measuring tweeter impedance

[Randall]

I just replaced a raspy sounding tweeter in a Fender AcoustaSonic. The new one is a direct replacement part. When I put my Fluke on it, it reads open, yet it works. Why is this? I think I got lucky this time, because the bad sounding one read open as well, and I assumed that was proof it needed to be replaced. Actually, I did get some erratic readings if I pressed the cone a bit.

[Mike Sulzer]

I just replaced a raspy sounding tweeter in a Fender AcoustaSonic. The new one is a direct replacement part. When I put my Fluke on it, it reads open, yet it works. Why is this? I think I got lucky this time, because the bad sounding one read open as well, and I assumed that was proof it needed to be replaced. Actually, I did get some erratic readings if I pressed the cone a bit.

I think it is piezoelectric: looks sort of like a capacitor rather than a resistor.

[Leo_Gnardo]

I think it is piezoelectric: looks sort of like a capacitor rather than a resistor.

What Mike said ^^^. When piezos go bad, they can go silent, drop noticeably in volume, or sound raspy. Well, more raspy than usual. The original cheapo Motorola piezos were notable for their "bark." Second generation piezos, what Community uses in the CS35 PA and monitor cabs & several companies use in amps for acoustic guitars, sound a lot smoother. And any tweeter will transmit unpleasant sounding noises when the amp driving it clips. When you hear that racket, it's time to back off, it's damaging to the tweeter/horn driver no matter what kind of driver it is.

[g1]

Also, some piezo tweeters will have internal resistors which can be either in series or parallel.
So there is no real rule about how they should measure with a meter.

[Leo_Gnardo]

Also, some piezo tweeters will have internal resistors which can be either in series or parallel.
So there is no real rule about how they should measure with a meter.

Series, definitely. I've dissected a couple of Motorola piezo drivers and found 30 ohm 1 or 2 watt series carbon comp resistor. I'm guessing that resistor is there to try and maintain power amp stability. The capacitance of a regular piezo iirc is about 0.1 uF. Without that resistor, the piezo driver would look like a near short circuit at high frequencies, say 5 KHz on up. Amps are bound to not like that, especially if there a number of piezo drivers in parallel.

[olddawg]

Series, definitely. I've dissected a couple of Motorola piezo drivers and found 30 ohm 1 or 2 watt series carbon comp resistor. I'm guessing that resistor is there to try and maintain power amp stability. The capacitance of a regular piezo iirc is about 0.1 uF. Without that resistor, the piezo driver would look like a near short circuit at high frequencies, say 5 KHz on up. Amps are bound to not like that, especially if there a number of piezo drivers in parallel.

It's also going to attenuate the signal and make them harder to trash. Ups the wattage rating.