New (to me) La Scalas--X-over won't drive tweeter

[tc535is]

Hey folks!

Just acquired a pair of La Scalas SNs: 4N385 & 7P216; the 4N385 won't drive the tweeter--mid and woofer working. It's not the driver; swaptronics came in handy. Also, the pair has had an aftermarket mod in the way of some potentiometers rigged in the ckt.

HOW SHOULD I PROCEED T-SHOOTING?

Thanks in advance,

tc

[djk]

Return to stock, and then go from there.

http://forums.klipsch.com/idealbb/files/Type%20AA.jpg

[tc535is]

Thanks to DJK for locating an AA schematic.

[tc535is]

Okay. Returned both crossover AA bal. networks to stock config (thanks to Steve Phillips and DJK for schematics.) Orig fault still exists: mid and woofer working, one tweeter inop, however it is not defective because I swapped it.

- Resoldered all joints

I'm still lost. I'm thinking it's either the two caps or the coil--what's left?

How do I check with just a V-Ohm-Meter?

Any help,

[WMcD]

Gee, I dunno.

Were these AA to begin with?

One thing you can do is go to an A style with the tweeter just being fed by a 2 uF cap. You should be able to find a couple of 1 uF tantalium caps at RS and wire them in parallel to make a 2 uF. Cost is about $3.00

I believe there is not too much you can do with a VOM.

Let's look at the AA. (I'm getting pedantic.) There are two zener diodes wired back to back. They are wired across the tweeter. That is a "clamp".

Regular diodes conduct when forward biased, and don't conduct when they're reverse biased.

Zener's are different. When back biased to the Zener voltage, they conduct. So, with the series circuit, once you get above the Zener voltage, the reverse biased one conducts ('cause it's a Zener) and the forward one conducts, like all diodes.

So, you say, what does this mean? It means that in normal circumstances, there is nothing (in the two Zener's) in parallel with the tweeter up to a certain voltage. Then, above the rated voltage, when the tweeter is in danger of being fried, there is a short circuit through the Zeners. So the voltage is clamped.

Now, maybe one or both of these have failed to a short circuit. I'd be a bit surprized because I believe most diodes fail to an open. However, it is worth considering.

Also, take a good look in there. Is there a device in series with the tweeter? Some designs put the equivalent of a fuse. That may have gone open.

I'll have to post the schematic of an A type.

Best,

Gil

[WMcD]

This shows the third order tweeter schematic.

It is a third order because there is a cap, feeding an inductor to ground, then another cap in series.

Then there is a "zobel" of a cap and inductor across the cirucit.

Importantly, it shows a protective device in series with the tweeter. The gizmo with a resistor across it.

Maybe you have something like this. The protective device is what I was saying acts like a fuse.

Gil

[sfogg]

" The protective device is what I was saying acts like a fuse."

It is a thermal fuse. If it gets enough current flow through it it opens and routes the signal through the resistor. Even if that is dead (open) the tweeter should still play.. just at a reduced level as it is going through the resistor.

With a multi-meter just measure the resistance at both ends of that resistor. If it is 100ohm the polyswitch is open. If you measure around 1/2 ohm the poly switch is closed.

Shawn

[WMcD]

This is the most simple of crossovers. It was used on the K-Horn and I don't know for a fact that it was used on the LS. One issue is whether the LS needs some filtering in the high end of the bass range.

However, look at the tweeter circuit. We see it gets a signal through the 16 uF cap which feeds the autotransformer which feeds the mid. That cap is mostly cutting off freqs below 400 Hz. Therefore, it is good conductor at tweeter frequencies and has no overall effect.

There is a 2 uF cap in series with the tweeter, and nothing else. It is a simple first order 6 dB per octave filter at about 6 kHz.

Overall, I can give little more advice. Try to dope out what you've got. I would suggest going to recreating the A in the mid and tweeter. Of course you will have to make sure that whatever is in the suspect "custom" crossover is out of circuit.

Best,

Gil

[WMcD]

Thanks for the info, sfogg. I'm always learning.

One question I have is whether the thermal fuse ever recovers. I infer from what you say, it does not. I see some stuff in Parts Express which does, but I suspect the thermal fuse, polyfuse, does not.

I suppose the good part of the design is that the tweeter continues on with some reduced power and the owner knows the voice coil is still good.

Gil

[JohnA]

Gil has started you in the right direction. Unbolt one of the diodes on the troublesome crossover and see if the tweeter works. If it doesn't remove the tweeter leads from both terminals to get it out of the circuit. With the one diode unbolted and the tweeter disconnected, check the resistance across the tweeter inductor. It should be just a few ohms, say one-half to 2. If it is shorted, it is likely your problem. If it is open, it needs replacement, but it is not the only problem. Next, you need to check the capacitors. I have a digital VOM that will measure capacitance. It was about $125. If you are unwilling to spend that, take the xover to a GOOD electronics supplier and see if they will let you test each cap (with all that stuff still disconnected).

I'll bet you have a bad capacitor. If you replace them, I recommend Hovland Musicaps of the same value. Hovlands are $$$, but great in tweeter circuits. The second best cap would be a metalized polypropylene film like the ones shown here:

http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=61

The 2 uF motor start caps are no longer made, so you won't be able to get one for replacement.

If you find the inductor is shorted (or open) replace it with a good air-core inductor, say 18 or 20 ga. Partsexpress or Madisound.com carries those, too.

[WMcD]

Echh. My distinguished collegue from the great state of Kentucky and me, from the great state of Illinois, are in a bit of a debate. John A. is very knowlegeable and gives good advice.

Still, I say, build an A. Then go back and troubleshoot. It is mostly a matter of form.

BTW, send us pictures. It is an interesting problem. Share with us.

Best,

Gil

[djk]

Network back to stock and the tweeter still doesn't play?

Dead tweeter works in the other speaker?

Lift one end of the zener diode pair.

Lift one end of the 245µH tweeter inductor.

Short out each of the 2µF caps one at a time.

At some point the tweeter should start playing.

Keep the volume low and you won't hurt anything.

You either have a shorted diode/inductor, or an open cap.

[sfogg]

"One question I have is whether the thermal fuse ever recovers.  I infer from what you say, it does not. "

Yes, it should when the current goes down and its temperature drops. At that point it should close again to be basically bypassing the large resistor.

Shawn

[tc535is]

SUCCESS! Thanks to everyone's input and recommendations. Followed DJK's advice: step by step T-shooting. Just like the man said, "at some point the tweeter will sound." Result: open 2uF capacitor adjacent (schematically) to the 13uF cap. R/R with Radio Shack metallic/poly (2) 1uF in parallel (temporary.) Will save my $$ for an ALK.

Thanks to all--livin' the dream.

tc