dodgy mordaunt short speakers

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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

Re: dodgy mordaunt short speakers


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You might have fried the tweeters by driving the
amp to clipping for a long time.

Re: dodgy mordaunt short speakers


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Nah, the amp was nowhere near clipping. And the tweeters still sound OK.

Cheers
Terry

Re: dodgy mordaunt short speakers


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I reread your original post carefully this time and
my first post is way off target.

Regards
Mark

Re: dodgy mordaunt short speakers


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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....

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Cheers
Terry

Re: dodgy mordaunt short speakers



"Terry Given"
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 **  The PTC is faulty.

Go look at the published specs - even 0.5 amp rated  PTCs have low cold
ohmages.



............  Phil




Re: dodgy mordaunt short speakers


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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

Re: dodgy mordaunt short speakers



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**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



Re: dodgy mordaunt short speakers


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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

Re: dodgy mordaunt short speakers



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**Better to err on the low side. Particularly for the bass driver
Polyswitch.

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**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



Re: dodgy mordaunt short speakers


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OK, will do.

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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

Re: dodgy mordaunt short speakers



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**I just tested a few. They do not 'switch'. They do, indeed, limit current,
depending on the temperature of the device. Naturally, since the fault
current causes heating, the internal resistance rises, which, in turn,
causes more heating and more temperature rises. Once the rated current is
reached, it shuts off power rapidly enough to protect devices which can
tolerate a modest overload (ie: Loudspeaker drivers).


--
Trevor Wilson
www.rageaudio.com.au



Re: dodgy mordaunt short speakers



"Trevor Wilson"
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**  Of course they do.


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**  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.





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...........   Phil



Re: dodgy mordaunt short speakers


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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 :)


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hows this for an outrageous statement: circuit breakers dont limit fault
current, fuses do :)

Cheers
Terry

Re: dodgy mordaunt short speakers



"Terry Given"
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**  Rates alongside the asinine one that says:


  "  guns do not kill people....  "




........   Phil







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Nah, you just need a nice low supply impedance to see it. With a fault
current of, say, 250,000A the difference between a circuit breaker and a
fuse is extremely obvious (if, of course, you have one hell of a good
CT). I've seen the aftermath of a dead short applied to such a system
whose CB wasnt rated for that level of fault current (and vaporised). oops.

its a bit harder to see in a house, where the fault current is 1,000 -
6,000A (depending on supply impedance, wire size etc). But I'm sure
you've seen many a blown fuse whose rupture current was well below that
(eg every 20mm glass fuse in existence). usually characterised by a
totally destroyed fuse.

have a read of some Gould-Shawmut (bought by Ferraz IIRC) app notes, or
maybe "Applying low-voltage fuses - classes & characteristics", H.W.
Reichenstein, McGraw-Hill, ISBN 07-606577-4


Cheers
Terry

Re: dodgy mordaunt short speakers



"Terry Given"
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**  How funking smartarse.

 You need to read the title of this NG.



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**  The ACTUAL short circuit current of a domestic AC outlet is rarely any
more than 250 amps rms.

Go measure the voltage regulation of one with a known load as see how
asinine your numbers are.



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 **  More asinine tripe.

Most glass equipment fuses *can*  break  250amps of current and do not need
to break any more since a domestic circuit breaker WILL  act instantly at
that current level.





..........   Phil




Re: dodgy mordaunt short speakers


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there are electronics companies in Australasia that make 1MW 3-phase
light dimmers, drives etc.


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depends on how close you are to the switchboard, and the nearest
distribution transformer (which might be right outside your house). New
suburbs tend to have nice low impedance (high efficiency) transformers
(5% cf 10%).

and just how do you measure the RMS short-circuit current? without a
very good CT (wide dynamic range) and a digital scope, it gets pretty
tricky.

then of course it makes a *big* difference when in the line cycle you
apply the short circuit.

peak current is a much more meaningful number when considering fault
currents. If the circuit interrupter works, over what time scale do you
make the RMS measurement?

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which wont tell you much about the short-circuit current at all.

those numbers, BTW, came via the EWRB, which ought to know.

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20mm glass fuses are rated at 125A.

how instantly? ms? us? ns? ps? fs? as?

you cant pull Trevor up for making an "instantly switch" claim for a
polyswitch, then do the same thing yourself for a circuit breaker - at
least not without looking silly :)

case in point: the CB switch mechanism has a finite, non-zero mass. it
requires infinitely high acceleration to move from one side to another
instantaneously. Physics 101 :)

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Cheers
Terry

Re: Take your meds - Terry !



"Terry Given"

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 **  No it is not.


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 **  Madness -  additional effects can only* reduce* the amp flow under
short circuit.


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**  No -   "manic"  is the term for your illness  - Terry.



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 **  The term is "manic" -   Terry  -   go look it up.


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**  Then you are totally stuffed.


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 **  You're a desperate lunatic.



...........  Phil








Re: Take your meds - Terry !


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OK, how big does an IGBT have to be before it stops beng electronics?

I've used 100mA SCRs, and 2,000A SCRs. Is the former electronics, if the
latter is not?

mighty peculiar definition.


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Really? I note you snipped out rotating loads on the same network. Oops,
every power systems engineer in the world is wrong.


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better than being pathetic enough to require a god.


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