dodgy mordaunt short speakers


My buddy and I have spent the afternoon generating horrible noises with
a waveform generator and a NAD 216THX amplifier, which we thought was
faulty. turns out its the speakers, a pair of 8 Ohm Mordaunt Short
MS45TI. We had set the speakers up on the deck, as we were working down
in the back paddock, and the sound faded away....it wasnt *that* loud
though.
Its the crossover network (MSP1B), which has PTC thermistors in series
with the mid/bass and tweeters. They both have a (measured) PTC
characteristic, bass/mid measures about 0.3 Ohms cold, the tweeter about
51 Ohms - which seems pretty high to me. Especially as its in series
with a 3.3Ohm 5W W/W resistor....
The bipolar caps measure about right, with a Fluke 12. The voice coils
look good, we've re-done any dodgy solder joints, and the choke windings
look OK.
my guess is that the PTC characteristic has changed, such that its
resistance increases before it ought to. But I am not familiar with this
sort of failure mode with PTCs - is it possible? common? It also seems a
bit odd to slap a 50R ptc in series with a 3R3 WW resistor....
we were sticking 17.3Vrms up its bum, at 100Hz (while wearing grade 4
earmuffs).
Any thoughts?
Cheers
Terry
Reply to
Terry Given
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You might have fried the tweeters by driving the amp to clipping for a long time.
Reply to
Mark Harriss
Duh,
Rcold has increased - perhaps from 0R1 to 0R3, so it gets hotter, faster. The real question now is, what is the correct part to replace it with? Its obviously not happy with 0R3, so I'd guess 0R1. Phil, you'd know....
Cheers Terry
Reply to
Terry Given
"Terry Given"
** The PTC is faulty.
Go look at the published specs - even 0.5 amp rated PTCs have low cold ohmages.
............ Phil
Reply to
Phil Allison
Nah, the amp was nowhere near clipping. And the tweeters still sound OK.
Cheers Terry
Reply to
Terry Given
Hi Phil,
spot on. The tweeter ptc is 8mm diameter, bass/mid 13mm. I figured 51R cold must be buggered, but was unsure about 0R3. A simple hand-wavey calc shows its probably too high (the speakers are 8R), and a measurement (with earmuffs, scope & freeze spray) confirms its the culprit.
whats the tweeter current rating likely to be? I have been told the speakers are rated at 100W, but cant track down any real data.
I can get a suitable small PTC from Farnell, but the best they have is 0R45 at 25C for the big PTC. My guess is I want 0R1. Any suggestions?
Thanks for your help, Terry
Reply to
Terry Given
I reread your original post carefully this time and my first post is way off target.
Regards Mark
Reply to
Mark Harriss
**MS was the first audio company (AFAIK) to use Polyswitches for protection of their drivers. Normally, the system is pretty foolproof, though Polyswitches can and do fail, usually due to over-Voltage. Naturally, you would expect to measure a DC resistance of around 5 Ohms (not 50 Ohms) for any driver which is rated at 8 Ohms. Polyswitches maintain a low resistance, until they hit their critical temperature. Then they 'switch' to a high resistance mode (sort of), whereby current is severely limited. Internal heating will keep the Polyswitch at this point, until the overload is removed.
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson
**Polyswitches can be purchased from a number of sources. WES Components and Jaycar are very cheap and they stock a wide range. In general, you would choose an 075 (or thereabouts) for the tweeter and a 1.15 for the bass. If they measure almost S/C when cold, then they're probably fine.
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson
** 1 amp rms continuous.
** You are looking at the ****wrong**** devices .
The PTCs that are used in speakers and battery packs are called "PolySwitch".
See Farnell cat: book 2 pages 345 to 347.
See the RXE series, 60 volt rated.
.......... Phil
Reply to
Phil Allison
I know you had fun out there on the deck, and you may not thank me for saying this, but pumping loud continuous tones into hifi speakers is evil. This is because domestic speakers are designed for real music which has very high peak values but and relatively low RMS (heating) values.
To test your amplifier, I suggest a CRO and a resistor dummy load. Many hifi amplifiers will also overheat when supplying high powers for long periods, so watch that. Otherwise, listen to real music.
Once your amplifier checked out OK, you could than have checked out the speakers. Another trick is to swap amplifier, swap speakers etc and eliminate the culprit.
By the same token don't plug a microphone into your hifi, because a single feedback squeal can toast something.
Things are a lot safer these days, thanks to polyswitches, but I would not want to bet my nice speakers on a polyswitch.
Roger Lascelles
Reply to
Roger Lascelles
Hi Trevor,
it was the tweeter polyswitch that measured 50R, I never measured the speaker impedance.
Thanks, Terry
Reply to
Terry Given
real music made it harder to pinpoint the problem.
Hi Roger,
point taken. Although it was LF, and I suspect modern music has fairly high peak-to-average LF content (I suspect most stereo buyers would be happy sans tweeters nowadays). The amp ran cool, and the actual power level was 1/3 the speaker rating.
Cheers Terry
Reply to
Terry Given
Bingo, thanks Phil & Trevor.
070 and 135 are the device markings - so an RXE075 and an RX135 ought to do the trick.
I've never used a polyswitch before, but did test some about 9 years ago - no good for that application, a +24Vdc supply that had to survive idiots connecting 400Vac to it, they were too slow.
Cheers Terry
Reply to
Terry Given
Hi Trevor,
075 to replace the 070, and a 160 to replace the 135. Jaycar it is, Kev will pick them up 2morrow. thanks for your help.
I'm just being pedantic here, but how S/C is S/C? A decent 300A semiconductor fuse is a few tens of micro-Ohms - 6 digits after the decimal point, written on the fuse. Of course they do cost $300 each.... inconvenient when you have to replace 18 at a time :)
When I get them, I'll take a few measurements and report back.
Cheers Terry
Reply to
Terry Given
**Better to err on the low side. Particularly for the bass driver Polyswitch.
**With average speakers, 0.3 Ohms is adequate. The advantage of a Polyswitch is that it remains at the low resistance, then 'switches' to high resistance. Fuses do not. A fuse will increase it's resistance with increases in current.
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson
OK, will do.
thats not how these behaved, I measured the resistance while heating it with my Hakko 850B hot air rework station, a nice gradual function of temperature. Which is why I thought it was a PTC. But that perhaps means nothing, as the device is (I think) faulty.
When the new parts arrive, I'll take some comparitive measurements, including temperature, and post the results.
Cheers Terry
Reply to
Terry Given
"Trevor Wilson"
** Of course they do.
** Rubbish - as bloody usual from a complete charlatan like TW.
FYI
A fuse and a PolySwitch both increase resistance with the TEMPERATURE of the element.
Temperature is a function of the current value squared - so until the rated current is reached little heating occurs in either case.
The MAJOR benefit of a PolySwitch is that it resets.
........... Phil
Reply to
Phil Allison
ayup.
like I said, I heated up the (albeit dodgy) polyswitch, and watched its resistance smoothly change with temperature. I've seen motor PTCs with a far sharper knee than that. I've also done the same with fuses, but of course the resistance change is roughly linear with temperature, up until the fuse wire melts.
A quick squiz at a polyswitch datasheet shows that too :)
hows this for an outrageous statement: circuit breakers dont limit fault current, fuses do :)
Cheers Terry
Reply to
Terry Given
"Terry Given"
** Rates al " guns do not kill people.... "
........ Phil
Reply to
Phil Allison

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