Can you identify a mystery component in a passive crossover?

[SteveWils]

I'm repairing the passive crossover in a PA speaker. Comparing it with
an identical second speaker I own, it seems there's a pair of
components that are missing. In fact, judging from the state of the
PCB track, I'd say they were never installed in the first place
(??!!).

They?? Well each speaker has 2 woofers and 2 tweeters, and the signal
is divided between them thru 2 identical crossovers (on the same
board). I've taken the liberty of attaching a pic of the circuit
forwarded by the manufacturers. The missing components are labelled
PTC, and are in a parallel with the 33R resistors on the way out to
the tweeters.

Why doesn't the company help? Because the speakers are obsolete, and
the guy who designed the crossover no longer works there.

The missing components are small blue disks (looking like capacitors)
and are marked 'PTR' with 'C950' below. Now C could refer to the
tolerance (+-25 pF) and 950 could be 95pF (95 followed by zero
zero's). And PTR could mean 'positive temperature coefficient', but
I've only heard of PTR's as thermistors. But then, I'm no expert :-)

There's also a symbol that could be a company logo. It consists of an
elongated 'S' shape with 2 small lines running close to and parallel
with the middle section. Does this ring a bell with you?

Can you help? Is this component a capacitor? A capacitor makes sense
to me in a crossover - it's allowing only the highest frequencies to
bypass that 33R resistor, and its absence would result in the duller
sound I've noticed. If a capacitor, is it 95pF? And is it really a PTR
device? Is the capacitance value affected by temperature? The harder it works, the hotter it gets, and the more low frequencies are filtered out.

Any theories gratefully received.

-SW

[kevinkr]

A PTC is a protective device, and might just be a thermistor with a strong positive temperature coefficient where the resistance increases rapidly as it heats up. The other possibility is it is a polyfuse which is a resistive protective device that initially exhibits a low resistance, and if the current through it becomes excessive it heats up and becomes an open circuit - once it cools off it resets.

Definitely not a capacitor, and if this speaker sounds dull it is probably because a cold PTC exhibits pretty low resistance and without it the 33 ohm resistor strongly attenuates the highs. I suspect they should have been installed and were forgotten.. Measure the ones in the other cabinet to determine their cold resistance, and get something that matches.

[simon7000]

http://media.digikey.com/pdf/Data%20...0PDFs/PFRA.pdf

You can use any PTC resetable fuse. For a small tweeter start out with around .1 Amp

[DF96]

Does this help?

In normal use it more or less shorts out the 33R resistor, but sustained high level signals will make it heat up and reduce the signal to the tweeter. If missing the tweeter signal is always reduced, so it will sound dull.

[Frank Berry]

The PTC (Positive Temperature Coefficient) device may very well be nothing more than an incandescent light bulb. JBL used them in many of their loudspeaker systems to provide tweeter protection at high volume levels.

[indianajo]

The blue disks with the S on them with the two slashes are overvoltage protectors. I'm familiar with the S07's that come in PCAT power supplies on the AC input, and the S15's (15 mm) that come on the 480V inputs to 3 phase variable frequency drives. PTCR is one description, but I thought they had a binder and metal powder that shorted over through the binder if the voltage got too high. Blue is the american manufacturer, green is epcos or some chinese co. See the picture, I got this from newark or mouser.com. You might get a hit on the part number from newark or mouser. I agree they are tweeter protectors against amp oscillation. I've lost two speakers to blown tweeters that can't be bought- ST70's aren't known to oscillate, but ???? If the manufacturer ran out of these one day, the customer's amp might blow up the tweeter- I can see the production manager saying "surely the service department can find a way to deny the repair under warrentee if the customer's amp blows this. Ship em".
I uploaded a .jpg picture but it only shows when I complain about it not showing up. shows as attached. ?

[Enzo]

C950 sounds to me like a date code - 50th week of 1979 or 1989 or 1999.

Bulbs are often used as PTC resistors, but since the poster knows they are small blue discs, one has to suspect the discs are not bulbs.

PTC tweeter protection is not rare. Not protecting against RF oscillation, it is just a way to prevent people from blowing the tweeters out with excess treble. How many times have I seen the "smiley face" curve on graphic EQs? That +12db at 10kHz is doing tweeters no favors.

Indiana likes Peavey examples, so look up their "Soundguard" as used in any number of PA speakers. The tiny disc in those is a "polyswitch."

[davidsrsb]

This looks like it
http://www.epcos.com/inf/55/db/PTC_0...V_C_B599_0.pdf
If this is it, it should have a cold resistance of about 4R

[DF96]

Perhaps I should have made it clear in my post #4 that C950 is an Epcos part number.

[DualTriode]

Hello,
Now that the missing PTC is identified take a look at the 33R resistor. I bet that it is not 33R but something much lower in value. If you install the missing PTC you may also need to replace the resistor that operates in parallel.
My guess is that the PTC was found to give unreliable results from one speaker box to the next and between warm days and cold days.
DT
All just for fun!