as with old cloth woofer surrounds that have lost their material?

Thanks,

Craig

Any observations? Also, at times and with certain material, I find this tweeter to be a bit harsh in the "Normal" mode. Switching to "Decrease" helps and the sound is smoother, but then a lot of presence is lost and the balance of the total sound seems lacking.

Has anyone played around with making/trying a foam ring or something similar to help smooth out the edges of this driver without sacrificing the SPL output?

Regards,

Craig