Sometimes, a sledge - hammer is the right tool ... :-)

[Arfa Daily]

Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of the
output bar leds alight. Other than this, it did not a lot. Oddly, at the
bottom of the two bar columns, are two more leds marked "DC" and "+15" and
"-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point, my
heart sank. You could measure a virtual dead short to ground on the -15v pin
at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in full
overload foldback, so not supplying enough current into the short.

So I decided it was make or break time, and got out my sledge - hammer in
the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp, hooked it
to the shorted rail, and settled back to wait for the smoke. There wasn't
any, so I went round with a MK 1 finger to see if I could find a hot IC. I
finally burnt my finger on a little surface mount 4880. Quicker than you
could say "soldering iron" I had it whipped off the board. Then I burnt my
finger again on the spot where it had been ... :-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap right
next to the IC, that was short circuit. It was pumping so much heat into the
board, that it was hotting up the IC as well. In the absence of a schematic,
I'm guessing that it's just an 0.1uF ceramic decoupler. That's what it looks
like anyway, so that's what it's got in it now as a replacement. In view of
the relatively small value of this mixer, I think that the sledge - hammer
'tune for maximum smoke' approach was about the only one that was
commercially viable, and in this case, it worked out nicely. Now all I need
is a few more faults like that. Don't know about anyone else, but I have
found the last three weeks to be the worst I have ever suffered in 20 years
of working for myself, both in terms of quantity, and quality of work. I
think we still have a very long way to go in recovering from the recent
recession. Anyone care to comment on how you're finding it on the
westpondian side ? Jeff, Mark , Meat ?

Arfa

 

On Oct 20, 6:05 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:

Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of the
output bar leds alight. Other than this, it did not a lot. Oddly, at the
bottom of the two bar columns, are two more leds marked "DC" and "+15" and
"-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point, my
heart sank. You could measure a virtual dead short to ground on the -15v pin
at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator  was in full
overload foldback, so not supplying enough current into the short.

So I decided it was make or break time, and got out my sledge - hammer in
the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp, hooked it
to the shorted rail, and settled back to wait for the smoke. There wasn't
any, so I went round with a MK 1 finger to see if I could find a hot IC. I
finally burnt my finger on a little surface mount 4880. Quicker than you
could say "soldering iron" I had it whipped off the board. Then I burnt my
finger again on the spot where it had been ...  :-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap right
next to the IC, that was short circuit. It was pumping so much heat into the
board, that it was hotting up the IC as well. In the absence of a schematic,
I'm guessing that it's just an 0.1uF ceramic decoupler. That's what it looks
like anyway, so that's what it's got in it now as a replacement. In view of
the relatively small value of this mixer, I think that the sledge - hammer
'tune for maximum smoke' approach was about the only one that was
commercially viable, and in this case, it worked out nicely. Now all I need
is a few more faults like that. Don't know about anyone else, but I have
found the last three weeks to be the worst I have ever suffered in 20 years
of working for myself, both in terms of quantity, and quality of work. I
think we still have a very long way to go in recovering from the recent
recession. Anyone care to comment on how you're finding it on the
westpondian side ? Jeff, Mark , Meat ?

Arfa

If the power is distributed with actual planes in the PCB it would be
difficult to do it any other way. If power is distributed in a 'tree'
you could have restricted the current to get a few hundred mV into the
board and then look for the Voltage differentials with a meter. OR if
its a 'tree', you can start measuring the ohms at each capacitor. With
my trusty old Fluke 8060 I correctly identified which hole was shorted
to the ground plane of a new PCB. The 2 holes were separated by 0.1
inch ( 2.54 mm. I know YOU know) connected with a 0.01 inch trace on 1
oz copper. Good meters are essential.

G˛

 

[Jeff Liebermann]

On Thu, 21 Oct 2010 02:05:18 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

It finally turned out to be a tiny gnat's-cock sized surface mount cap right
next to the IC, that was short circuit. It was pumping so much heat into the
board, that it was hotting up the IC as well.

If it was belching that much heat, you could have seen some of it with
a digital camera that's sensitive to infrared. Granted, the typical
camera is only sensitive to near IR, and not the far IR that's need
for thermal viewing. Lacking a suitable camera, I've used a gun style
IR thermometer, with a paper tube over the lens to limit the exposure.
It takes a while to scan the board, but it works. I also have several
sheets of liquid crystals, that clearly show hot spots if I can get
them close to the board.
<http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1642>

Don't know about anyone else, but I have
found the last three weeks to be the worst I have ever suffered in 20 years
of working for myself, both in terms of quantity, and quality of work. I
think we still have a very long way to go in recovering from the recent
recession. Anyone care to comment on how you're finding it on the
westpondian side? Jeff, Mark , Meat ?

That's the left coast please.

What I'm seeing is really marginal design. That's the real benefit of
computers in electronic equipment design. With modeling and
simulation, it's possible to select components that are ever closer to
the bitter edge of self destruction. The days of safety factors and
over current/voltage/power/whatever overhead are long gone. Everything
is designed to blow up should any parameter be exceeded. If the
transistor goes into thermal runaway at 100C case temp, then the heat
sink will be minimally sized to run it at 99C. It's enough to make me
cry, but there's no way to stop the trend.

As for the recession, it's still happening. Home mortgage rates are
about 3.75% which is as low as it's ever been and only half a point
above the official 3.25% prime rate. My office complex has 3
vacancies out of 6 offices. I'm having to negotiate payment plans
with customers. I'm also loaning money to friends, which is really a
bad idea. I'm bidding on design contracts against engineers who are
willing to work almost for free. My profits are basically supporting
my office expenses and taxes. Not good, and no improvements in sight.
Doom and gloom.

On the other foot, I'm finding more time to tinker, play, and loaf,
which was not the case when I was busy. I'm even getting some
exercise and bike rides during working hours. Maybe recessions aren't
so bad after all.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[N_Cook]

Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news:jlMvo.56612$iY6.6495@newsfe12.ams2...

Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of
the
output bar leds alight. Other than this, it did not a lot. Oddly, at the
bottom of the two bar columns, are two more leds marked "DC" and "+15" and
"-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point,
my
heart sank. You could measure a virtual dead short to ground on the -15v
pin
at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor
anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in
full
overload foldback, so not supplying enough current into the short.

So I decided it was make or break time, and got out my sledge - hammer in
the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp, hooked
it
to the shorted rail, and settled back to wait for the smoke. There wasn't
any, so I went round with a MK 1 finger to see if I could find a hot IC. I
finally burnt my finger on a little surface mount 4880. Quicker than you
could say "soldering iron" I had it whipped off the board. Then I burnt my
finger again on the spot where it had been ... :-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap
right
next to the IC, that was short circuit. It was pumping so much heat into
the
board, that it was hotting up the IC as well. In the absence of a
schematic,
I'm guessing that it's just an 0.1uF ceramic decoupler. That's what it
looks
like anyway, so that's what it's got in it now as a replacement. In view
of
the relatively small value of this mixer, I think that the sledge - hammer
'tune for maximum smoke' approach was about the only one that was
commercially viable, and in this case, it worked out nicely. Now all I
need
is a few more faults like that. Don't know about anyone else, but I have
found the last three weeks to be the worst I have ever suffered in 20
years
of working for myself, both in terms of quantity, and quality of work. I
think we still have a very long way to go in recovering from the recent
recession. Anyone care to comment on how you're finding it on the
westpondian side ? Jeff, Mark , Meat ?

Arfa

The likely MLCC and SM failure mechanism
http://www.era.co.uk/news/rfa_feature_07b.asp
even more likely with the extra 30 deg C or more of PbF soldering

memo to myself, must get one of those IR and sighting laser-spot thermometer
guns

 

[Arfa Daily]

<stratus46@yahoo.com> wrote in message
news:d076f765-05c3-441e-a33c-7f4c10e34cae@m7g2000yqm.googlegroups.com...

On Oct 20, 6:05 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of
the
output bar leds alight. Other than this, it did not a lot. Oddly, at the
bottom of the two bar columns, are two more leds marked "DC" and "+15"
and
"-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point,
my
heart sank. You could measure a virtual dead short to ground on the -15v
pin
at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor
anywhere
on the board, or any IC. I tried all the usual stuff like using a low
ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in
full
overload foldback, so not supplying enough current into the short.

So I decided it was make or break time, and got out my sledge - hammer in
the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp, hooked
it
to the shorted rail, and settled back to wait for the smoke. There wasn't
any, so I went round with a MK 1 finger to see if I could find a hot IC.
I
finally burnt my finger on a little surface mount 4880. Quicker than you
could say "soldering iron" I had it whipped off the board. Then I burnt
my
finger again on the spot where it had been ... :-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap
right
next to the IC, that was short circuit. It was pumping so much heat into
the
board, that it was hotting up the IC as well. In the absence of a
schematic,
I'm guessing that it's just an 0.1uF ceramic decoupler. That's what it
looks
like anyway, so that's what it's got in it now as a replacement. In view
of
the relatively small value of this mixer, I think that the sledge -
hammer
'tune for maximum smoke' approach was about the only one that was
commercially viable, and in this case, it worked out nicely. Now all I
need
is a few more faults like that. Don't know about anyone else, but I have
found the last three weeks to be the worst I have ever suffered in 20
years
of working for myself, both in terms of quantity, and quality of work. I
think we still have a very long way to go in recovering from the recent
recession. Anyone care to comment on how you're finding it on the
westpondian side ? Jeff, Mark , Meat ?

Arfa

If the power is distributed with actual planes in the PCB it would be
difficult to do it any other way. If power is distributed in a 'tree'
you could have restricted the current to get a few hundred mV into the
board and then look for the Voltage differentials with a meter. OR if
its a 'tree', you can start measuring the ohms at each capacitor. With
my trusty old Fluke 8060 I correctly identified which hole was shorted
to the ground plane of a new PCB. The 2 holes were separated by 0.1
inch ( 2.54 mm. I know YOU know) connected with a 0.01 inch trace on 1
oz copper. Good meters are essential.

G˛

As far as I could see, it was a tree. Unfortunately, this thing is quite
'compact and bijou' which, coupled with the fact that the opamps are mostly
quite tall SILs, nestling in amongst the rotary and slider pots, makes it
difficult to even get the meter probes in anywhere on the business side.
Interesting your comment on restricting the current. I did in fact initially
put a 150 ohm resistor in the line, but the short was, as it turned out only
a few copper cms from the connector carrying the power in, and the short was
so complete that all that was left at the first place on the board that you
could conveniently measure, was actually only a few mV, let alone a few
*hundred* mV. This made me suspect that the problem was close-in, and I
immediately checked all the nearby electrolytics. However, as it turned out,
it was a ceramic cap that had failed. That fact alone contributed to to
making the job 'less than normal' as I did not for one moment think that it
was going to be down to one of them. Yes, of course I've had these caps go
both leaky and short, but this one really had gone *so* short that it may as
well have been 1mm of wire ... !!

Under normal circumstances, I'm with you on doing the job 'properly', but
in today's financial climate, the value of the equipment - not much in this
case - and how much the owner has in his pocket, and how much a successful
repair at a reasonable cost, is likely to bring him back in the future, are
all factors that need to be taken into consideration commercially.

It was a bit of a 'last resort' method, based on those criteria, I freely
admit. Still, it was a good feeling to 'win' for a change. I'm sure we've
all been there !

Arfa

 

[JW]

On Thu, 21 Oct 2010 02:05:18 +0100 "Arfa Daily" <arfa.daily@ntlworld.com>
wrote in Message id: <jlMvo.56612$iY6.6495@newsfe12.ams2>:

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in full
overload foldback, so not supplying enough current into the short.

Curious - what do you use for a low ohms meter?

 

[N_Cook]

I just bought a small non contact IR thermometer, badged Maplin N19FR. It
has scan mode, not mentioned in the instructions. Setting on deg F rather
than C gives resolution of .1 deg F ,its bigger brothers usually have 1 deg
F as best resolution , as higher temp range. Also cone monitoring rather
than spot monitoring so probably better for this purpose, but limited to
divining to an area of 10mm diameter rather than a particular SMD say.
Anyone know of a source of IR transmissive lens ?, glass is totally opaque
to room temp objects. Or would a truncated metal cone inverted over the
sensing one work well enough as a "macro lens" accessory after homing in to
a target area.
I am faced with a similar situation. 2x1100W Crest amp that intermittently
falesely trips out as brownout when above a few tens of watts of output.
Something is loading the + or - 15V rails, like thermal runaway, loading
something like exponentially but always recovers , whatever it is. Opamps
all over the place and branching rails . I was thinking of defeating the
brownout line and let whatever overheat to the point of smoking but will now
have a non-destructive extra option when I can clear some 2 foot x 2 foot
bench space.
Previously I'd tried a PIR pyro sensor but not suitable for this purpose as
wavering output for static "image".

 

[N_Cook]

JW <none@dev.null> wrote in message
news:6d20c6d2iiiukp1h9vuefpnmr5t1jhdnki@4ax.com...

On Thu, 21 Oct 2010 02:05:18 +0100 "Arfa Daily" <arfa.daily@ntlworld.com
wrote in Message id: <jlMvo.56612$iY6.6495@newsfe12.ams2>:

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor
anywhere
on the board, or any IC. I tried all the usual stuff like using a low
ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in
full
overload foldback, so not supplying enough current into the short.

Curious - what do you use for a low ohms meter?

I use a 1KHz RLC meter Avo B183 , resolution .01 ohm, with 2 diodes across
the probe terminals if the 10V pk-pk is excessive, still usable info in a
comparative if not absolute sense

 

[N_Cook]

Found a long-tail pair of TO92 running about 25 deg F higher in one amp than
the other amp, while running normally. I will go with a problem in that area
to start with. A previous amp had burnt out R and Tr there so presumably a
stock fault with leaky transistors, previous were leaky anyway. I doubt if
I'd picked up the difference between warm and extra warm by 25 deg just by
"digital" testing. Putting a plastic Din plug cover over the pyro with a
rubber sleeve holding in place and IR entering the cord hole, narrows the
search area but substantially reduces the apparent temp.

 

[William Sommerwerck]

I'm not sure "sledgehammer" is quite the right term.

It would be more correct to say that you handed the device sufficient rope
to hang itself.

 

[Meat Plow]

On Thu, 21 Oct 2010 02:05:18 +0100, Arfa Daily wrote:

Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of
the output bar leds alight. Other than this, it did not a lot. Oddly, at
the bottom of the two bar columns, are two more leds marked "DC" and
"+15" and "-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point,
my heart sank. You could measure a virtual dead short to ground on the
-15v pin at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor
anywhere on the board, or any IC. I tried all the usual stuff like using
a low ohms meter to see if I could get close to the location, but the
results were inconclusive. Nothing was running hot, as the linear
regulator was in full overload foldback, so not supplying enough
current into the short.

So I decided it was make or break time, and got out my sledge - hammer
in the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp,
hooked it to the shorted rail, and settled back to wait for the smoke.
There wasn't any, so I went round with a MK 1 finger to see if I could
find a hot IC. I finally burnt my finger on a little surface mount 4880.
Quicker than you could say "soldering iron" I had it whipped off the
board. Then I burnt my finger again on the spot where it had been ...
:-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap
right next to the IC, that was short circuit. It was pumping so much
heat into the board, that it was hotting up the IC as well. In the
absence of a schematic, I'm guessing that it's just an 0.1uF ceramic
decoupler. That's what it looks like anyway, so that's what it's got in
it now as a replacement. In view of the relatively small value of this
mixer, I think that the sledge - hammer 'tune for maximum smoke'
approach was about the only one that was commercially viable, and in
this case, it worked out nicely. Now all I need is a few more faults
like that. Don't know about anyone else, but I have found the last three
weeks to be the worst I have ever suffered in 20 years of working for
myself, both in terms of quantity, and quality of work. I think we still
have a very long way to go in recovering from the recent recession.
Anyone care to comment on how you're finding it on the westpondian side
? Jeff, Mark , Meat ?

Arfa

LOL I've done the same thing looking for fuse blowing shorts. As far as
work goes, I earn earn enough to live with food on the table and a roof
over my head. Wish I could get my lazy-ass now 18 yr old son to get any
kind of a job, burger flipper, stock-boy, whatever to help out. He's not
stupid with an IQ of around 135 just lay-ZEE! Hell when I was his age,
even before I couldn't wait to get a job.



--
Live Fast, Die Young and Leave a Pretty Corpse

 

[Arfa Daily]

On 21/10/2010 10:37, JW wrote:

On Thu, 21 Oct 2010 02:05:18 +0100 "Arfa Daily"<arfa.daily@ntlworld.com
wrote in Message id:<jlMvo.56612$iY6.6495@newsfe12.ams2>:

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in full
overload foldback, so not supplying enough current into the short.

Curious - what do you use for a low ohms meter?

A Bob Parker / Dick Smith ESR/Low ohms meter. Resolves down to a few
miliohms. I have used it with some success for this purpose before, but
in this case, the results were 'vague' ...

Arfa

 

[Arfa Daily]

On 21/10/2010 15:58, Meat Plow wrote:

On Thu, 21 Oct 2010 02:05:18 +0100, Arfa Daily wrote:

Well, figuratively speaking, anyway !

Today, a Carlsbro mixer desk landed on my bench. At power up, a few leds
flickered and flashed pathetically, before it settling with a couple of
the output bar leds alight. Other than this, it did not a lot. Oddly, at
the bottom of the two bar columns, are two more leds marked "DC" and
"+15" and "-15". The "+15" was alight, but not the "-15".

The power supply is a separate linear unit, screwed to the bottom of the
desk, and connected to the main board by a ribbon cable. I was able to
measure +15v and +5v at this connector, but no -15v, which seemed to go
along with with what the diagnostic leds on the front panel were saying.
However, with the power supply unplugged, -15v returned, at which point,
my heart sank. You could measure a virtual dead short to ground on the
-15v pin at the board end.

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor
anywhere on the board, or any IC. I tried all the usual stuff like using
a low ohms meter to see if I could get close to the location, but the
results were inconclusive. Nothing was running hot, as the linear
regulator was in full overload foldback, so not supplying enough
current into the short.

So I decided it was make or break time, and got out my sledge - hammer
in the form of a bench power supply. I disconnected the unit's own power
supply, and set my bench one to -12v and a current limit of 1 amp,
hooked it to the shorted rail, and settled back to wait for the smoke.
There wasn't any, so I went round with a MK 1 finger to see if I could
find a hot IC. I finally burnt my finger on a little surface mount 4880.
Quicker than you could say "soldering iron" I had it whipped off the
board. Then I burnt my finger again on the spot where it had been ...
:-(

It finally turned out to be a tiny gnat's-cock sized surface mount cap
right next to the IC, that was short circuit. It was pumping so much
heat into the board, that it was hotting up the IC as well. In the
absence of a schematic, I'm guessing that it's just an 0.1uF ceramic
decoupler. That's what it looks like anyway, so that's what it's got in
it now as a replacement. In view of the relatively small value of this
mixer, I think that the sledge - hammer 'tune for maximum smoke'
approach was about the only one that was commercially viable, and in
this case, it worked out nicely. Now all I need is a few more faults
like that. Don't know about anyone else, but I have found the last three
weeks to be the worst I have ever suffered in 20 years of working for
myself, both in terms of quantity, and quality of work. I think we still
have a very long way to go in recovering from the recent recession.
Anyone care to comment on how you're finding it on the westpondian side
? Jeff, Mark , Meat ?

Arfa

LOL I've done the same thing looking for fuse blowing shorts. As far as
work goes, I earn earn enough to live with food on the table and a roof
over my head. Wish I could get my lazy-ass now 18 yr old son to get any
kind of a job, burger flipper, stock-boy, whatever to help out. He's not
stupid with an IQ of around 135 just lay-ZEE! Hell when I was his age,
even before I couldn't wait to get a job.



My son was the same at that age, but I had enough work to 'employ' him

in my business, doing the crap stuff like dismantling and reassembling
units, and replacing lasers. I put "employ" in single inverteds, because
he never used to surface from his pit until I called him mid morning,
and by 2pm, he was burnt out again ...

When work dropped off at the beginning of this recession when all the
banks started exploding, I could no longer justify the cost of him. That
seemed to do the trick. He went out and got himself a job in a betting
shop (yes, they are legal this side of the pond !) and then went on to
get a job with the national electricity and gas distribution
infrastructure company, where he still is, and is doing very well. Like
your boy, he is very intelligent, and a computer genius when he puts his
mind to it. It's just that he was, as your boy is, very lazy. Once he's
home from work, it's still hard to get him to move from the couch, but
at least he goes every day, and is on time, so maybe there's hope for
your lad too ....

Arfa

 

[William Sommerwerck]

It's just that he was, as your boy is, very lazy. Once he's
home, it's still hard to get him to move from the couch...

I wonder if there's anything biological in this. I'm not the only person
who's observed that, when visiting my parents, I feel as if all the energy
has been drained out of me.

 

[Arfa Daily]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:i9pn2m$52j$1@news.eternal-september.org...

I'm not sure "sledgehammer" is quite the right term.

It would be more correct to say that you handed the device sufficient rope
to hang itself.

It was the right term for me. I was trying to conjour the image of a piece
of delicate electronic equipment, dealt with (successfully) by the use of a
bludgeoning and completely inappropriate 'tool' ...

Probably one of those east - west humour things again.

Arfa

 

[William Sommerwerck]

I'm not sure "sledgehammer" is quite the right term.
It would be more correct to say that you handed the
device sufficient rope to hang itself.

It was the right term for me. I was trying to conjour the
image of a piece of delicate electronic equipment, dealt
with (successfully) by the use of a bludgeoning and
completely inappropriate 'tool' ...

We're having some fun, so don't anybody get too serious...

I don't see what was inappropriate about the tool, nor do I consider what
you did "bludgeoning". If anything, it was quite the opposite -- not unlike
handing a man with a leg injury a cane so he could stand up. Once he's
standing, you can more-easily locate the injury.

 

[Arfa Daily]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:i9qng8$r3$1@news.eternal-september.org...

I'm not sure "sledgehammer" is quite the right term.
It would be more correct to say that you handed the
device sufficient rope to hang itself.

It was the right term for me. I was trying to conjour the
image of a piece of delicate electronic equipment, dealt
with (successfully) by the use of a bludgeoning and
completely inappropriate 'tool' ...

We're having some fun, so don't anybody get too serious...

I don't see what was inappropriate about the tool, nor do I consider what
you did "bludgeoning". If anything, it was quite the opposite -- not
unlike
handing a man with a leg injury a cane so he could stand up. Once he's
standing, you can more-easily locate the injury.

Well, normally, 'appropriate' tools for this type of fault diagnosis, would
be considered to be meters and 'scopes and small soldering irons, and so on.
A bloody great power supply with a current limit set only to protect itself,
seemed to me both 'inappropriate' in the normal range of tools, and
'bludgeoning' in that it was capable of doing serious physical damage to
tracks and vias, in the process of trying to persuade something to declare
its faultiness by releasing its magic smoke.

All I sought to show, along with an amusing image, was that sometimes,
thinking outside of the box that would normally be taught to students in
college, and using what less experienced people would probably think of as
outrageously inappropriate tools and methods, can result in the 'cure' side
of a 'kill or cure' equation.

I do, however, take your meaning, William

Arfa

 

[JW]

On Thu, 21 Oct 2010 19:42:12 +0100 Arfa Daily <arfa.daily@ntlworld.com>
wrote in Message id: <8Q%vo.47818$no2.25435@newsfe28.ams2>:

On 21/10/2010 10:37, JW wrote:
On Thu, 21 Oct 2010 02:05:18 +0100 "Arfa Daily"<arfa.daily@ntlworld.com
wrote in Message id:<jlMvo.56612$iY6.6495@newsfe12.ams2>:

It's a single board, so the next 20 minutes were spent removing knobs,
screws and jack nuts to get the board out. It was double sided with
thousands of tiny vias, and a mixture of surface mount and through hole
technology, including many through hole SIL opamps. Shorts like this are
very difficult to track down. It could be any decoupling capacitor anywhere
on the board, or any IC. I tried all the usual stuff like using a low ohms
meter to see if I could get close to the location, but the results were
inconclusive. Nothing was running hot, as the linear regulator was in full
overload foldback, so not supplying enough current into the short.

Curious - what do you use for a low ohms meter?


A Bob Parker / Dick Smith ESR/Low ohms meter. Resolves down to a few
miliohms. I have used it with some success for this purpose before, but
in this case, the results were 'vague' ...

Might want to try using a higher end DMM which supports four wire
resistance measurements. I currently use a 6.5 digit Keithley 2000 which
supports this, and has a resolution of 100 micro ohms. It's worked pretty
well for me finding low resistance shorts, but they're a bit pricey on the
used market. An HP 3478A 5.5 digit DMM can be had for $125 USD or so, and
has the same 100 micro ohm resolution as well as 4 wire support.

 

[JW]

On Fri, 22 Oct 2010 09:29:18 +0100 "Arfa Daily" <arfa.daily@ntlworld.com>
wrote in Message id: <HXbwo.18591$VS3.18171@newsfe14.ams2>:

A bloody great power supply with a current limit set only to protect itself,
seemed to me both 'inappropriate' in the normal range of tools, and
'bludgeoning' in that it was capable of doing serious physical damage to
tracks and vias, in the process of trying to persuade something to declare
its faultiness by releasing its magic smoke.

The problem with using a power supply is that after the short blows there
may be a sudden voltage overshoot as the supply recovers. Here at work I
just hooked a DSO up to a variable power supply set to 5 volts with a
current limit of 3 amps driving a short. I removed the short a number of
times and observed that there were spikes greater than 35 volts which
lasted several microseconds. I would think that there'd be some potential
to blow some more sensitive circuitry. I would use a power supply as a
last resort because of this.

 

[Arfa Daily]

"JW" <none@dev.null> wrote in message
news:3lo2c6pvssqhlh42vbu8itujppg9ksbe5s@4ax.com...

On Fri, 22 Oct 2010 09:29:18 +0100 "Arfa Daily" <arfa.daily@ntlworld.com
wrote in Message id: <HXbwo.18591$VS3.18171@newsfe14.ams2>:

A bloody great power supply with a current limit set only to protect
itself,
seemed to me both 'inappropriate' in the normal range of tools, and
'bludgeoning' in that it was capable of doing serious physical damage to
tracks and vias, in the process of trying to persuade something to declare
its faultiness by releasing its magic smoke.

The problem with using a power supply is that after the short blows there
may be a sudden voltage overshoot as the supply recovers. Here at work I
just hooked a DSO up to a variable power supply set to 5 volts with a
current limit of 3 amps driving a short. I removed the short a number of
times and observed that there were spikes greater than 35 volts which
lasted several microseconds. I would think that there'd be some potential
to blow some more sensitive circuitry. I would use a power supply as a
last resort because of this.

Yes, agreed. It was a method of last resort in this case, as the value of
the item, and what it's owner would be prepared to pay, precluded putting an
awful lot more labour time into a problem that many years experience of
working on similar equipment, told me was going to be tricky and
time-consuming to locate, using 'traditional' methods ...

In terms of overshoot from the short blowing itself off, I set the current
limit at 1 amp in the hope that this would allow enough current to get the
faulty item smoking, without actually blowing it back open again.

Arfa