Tracking down "excessive current"

[D Yuniskis]

Hi,

I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

Thx,
--don

 

[GregS]

In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis <not.going.to.be@seen.com> wrote:

Hi,

I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

Thx,
--don

I often use light bulbs on the mains to limit current.
i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.
I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

greg

 

[D Yuniskis]

Hi Greg,

GregS wrote:

In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis <not.going.to.be@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

 

[D Yuniskis]

Hi Neil,

nesesu wrote:

Perhaps if you said what the problem turned out to be, we could judge
if there might be any 'easy' way to track such a problem current. For

Ah, but that's not fair! "Monday morning quarterback".
The point of my question is, knowing *exactly* what I
knew when I sat down with this problem, what's the *best*
way to proceed.

It's sort of like asking *how* to locate your (lost) car keys
and being asked "Where did you *find* them?" :>

example, it would be almost impossible to locate excess leakage
current in the PS output filter cap since it is 'inboard' of any
circuits that might have voltage/resistance given such that proper
current could be calculated from. It would also be difficult to use
heating of the cap as an indicator unless the 'normal' temperature was
known since such a cap can heat from ripple current/ESR [I^2*R],
leakage dissipation or just high ESR. Where practical, I substitute a

That's true of all components! I.e., you can look at a
schematic and not (easily) determine what the current
flowing through a particular node will be (unless it is
a simple passive network). Or how warm a component would
be, etc.

bench power supply for the unit's built in supply so I can monitor the
current drawn, but where the unit being tested has a multi-output
supply such as you describe, that may not be feasable.

 

[nesesu]

On Jun 11, 1:29 pm, D Yuniskis <not.going.to...@seen.com> wrote:

Hi Greg,





GregS wrote:
In article <huu0rv$68...@speranza.aioe.org>, D Yuniskis <not.going.to....@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw.  But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set.  I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set.  The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here.  Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful.  Though not
sure if even that would have helped (though it probably would
for some types of failures)- Hide quoted text -

- Show quoted text -

Perhaps if you said what the problem turned out to be, we could judge
if there might be any 'easy' way to track such a problem current. For
example, it would be almost impossible to locate excess leakage
current in the PS output filter cap since it is 'inboard' of any
circuits that might have voltage/resistance given such that proper
current could be calculated from. It would also be difficult to use
heating of the cap as an indicator unless the 'normal' temperature was
known since such a cap can heat from ripple current/ESR [I^2*R],
leakage dissipation or just high ESR. Where practical, I substitute a
bench power supply for the unit's built in supply so I can monitor the
current drawn, but where the unit being tested has a multi-output
supply such as you describe, that may not be feasable.

Neil S.

 

[Phil Allison]

"D Yuniskis"

I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

** So this fuse was not being stressed all the time - ie bending or faintly
glowing ?

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

** So ONE of these loads was intermittently drawing high current ?

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

** The good, old fashioned way is to use a larger fuse and wait for
something to get hot or start smoking.

Dunno about you, but I use a current meter in the AC supply for all bench
service jobs - it is sensitive enough ( 1mA resolution) to observe small
changes in the AC current draw and indicate if the current is creeping up.
I also keep a book recording the normal current draws ( of and on load for
amplifiers ) of everything I see.

In your case, the AC draw was slowly or intermittently increasing by a
significant percentage and that means something MUST dissipating all that
additional energy as HEAT.

Normally, there would be other observable symptoms in the performance of the
unit - as whatever it is overheats.



...... Phil

 

[N_Cook]

D Yuniskis <not.going.to.be@seen.com> wrote in message
news:huu68u$ile$1@speranza.aioe.org...

Hi Greg,

GregS wrote:
In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis
not.going.to.be@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

I happened to be playing with the innards of a domestic PIR unit for hotspot
"divining". Not too sure whether it would be useable. Uses RPY96 6 to 14
micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324 output
for a small component 8 degree C over ambient, gave about 50mV swing up and
down of a nominal not too steady quiescent long term level. So if static at
2.3V would swing up to about 2.35 and then down to about 2.25 with a time
constant of a second or so , so again inconvenient. You have to scan across
as not a static process of generation of signal. There is a piezo effect as
well with the sensor so you would have to be very gentle in movement to
monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a 1.5
inch long dense foam rubber tube but matt metal maybe better, any thoughts?

 

[N_Cook]

N_Cook <diverse@tcp.co.uk> wrote in message
news:huvcba$rgi$1@news.eternal-september.org...

D Yuniskis <not.going.to.be@seen.com> wrote in message
news:huu68u$ile$1@speranza.aioe.org...
Hi Greg,

GregS wrote:
In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis
not.going.to.be@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)


I happened to be playing with the innards of a domestic PIR unit for
hotspot
"divining". Not too sure whether it would be useable. Uses RPY96 6 to 14
micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324
output
for a small component 8 degree C over ambient, gave about 50mV swing up
and
down of a nominal not too steady quiescent long term level. So if static
at
2.3V would swing up to about 2.35 and then down to about 2.25 with a time
constant of a second or so , so again inconvenient. You have to scan
across
as not a static process of generation of signal. There is a piezo effect
as
well with the sensor so you would have to be very gentle in movement to
monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a
1.5
inch long dense foam rubber tube but matt metal maybe better, any
thoughts?

A more practicle solution is probably a rotary vane in the viewing path to
avoid the need for manual scanning to get down to the 1 or 2 deg C
resolution level

 

[PeterD]

On Fri, 11 Jun 2010 13:29:06 -0700, D Yuniskis
<not.going.to.be@seen.com> wrote:

Hi Greg,

GregS wrote:
In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis <not.going.to.be@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

Have you tried putting a meter on the output of the power supply to
see if the current draw is in side the monitor, and how much more than
expected the current is? You also may want to check to see if the
backlight system is what is drawing the excessive current.

Trying to current limit the input of a SMPS is usually not viable. As
input voltage drops, the SMPS simply tries to comphensate, until it
reaches an unstable point then (often bad) unexpected things happen
(or it simply shuts down).

 

[D Yuniskis]

Hi Peter,

PeterD wrote:

On Fri, 11 Jun 2010 13:29:06 -0700, D Yuniskis
not.going.to.be@seen.com> wrote:
GregS wrote:
In article <huu0rv$68h$1@speranza.aioe.org>, D Yuniskis <not.going.to.be@seen.com> wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?
I often use light bulbs on the mains to limit current.
Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.
Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.
I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

Have you tried putting a meter on the output of the power supply to
see if the current draw is in side the monitor,

Set consists of a "brick" and the display itself.
Brick provides three (?) separate supplies to the set:
- inverter power
- audio amp power (?)
- the rest of the set

Inverters are fused separately and individually.
Fuse that was failing was in the "rest of the set" branch.

That fuse feeds the main switcher *inside* the set
(not to be confused with the switcher in the *brick*!).
The internal switcher seems to have five or six outputs
(i.e., all the various supplies needed by "the rest of
the set")

and how much more than expected the current is?

But that's the point -- you have no way of knowing what
the "expected" current is! The nameplate will tell you what
the nominal power requirements are (from which you can deduce
the nominal current). But, that applies to the entire set!
How much current should flow through each of the fuses
(two for the inverters plus two for the audio + rest_of_set)
isn't something you can know with any certainty.

Except to say that the fuses' ratings should exceed the
nominal current they are each expected to pass! :>

Given that the set runs for a day or more without blowing
the fuse in question suggests the "excess" isn't that
severe (or, the fuse is significantly overrated *and*
the brick is overdesigned to be capable of supplying
that "excess").

You also may want to check to see if the
backlight system is what is drawing the excessive current.

See above.

Trying to current limit the input of a SMPS is usually not viable. As
input voltage drops, the SMPS simply tries to comphensate, until it
reaches an unstable point then (often bad) unexpected things happen
(or it simply shuts down).

Correct. All you do is change duty cycles.

My question regards how you can get this sort of information
from service documents. I.e., at best, you'll see voltages
marked at various test points in the circuit. And *maybe*
some *nominal* waveforms. But, nothing that would clearly
allow you to diagnose this type of problem.

(e.g., drawing enough extra current to blow the fuse implies
the duty cycle of the internal switcher is higher than
intended. But, nothing in the types of documentation
that I just mentioned would tell you what the duty cycle
*should* be "for a typical load")

 

[Winston]

On 6/12/2010 12:18 AM, N_Cook wrote:

(...)

I happened to be playing with the innards of a domestic PIR unit for hotspot
"divining".

That project is done. And it is cheap too!

http://www.harborfreight.com/non-contact-laser-thermometer-96451.html

--Winston

 

[Cydrome Leader]

Phil Allison <phil_a@tpg.com.au> wrote:

"D Yuniskis"

I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

** So this fuse was not being stressed all the time - ie bending or faintly
glowing ?


The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.


** So ONE of these loads was intermittently drawing high current ?


After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?


** The good, old fashioned way is to use a larger fuse and wait for
something to get hot or start smoking.

Dunno about you, but I use a current meter in the AC supply for all bench
service jobs - it is sensitive enough ( 1mA resolution) to observe small
changes in the AC current draw and indicate if the current is creeping up.
I also keep a book recording the normal current draws ( of and on load for
amplifiers ) of everything I see.

this is an interesting practice- logging such info.

 

[D Yuniskis]

N_Cook wrote:

D Yuniskis <not.going.to.be@seen.com> wrote in message
I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

I happened to be playing with the innards of a domestic PIR unit for hotspot
"divining". Not too sure whether it would be useable. Uses RPY96 6 to 14

Sorry, I meant *imager* (not just a "thermometer").
I.e., look at large areas and see different temperature
ranges in pseudo-colors.

"Gee, why is this section so 'red'?"

micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324 output
for a small component 8 degree C over ambient, gave about 50mV swing up and
down of a nominal not too steady quiescent long term level. So if static at
2.3V would swing up to about 2.35 and then down to about 2.25 with a time
constant of a second or so , so again inconvenient. You have to scan across
as not a static process of generation of signal. There is a piezo effect as
well with the sensor so you would have to be very gentle in movement to
monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a 1.5
inch long dense foam rubber tube but matt metal maybe better, any thoughts?

 

[Phil Allison]

"Cydrome Leader"

Phil Allison

** The good, old fashioned way is to use a larger fuse and wait for
something to get hot or start smoking.

Dunno about you, but I use a current meter in the AC supply for all bench
service jobs - it is sensitive enough ( 1mA resolution) to observe small
changes in the AC current draw and indicate if the current is creeping
up.
I also keep a book recording the normal current draws ( of and on load
for
amplifiers ) of everything I see.

this is an interesting practice- logging such info.

** Keep in all in my " little red book" - aka a telephone index.

Brand and model name/number, followed by idle current and typical on-load
current if any - makes it a doddle to check adjust bias settings on most
tube and SS amps.

I also list the primary ohms for transformer PSU items in the same index.



..... Phil

 

[Phil Allison]

"D Yuniskis"

The nameplate will tell you what
the nominal power requirements are (from which you can deduce
the nominal current).

** Only very rarely is that true.

The amp ratings shown on the back of things like CRT monitors and audio
amplifiers give almost no clue as to the actual current draw when in use.

Eg; My 17 inch CRT monitor has a back panel rating of 1.5 amps but in fact
draws only 345mA rms from the 240 volt AC supply - the figure of 1.5 amps
appears to come from the fuse size used which must cope with inrush surges.




..... Phil

 

[Phil Allison]

"Alex Simpson"

But since it is already fixed I think the best answer to your question
would be if we knew how you fix it.


** Of course, once the nature of the INTERMITTENT fault scenario is known
one can usually determine a faster way of discovering it.

But the OP is playing a childish game with us and LYING about what really
went on.



..... Phil

 

[Alex Simpson]

Again, no shorts here.  Everything *worked"

Pardon me but if everything "worked" fuse would Not be blowing.

My first thought to a 2-3 day fuse is an "intermittent" problem.

Some ways I handle the intermittent is by close up observation of the
boards, heating and or tapping components out.

After tracking down the problem, it occurred to me just
how hard it is to do such things.

We all know if it is not a loose connection Intermittent are not easy.

So, how *should* this problem have been approached (without
risk to the set),

As much as I hate saying/doing this - Sometimes risk is the only way
to the solution....



But since it is already fixed I think the best answer to your question
would be if we knew how you fix it.


Gabe

 

[D Yuniskis]

Alex Simpson wrote:

Again, no shorts here. Everything *worked"

Pardon me but if everything "worked" fuse would Not be blowing.

Sure they can! All the fuse blowing means is the load
was drawing *nominally* more power than the fuse was
rated. E.g., a typical 3AG fuse will carry 110% of
its rated load for several hours. Given that this
fuse lasted *days*, means it was operating near its
rated capacity.

I.e., nothing *severe*.

Since there is no way of knowing (from the service
documents) what the *intended* load was, it could be
the system was just drawing 10-20% more than expected
(though I doubt that since proper derating for ambient
would typically have the fuse operating at ~60-70% of
it's rated current).

My first thought to a 2-3 day fuse is an "intermittent" problem.

No. An intermittent wouldn't take (roughly) the same amount
of time to show up each time the fuse blew.

Some ways I handle the intermittent is by close up observation of the
boards, heating and or tapping components out.

After tracking down the problem, it occurred to me just
how hard it is to do such things.

We all know if it is not a loose connection Intermittent are not easy.

So, how *should* this problem have been approached (without
risk to the set),

As much as I hate saying/doing this - Sometimes risk is the only way
to the solution....

But since it is already fixed I think the best answer to your question
would be if we knew how you fix it.

You're missing the point of my question.

You are suggesting that, in fact, the way to "solve" such problems
is to ask someone who solved a similar problem previously! So,
when you come up with a problem that someone else *hasn't*
solved, you're SOL.

My comments regarding the lack of information necessary to
track down this sort of problem suggests that service
documents should carry additional information. And,
possibly, minor changes to circuit topologies to make
this sort of troubleshooting easier.

E.g., if you have a (removable) fuse, you have an *easy*
way to monitor current through that branch of a circuit.
Adding a note on the drawing set indicating what this
current should be, nominally, goes a long way to sorting
out what problems may exist "in the field".

Likewise, *adding* disposable components to circuit
paths to facilitate opening that path to monitor current
can be a big win (e.g., FB's or 0 ohm shunts in the
secondaries of the switching transformer in my example...
easy to replace)

Of course, the conditions for all measurements have to be
codified else low line, high line, component variations,
etc. can make them meaningless (if the primary increases
by 10%, current decreases inverse proportionately, etc.).

Likewise, scope traces with real times/duty cycles
noted gives you an idea if things are operating at (or
near) their correct operating point.

Hoping to see repeat failures of the same sort is
a losing strategy (as it relies on design flaws instead
of component failures)