I'm checking a couple of PA speaker cabinets looking for a possible fault - 
I won't bore you with the all details.
The speakers have are 12"+horn type and I think the compression drivers for 
the horns are going tits up.

Checked the impedance of the 12" drivers and get around 8 Ohms.
Fairy Nuff all seems normal there.

Checked the impedance of the compression drivers and I get an open circuit. 
i.e. No reading on the multimeter.
Is that normal?
Or should I get an impedance of around 8 Ohms?

TIA
Dave

"Dave Benj"  wrote in message 
news:xvWdnUJolcCT4CvXnZ2dnUVZ8qKdnZ2d@bt.com...
> I'm checking a couple of PA speaker cabinets looking for a possible 
> fault - I won't bore you with the all details.
> The speakers have are 12"+horn type and I think the compression drivers 
> for the horns are going tits up.
>
> Checked the impedance of the 12" drivers and get around 8 Ohms.
> Fairy Nuff all seems normal there.
>
> Checked the impedance of the compression drivers and I get an open 
> circuit. i.e. No reading on the multimeter.
> Is that normal?
> Or should I get an impedance of around 8 Ohms?
>
> TIA
> Dave


If they are piezo tweeters like:
http://www.jpgelectronics.com/products.asp?partno=L060AD
or
http://www.maplin.co.uk/searchtemplate.asp?criteria=PIEZO%20TWEETER
... they would show an open circuit.  These devices work by having the 
applied voltage develop an electric field across a piezo-ceramic plate, 
which causes the plate to distort, and the distortion (movement) is coupled 
to a thin diaphragm at the throat of the horn.  A popular make of piezo 
tweeters used to be Motorola but they're probably all rookee rikee now.

If they are moving coil tweeters then you would expect to find 8 ohms, or 
whatever, but they would have to be connected via a crossover network (i.e. 
a circuit containing at least a series capacitor, and probably also a shunt 
inductor, or coil) - which would be obvious.  Piezo tweeters don't need a 
crossover so they are connected in parallel with the bass/midrange 
speakers - and this should be obvious.

If you connect a 9 volt battery to a piezo tweeter it will make a click if 
it's workable.  If you do the same to a moving coil tweeter it will also 
click if it's workable, but if you leave the battery connected the voice 
coil may burn out!

Good luck,

Chris

"christofire"  wrote in message 
news:I9CdnQBeHJ8EHCvXnZ2dnUVZ8iqdnZ2d@bt.com...
>
> "Dave Benj"  wrote in message 
> news:xvWdnUJolcCT4CvXnZ2dnUVZ8qKdnZ2d@bt.com...

>> The speakers have are 12"+horn type and I think the compression drivers 
>> for the horns are going tits up.
>>

>
> If they are piezo tweeters like:
> http://www.jpgelectronics.com/products.asp?partno=L060AD

Nope,

> http://www.maplin.co.uk/searchtemplate.asp?criteria=PIEZO%20TWEETER

Similar.

> ... they would show an open circuit.

That's what I'm getting

> If they are moving coil tweeters then you would expect to find 8 ohms, or 
> whatever, but they would have to be connected via a crossover network 
> (i.e. a circuit containing at least a series capacitor, and probably also 
> a shunt inductor, or coil)

There's a cross-over of sorts, containing a (wire wound) resistor and a 
capcitor, presumably acting as a "tone control" to roll the low frequencies 
off the tweeter.

> If you connect a 9 volt battery to a piezo tweeter it will make a click if 
> it's workable.  If you do the same to a moving coil tweeter it will also 
> click if it's workable,

That'll be the next thing to try then.

"Dave Benj"  wrote in message 
news:xvWdnUJolcCT4CvXnZ2dnUVZ8qKdnZ2d@bt.com...
> I'm checking a couple of PA speaker cabinets looking for a possible 
> fault - I won't bore you with the all details.

I've been thinking about this again...maybe I will have to bore you :-)

The "fault" is an intermittent crackling on the tweeters/horns only.
It seems to begin after 2 or three hours of use (with a pair of CD players 
plumbed  into the PA in "mobile disco" mode).
The cracking is present on both speakers, but not to same extent and not at 
the same time.
There's nothing amiss on the bass/mid-range drivers.
Nothing is being run "flat out" - none of the faders ever get above 0db and 
the "clip" indicator on the power amp doesn't come on.

If the fault were in the power amp or mixer, then the crackling would be 
present across both bass/mid and tweeter, right?
Piezo tweeters either work or they don't so a fault there is unlikely.
I'm starting to suspect a fault in the cross-overs (such as they are).
Any other ideas?

For info, the system is this:

Numark CD-1 twin deck thingy + a Laptop + an iPod
plugged into a Yamaha MG16/6FX mixer plus:
IMG STA-700 power amp
D Class speakers (they look like fairly generic Chinese manufatured units)

"Dave Benj"  wrote in message 
news:i_CdnZfc9eoHEivXnZ2dnUVZ8hCdnZ2d@bt.com...
>
> "Dave Benj"  wrote in message 
> news:xvWdnUJolcCT4CvXnZ2dnUVZ8qKdnZ2d@bt.com...
>> I'm checking a couple of PA speaker cabinets looking for a possible 
>> fault - I won't bore you with the all details.
>
> I've been thinking about this again...maybe I will have to bore you :-)
>
> The "fault" is an intermittent crackling on the tweeters/horns only.
> It seems to begin after 2 or three hours of use (with a pair of CD players 
> plumbed  into the PA in "mobile disco" mode).
> The cracking is present on both speakers, but not to same extent and not 
> at the same time.
> There's nothing amiss on the bass/mid-range drivers.
> Nothing is being run "flat out" - none of the faders ever get above 0db 
> and the "clip" indicator on the power amp doesn't come on.
>
> If the fault were in the power amp or mixer, then the crackling would be 
> present across both bass/mid and tweeter, right?
> Piezo tweeters either work or they don't so a fault there is unlikely.
> I'm starting to suspect a fault in the cross-overs (such as they are).
> Any other ideas?
>
> For info, the system is this:
>
> Numark CD-1 twin deck thingy + a Laptop + an iPod
> plugged into a Yamaha MG16/6FX mixer plus:
> IMG STA-700 power amp
> D Class speakers (they look like fairly generic Chinese manufatured units)


Well, your next step is to try some different speakers with the rest of the 
set-up as it is.  It could be a fault in the amplifier or the mixer, 
especially if it affects _both_ channels (unlikely both speakers would 
develop the same fault), and it would be worth eliminating this first.

I once had a fault in a Mackie 1604 that produced lots of high-pitched 
crackling (no pun intended - swine flue help line, etc.) that I heard 
predominantly through tweeters.  The cause of the fault was electrolytic 
capacitors that had burst and deposited semi-conducting gunk at 
strategically bad places on the PCB.  I ended up buying another mixer!

Chris

"christofire"  wrote in message 
news:5vWdnffUbvKZCyvXnZ2dnUVZ8nCdnZ2d@bt.com...

> Well, your next step is to try some different speakers with the rest of 
> the set-up as it is.  It could be a fault in the amplifier or the mixer, 
> especially if it affects _both_ channels (unlikely both speakers would 
> develop the same fault), and it would be worth eliminating this first.

I've had it happen in the past...
:-)

"Dave Benj"  wrote in message 
news:WpmdnfhwOLYUBCvXnZ2dnUVZ8tydnZ2d@bt.com...
>
> "christofire"  wrote in message 
> news:5vWdnffUbvKZCyvXnZ2dnUVZ8nCdnZ2d@bt.com...
>
>> Well, your next step is to try some different speakers with the rest of 
>> the set-up as it is.  It could be a fault in the amplifier or the mixer, 
>> especially if it affects _both_ channels (unlikely both speakers would 
>> develop the same fault), and it would be worth eliminating this first.
>
> I've had it happen in the past...
> :-)
>

OK, I guess it can happen, but you might save a lot of later head scratching 
by trying some other speakers now, or even just one speaker on each channel 
in turn - your solid-state power amp probably won't complain about not being 
loaded on one channel.

Chris