Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no obvious
burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg
B is the original blue body colour, C is part of the ceramic coating that
got left after my scraping. My scraping was axial so not the cause of the
tapered loss of metal oxide in the spiral between * and *, actual break just
off pic and a slight trace of this loss on the spiral above *-*. What caused
this tapering loss ? electrochemical? manufacturing fault ? All other 1/3 W
resistors in the amp look the same manufacture so wonder if being in the HT
line is significant or could any/all of the other 1/3W resistors fail in
similar fashion


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" wrote:

:Was in a valve /tube amp HT line as a dropper carying about 250V , no
bvious heating of the body or surround of this 10K resistor and no obvious
:burning of the resistor track, microscope view
:http://home.graffiti.net/diverse:gra...O_resistor.jpg
:B is the original blue body colour, C is part of the ceramic coating that
:got left after my scraping. My scraping was axial so not the cause of the
:tapered loss of metal oxide in the spiral between * and *, actual break just
ff pic and a slight trace of this loss on the spiral above *-*. What caused
:this tapering loss ? electrochemical? manufacturing fault ? All other 1/3 W
:resistors in the amp look the same manufacture so wonder if being in the HT
:line is significant or could any/all of the other 1/3W resistors fail in
:similar fashion


Could it be the voltage rating of the resistor and the possibility of higher
than normal transient peaks?

I had a similar case where I used a Beyschlag 0.25W/70C MF resistor in a 240Vac
mains driven circuit and after about 6 months the device stopped working. I
discovered this resistor, which looked perfectly normal, was open circuit. I
replaced it with an identical resistor and some months later, similar fault. I
decided to replace it this time with a hi-stab 0.5W carbon film which was
physically larger and it hasn't failed since. Checking the specs for the MF
resistor showed it had a max voltage rating of only 250V, and it would have been
exposed to peaks of more than 300V from inductive components during operation.
The carbon film resistor replacing it was rated at 350V.

Perhaps your MO resistor has suffered a similar failure mode. But unless you
know the manf specs for the resistor in question it may be difficult to
determine.

On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" put
finger to keyboard and composed:

Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no obvious
burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg
B is the original blue body colour, C is part of the ceramic coating that
got left after my scraping. My scraping was axial so not the cause of the
tapered loss of metal oxide in the spiral between * and *, actual break just
off pic and a slight trace of this loss on the spiral above *-*. What caused
this tapering loss ? electrochemical? manufacturing fault ? All other 1/3 W
resistors in the amp look the same manufacture so wonder if being in the HT
line is significant or could any/all of the other 1/3W resistors fail in
similar fashion

I recently had a 1/4W 100K resistor go O/C in a Pioneer SX-202R
receiver. It straddled the +/-45 rails (in series with two diodes) and
saw 90V for its entire life. That's only 81mW. The coating was only
slightly discoloured. There was no other damage to the amp.

I've also seen resistors go open in 5V circuits without good cause.

Then there are the high value startup resistors that go open in PSUs
despite been operated well within their ratings ...

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Ross Herbert wrote in message
...
On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" wrote:

:Was in a valve /tube amp HT line as a dropper carying about 250V , no
bvious heating of the body or surround of this 10K resistor and no
obvious
:burning of the resistor track, microscope view
:http://home.graffiti.net/diverse:gra...O_resistor.jpg
:B is the original blue body colour, C is part of the ceramic coating that
:got left after my scraping. My scraping was axial so not the cause of the
:tapered loss of metal oxide in the spiral between * and *, actual break
just
ff pic and a slight trace of this loss on the spiral above *-*. What
caused
:this tapering loss ? electrochemical? manufacturing fault ? All other 1/3
W
:resistors in the amp look the same manufacture so wonder if being in the
HT
:line is significant or could any/all of the other 1/3W resistors fail in
:similar fashion


Could it be the voltage rating of the resistor and the possibility of
higher
than normal transient peaks?

I had a similar case where I used a Beyschlag 0.25W/70C MF resistor in a
240Vac
mains driven circuit and after about 6 months the device stopped working.
I
discovered this resistor, which looked perfectly normal, was open circuit.
I
replaced it with an identical resistor and some months later, similar
fault. I
decided to replace it this time with a hi-stab 0.5W carbon film which was
physically larger and it hasn't failed since. Checking the specs for the
MF
resistor showed it had a max voltage rating of only 250V, and it would
have been
exposed to peaks of more than 300V from inductive components during
operation.
The carbon film resistor replacing it was rated at 350V.

Perhaps your MO resistor has suffered a similar failure mode. But unless
you
know the manf specs for the resistor in question it may be difficult to
determine.

That was part of my thinking. Whatever the rating, they were of a type used
throughout the amp so presumably whatever is cheapest so not HV rating.
What makes the difference in construction or whatever , between HV and LV
use , afterall the value of resistance and wattage sets the volts across the
length of the resistor. I've always observed the policy for HV ones as
droppers for the supply for zero-crossing ICs etc for triac circuits etc,
but have never known the physiscal reason, other than the wrong , ie LV
ones, will fail in such circumstances.

Anyone know the chemical/physical reasoin for the tapering failure, I
suspect some very sophisticated magnetic field distribution effect, but just
a guess.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

I should have said there is a waveyness to the terminator between surviving
MO track and the erroded section of the taper on the original full
resolution pic for this
http://home.graffiti.net/diverse:gra...O_resistor.jpg
shown here
http://home.graffiti.net/diverse:gra..._resistor2.jpg


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On Mon, 16 Mar 2009 17:10:38 +1100, Franc Zabkar wrote:

On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" put
finger to keyboard and composed:

Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no
obvious burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg B is the
original blue body colour, C is part of the ceramic coating that got
left after my scraping. My scraping was axial so not the cause of the
tapered loss of metal oxide in the spiral between * and *, actual break
just off pic and a slight trace of this loss on the spiral above *-*.
What caused this tapering loss ? electrochemical? manufacturing fault ?
All other 1/3 W resistors in the amp look the same manufacture so wonder
if being in the HT line is significant or could any/all of the other
1/3W resistors fail in similar fashion

I recently had a 1/4W 100K resistor go O/C in a Pioneer SX-202R
receiver. It straddled the +/-45 rails (in series with two diodes) and
saw 90V for its entire life. That's only 81mW. The coating was only
slightly discoloured. There was no other damage to the amp.

I've also seen resistors go open in 5V circuits without good cause.

Then there are the high value startup resistors that go open in PSUs
despite been operated well within their ratings ...

- Franc Zabkar

What was the voltage rating of the resistor? Even though it is
dissipating less than the max. power rating, things like electromigration
may causing the open.

Al

alchazz wrote in message
news
On Mon, 16 Mar 2009 17:10:38 +1100, Franc Zabkar wrote:

On Sun, 15 Mar 2009 20:58:36 -0000, "N_Cook" put
finger to keyboard and composed:

Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no
obvious burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg B is the
original blue body colour, C is part of the ceramic coating that got
left after my scraping. My scraping was axial so not the cause of the
tapered loss of metal oxide in the spiral between * and *, actual break
just off pic and a slight trace of this loss on the spiral above *-*.
What caused this tapering loss ? electrochemical? manufacturing fault ?
All other 1/3 W resistors in the amp look the same manufacture so wonder
if being in the HT line is significant or could any/all of the other
1/3W resistors fail in similar fashion

I recently had a 1/4W 100K resistor go O/C in a Pioneer SX-202R
receiver. It straddled the +/-45 rails (in series with two diodes) and
saw 90V for its entire life. That's only 81mW. The coating was only
slightly discoloured. There was no other damage to the amp.

I've also seen resistors go open in 5V circuits without good cause.

Then there are the high value startup resistors that go open in PSUs
despite been operated well within their ratings ...

- Franc Zabkar

What was the voltage rating of the resistor? Even though it is
dissipating less than the max. power rating, things like electromigration
may causing the open.

Al

I've no idea but it was the same 5 band, 1/3W, type used all over the amp ,
I assume 5 band does not necessarily mean suitable for 200 volt use.
Electromigration relative to a nearby ground plane if that was the case (not
here. polyester pcb and chassis 20 mm away) but why a problem with a
standing DC of 250V and only a few volts drop along the resistor.

"N_Cook" wrote in
:

Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no
obvious burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg
B is the original blue body colour, C is part of the ceramic coating
that got left after my scraping. My scraping was axial so not the cause
of the tapered loss of metal oxide in the spiral between * and *, actual
break just off pic and a slight trace of this loss on the spiral above
*-*. What caused this tapering loss ? electrochemical? manufacturing
fault ? All other 1/3 W resistors in the amp look the same manufacture
so wonder if being in the HT line is significant or could any/all of the
other 1/3W resistors fail in similar fashion

I used to design and oversee the making of resistors and capacitors for
Sprague in the late 60's and early 70's.

We 'adjusted' the value of our resistors and capacitors by abrading the
material.

In the late 60's, we used sand blasting to remove material.
In the 70's, we used lasers on many resistors (and still used sand for
capacitors).

It looks to me as if the spiral cut, that adjusted the resistance, varies
in width.
It should be UNIFORM all along the length of the cut.

It looks like poor quality control allowed the conductive track to be too
thin in one area.

The thin track had an area of high power dissipation because of the high
resistance in that area.
Most of the voltage drop would ALSO be along that region and lead to
failure.

Good design and good quality control would have rejected the resistor.

Of course, it survived many hours of use, so the design wasn't all THAT
bad.





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

bz wrote in message
98.139...
"N_Cook" wrote in
:

Was in a valve /tube amp HT line as a dropper carying about 250V , no
obvious heating of the body or surround of this 10K resistor and no
obvious burning of the resistor track, microscope view
http://home.graffiti.net/diverse:gra...O_resistor.jpg
B is the original blue body colour, C is part of the ceramic coating
that got left after my scraping. My scraping was axial so not the cause
of the tapered loss of metal oxide in the spiral between * and *, actual
break just off pic and a slight trace of this loss on the spiral above
*-*. What caused this tapering loss ? electrochemical? manufacturing
fault ? All other 1/3 W resistors in the amp look the same manufacture
so wonder if being in the HT line is significant or could any/all of the
other 1/3W resistors fail in similar fashion

I used to design and oversee the making of resistors and capacitors for
Sprague in the late 60's and early 70's.

We 'adjusted' the value of our resistors and capacitors by abrading the
material.

In the late 60's, we used sand blasting to remove material.
In the 70's, we used lasers on many resistors (and still used sand for
capacitors).

It looks to me as if the spiral cut, that adjusted the resistance, varies
in width.
It should be UNIFORM all along the length of the cut.

It looks like poor quality control allowed the conductive track to be too
thin in one area.

The thin track had an area of high power dissipation because of the high
resistance in that area.
Most of the voltage drop would ALSO be along that region and lead to
failure.

Good design and good quality control would have rejected the resistor.

Of course, it survived many hours of use, so the design wasn't all THAT
bad.





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap


All the MO resistors I've had a close look at seem to have the spirals
formed by a milling process (ends are semicircles) rather than some ablating
process as used in precision Rs. But some sort of runout error in the
milling could cause a taper error. But I would expect a straight, but
misplaced cut, rather than that wavy edge. Reducing any part of the track
sets the resistance value and I would have thought a taper down to 20
percent of intended would push it up from 10K to 50K or more and early
failure of the resistor in circuit.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On 3/15/2009 11:38 PM N_Cook spake thus:

Anyone know the chemical/physical reasoin for the tapering failure, I
suspect some very sophisticated magnetic field distribution effect,
but just a guess.

Oh, come on; there's a very simple explanation: the resistor was *made*
that way. **** happens.

Sheesh.


--
Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears: One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)

"N_Cook" wrote in
:

.....

All the MO resistors I've had a close look at seem to have the spirals
formed by a milling process (ends are semicircles) rather than some
ablating process as used in precision Rs. But some sort of runout error
in the milling could cause a taper error. But I would expect a straight,
but misplaced cut, rather than that wavy edge. Reducing any part of the
track sets the resistance value and I would have thought a taper down to
20 percent of intended would push it up from 10K to 50K or more and
early failure of the resistor in circuit.

And this one may be 'an early failure', compared to similar resistors
correctly milled.

-------bonus information section---------------
Take a tubular resistor that is uncut and measures 1000 ohms.

Take one with circumference is equal to length.
This makes it equivalent to a 'square' flat resistor.

Resistance is proportional to length and inversely proportional to width.

Designers use 'ohms per square' in designing resistors.
It doesn't matter if it is a square inch or a square cm or a square mile,
the resistance will be the same.
(the size, however, would effect the POWER rating)

The above resistor would have 1000 ohms per square.

If you cut a single narrow grove straight from one end to the other, you
have not changed the resistance significantly.

Cut a single spiral around the resistor along the length.
You have just made the resistor LONGER and Narrower.**
You have NOT significantly changed the amount of power it can dissipate.
You have raised its value. The effective length will be sqrt(2) x original
length.
The effective width will be sqrt(1-sqrt(2)/2) x original length.
So the ratio of length to width will be ~ 2.613, or the resistance will be
2613 ohms.

Change the pitch of the spiral until you have as many turns as needed to
get the desired value.

Again, if the cut is narrow, you have not changed the power rating but you
have increased the resistance.

Now, imagine that you made a mistake and the resistor material had too low
an ohm per square value.
You didn't notice this until after the spiral groove had already been cut.
The resistor is too low in value.
You can throw it in the trash or you can go back and raise the resistance
by grinding away and making the groove wider.

If the heat conductivity of the substrate is good then you will probably
get away with this.

It looks like that is what they did, until now.

** Note, the situation is NOT quite as simple as given above. It assumes
all current follow equal length paths.
If you only cut a single spiral, some current paths are MUCH shorter than
others. Several turns are required to ensure that the current flow is
uniform across the width of the spiral.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

On Mon, 16 Mar 2009 07:50:14 -0000, "N_Cook" wrote:

:I should have said there is a waveyness to the terminator between surviving
:MO track and the erroded section of the taper on the original full
:resolution pic for this
:http://home.graffiti.net/diverse:gra...O_resistor.jpg
:shown here
:http://home.graffiti.net/diverse:gra..._resistor2.jpg


It is difficult to know whether or not that this particular resistor suffered
from a manufacturing defect which produced a thinner than normal track at the
point of failure.

There is some literature related to metal film resistor failures due to single
shot high voltage pulses but it is not known if a manufacturing defect may have
contributed to such failures.
http://www.iop.org/EJ/abstract/0022-3727/20/11/014

There has been a case where an ineffective cleaning process left small traces of
chlorine on the resistor body which caused etching after the the encapsulation
or coating process.
http://www3.interscience.wiley.com/j...TRY=1&SRETRY=0

While not on the same subject there is also a x-ray study of a metal film
resistor which exhibited a drop in resistance due to foreign matter shorting
adjacent tracks. It highlights the possibility of manufacturing defects though.
http://eeepitnl.tksc.jaxa.jp/jp/even...xt/209_oka.pdf

bz wrote in message
98.139...
"N_Cook" wrote in
:

....

All the MO resistors I've had a close look at seem to have the spirals
formed by a milling process (ends are semicircles) rather than some
ablating process as used in precision Rs. But some sort of runout error
in the milling could cause a taper error. But I would expect a straight,
but misplaced cut, rather than that wavy edge. Reducing any part of the
track sets the resistance value and I would have thought a taper down to
20 percent of intended would push it up from 10K to 50K or more and
early failure of the resistor in circuit.

And this one may be 'an early failure', compared to similar resistors
correctly milled.

-------bonus information section---------------
Take a tubular resistor that is uncut and measures 1000 ohms.

Take one with circumference is equal to length.
This makes it equivalent to a 'square' flat resistor.

Resistance is proportional to length and inversely proportional to width.

Designers use 'ohms per square' in designing resistors.
It doesn't matter if it is a square inch or a square cm or a square mile,
the resistance will be the same.
(the size, however, would effect the POWER rating)

The above resistor would have 1000 ohms per square.

If you cut a single narrow grove straight from one end to the other, you
have not changed the resistance significantly.

Cut a single spiral around the resistor along the length.
You have just made the resistor LONGER and Narrower.**
You have NOT significantly changed the amount of power it can dissipate.
You have raised its value. The effective length will be sqrt(2) x original
length.
The effective width will be sqrt(1-sqrt(2)/2) x original length.
So the ratio of length to width will be ~ 2.613, or the resistance will be
2613 ohms.

Change the pitch of the spiral until you have as many turns as needed to
get the desired value.

Again, if the cut is narrow, you have not changed the power rating but you
have increased the resistance.

Now, imagine that you made a mistake and the resistor material had too low
an ohm per square value.
You didn't notice this until after the spiral groove had already been cut.
The resistor is too low in value.
You can throw it in the trash or you can go back and raise the resistance
by grinding away and making the groove wider.

If the heat conductivity of the substrate is good then you will probably
get away with this.

It looks like that is what they did, until now.

** Note, the situation is NOT quite as simple as given above. It assumes
all current follow equal length paths.
If you only cut a single spiral, some current paths are MUCH shorter than
others. Several turns are required to ensure that the current flow is
uniform across the width of the spiral.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap


Would you be able to confirm my interpretation of fusible resistor
construction, from the analysis of a failed one (reason known that time). As
1 minute timing does not ring true to me, unless under 1 minute can mean 2
seconds.
From a makers data (not for the one in question)
http://www.peterparts.com/%5CCatalog...C73%5C3123.pdf
and 5 band colour coding
Markings are to be shown on the resistor body by color coding.
1st, 2nd and 3rd color codes : nominal resistance value.
4th color code: resistance tolerance-J (±5%) - gold color.
5 th color code : fusing characteristics
green color -8 times
blue color -12 times
white color -16 times
violet color -32 times
if 1W then white band means subjecting to 16W will fuse in under 1 minute
Surely any normal 1W resistor subjected to that sort of overload will have
burnt up quicker than in 1 minute or have I misread something ?
Removed the grey coating and remnant parts of the metalisation/ MO sum to
about 1.5 ohm. The joining section is about 1 x 1 mm with a neat hair-line
crack across it and no sign of any heating there.
So mechanical breakage, probably a torsional failure due to
one end of the resistor swinging one way and the other end
swinging the other way, when the amp was dropped.
http://www.diversed.fsnet.co.uk/crack2.jpg
Before removing coating, was definitely 2.2 ohm markings with white band
(if) for x16 presumably referring to that 1 x 1 mm section, so x32 violet
banded one would perhaps have a something like 0.7 x 0.7mm intercept region
and a
green band x8 would have more like 1.4 x 1.4 mm fusing area.
http://www.diversed.fsnet.co.uk/crack.jpg
Is a microscope view of the crack. Viewing magnified it can now be seen
the fault developement.
The just left of centre part of the crack must be part of a crack that
formed initially but enough current flowed in the other parts making
touching contact until there was too much localised fusing.
1 to 1 marks the 1mm or so of conductor between the 2 straight line
etching/millings ? into the conductor to form part spiral paths.
The C marks the end of one of these formed breaks and there is another one
on the other track , off the top left of the image, giving about 1 x 1mm
central fusible conductor.
Nowhere on the remaining conductor is there any discolouration due to
overheating.

"N_Cook" wrote in
:




Would you be able to confirm my interpretation of fusible resistor

Not an expert of fusible resistors. Sorry.

construction, from the analysis of a failed one (reason known that
time). As 1 minute timing does not ring true to me, unless under 1
minute can mean 2 seconds.
From a makers data (not for the one in question)
http://www.peterparts.com/%5CCatalog...C73%5C3123.pdf
and 5 band colour coding
Markings are to be shown on the resistor body by color coding.
1st, 2nd and 3rd color codes : nominal resistance value.
4th color code: resistance tolerance-J (±5%) - gold color.
5 th color code : fusing characteristics
green color -8 times
blue color -12 times
white color -16 times
violet color -32 times

I would think it means multiples of the length of time it will sustain the
overload rather than multiple of the power.

if 1W then white band means subjecting to 16W will fuse in under 1
minute Surely any normal 1W resistor subjected to that sort of overload
will have burnt up quicker than in 1 minute or have I misread something
? Removed the grey coating and remnant parts of the metalisation/ MO sum
to about 1.5 ohm. The joining section is about 1 x 1 mm with a neat
hair-line crack across it and no sign of any heating there.
So mechanical breakage, probably a torsional failure due to
one end of the resistor swinging one way and the other end
swinging the other way, when the amp was dropped.
http://www.diversed.fsnet.co.uk/crack2.jpg
Before removing coating, was definitely 2.2 ohm markings with white band
(if) for x16 presumably referring to that 1 x 1 mm section, so x32
violet banded one would perhaps have a something like 0.7 x 0.7mm
intercept region and a
green band x8 would have more like 1.4 x 1.4 mm fusing area.
http://www.diversed.fsnet.co.uk/crack.jpg
Is a microscope view of the crack. Viewing magnified it can now be seen
the fault developement.
The just left of centre part of the crack must be part of a crack that
formed initially but enough current flowed in the other parts making
touching contact until there was too much localised fusing.

The vaporized material may have gone with the cover coating.

1 to 1 marks the 1mm or so of conductor between the 2 straight line
etching/millings ? into the conductor to form part spiral paths.
The C marks the end of one of these formed breaks and there is another
one on the other track , off the top left of the image, giving about 1 x
1mm central fusible conductor.
Nowhere on the remaining conductor is there any discolouration due to
overheating.

Not an expert on fusable resistors. Sorry.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap