dodgy mordaunt short speakers

T

Terry Given

Guest
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
 
Terry Given wrote:
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....
Click to expand...
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....

we were sticking 17.3Vrms up its bum, at 100Hz (while wearing grade 4
earmuffs).

Any thoughts?
Click to expand...
Cheers
Terry
 
"Terry Given"
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.
Click to expand...


** The PTC is faulty.

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



............. Phil
 
Mark Harriss wrote:
Terry Given wrote:


Any thoughts?



You might have fried the tweeters by driving the
amp to clipping for a long time.
Click to expand...
Nah, the amp was nowhere near clipping. And the tweeters still sound OK.

Cheers
Terry
 
Phil Allison wrote:
"Terry Given"

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.




** The PTC is faulty.

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

............ Phil
Click to expand...
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
 
Terry Given wrote:
Mark Harriss wrote:

Terry Given wrote:


Any thoughts?



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


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

Cheers
Terry
Click to expand...

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

Regards
Mark
 
"Terry Given" <my_name@ieee.org> wrote in message
news:gOzQe.6821$iM2.675747@news.xtra.co.nz...
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?
Click to expand...
**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
 
"Terry Given" <my_name@ieee.org> wrote in message
news:2%AQe.6851$iM2.680368@news.xtra.co.nz...
Phil Allison wrote:
"Terry Given"

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.




** The PTC is faulty.

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

............ Phil

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?
Click to expand...
**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
 
"Terry Given"
Phil Allison wrote:

** The PTC is faulty.

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


whats the tweeter current rating likely to be?
Click to expand...

** 1 amp rms continuous.



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?
Click to expand...


** 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
 
"Terry Given" <my_name@ieee.org> wrote in message
news:gOzQe.6821$iM2.675747@news.xtra.co.nz...
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 were sticking 17.3Vrms up its bum, at 100Hz (while wearing grade 4
earmuffs).
Click to expand...
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
 
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:gOzQe.6821$iM2.675747@news.xtra.co.nz...

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?


**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.
Click to expand...
Hi Trevor,

it was the tweeter polyswitch that measured 50R, I never measured the
speaker impedance.

Thanks,
Terry
 
Roger Lascelles wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:gOzQe.6821$iM2.675747@news.xtra.co.nz...

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 were sticking 17.3Vrms up its bum, at 100Hz (while wearing grade 4
earmuffs).


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.
Click to expand...
real music made it harder to pinpoint the problem.

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
Click to expand...
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
 
Phil Allison wrote:
"Terry Given"

Phil Allison wrote:

** The PTC is faulty.

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


whats the tweeter current rating likely to be?



** 1 amp rms continuous.




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?




** 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
Click to expand...
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
 
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:2%AQe.6851$iM2.680368@news.xtra.co.nz...

Phil Allison wrote:

"Terry Given"


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.




** The PTC is faulty.

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

............ Phil

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?


**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.
Click to expand...
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
 
"Terry Given" <my_name@ieee.org> wrote in message
news:CPCQe.6885$iM2.684414@news.xtra.co.nz...
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:2%AQe.6851$iM2.680368@news.xtra.co.nz...

Phil Allison wrote:

"Terry Given"


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.




** The PTC is faulty.

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

............ Phil

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?


**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.

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

I'm just being pedantic here, but how S/C is S/C?
Click to expand...
**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
 
Trevor Wilson wrote:
"Terry Given" <my_name@ieee.org> wrote in message
news:CPCQe.6885$iM2.684414@news.xtra.co.nz...

Trevor Wilson wrote:

"Terry Given" <my_name@ieee.org> wrote in message
news:2%AQe.6851$iM2.680368@news.xtra.co.nz...


Phil Allison wrote:


"Terry Given"



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.




** The PTC is faulty.

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

............ Phil

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?


**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.

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.


**Better to err on the low side. Particularly for the bass driver
Polyswitch.
Click to expand...
OK, will do.

I'm just being pedantic here, but how S/C is S/C?


**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.
Click to expand...
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
 
"Trevor Wilson"
**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.
Click to expand...
** Of course they do.


A fuse will increase it's resistance with increases in current.
Click to expand...


** 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.





Trevor Wilson
www.outrageaudio.com.au
Click to expand...


............ Phil
 
Phil Allison wrote:
"Trevor Wilson"

**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.


** Of course they do.



A fuse will increase it's resistance with increases in current.




** 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.
Click to expand...
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 :)


Trevor Wilson

........... Phil
Click to expand...
hows this for an outrageous statement: circuit breakers dont limit fault
current, fuses do :)

Cheers
Terry
 
"Terry Given"
hows this for an outrageous statement: circuit breakers dont limit fault
current, fuses do :)
Click to expand...

** Rates alongside the asinine one that says:


" guns do not kill people.... "




......... Phil
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top