Gentlemen,

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on, what/which
is/are the most likely suspects to be considered blameworthy? I'm
guessing dry joints has to be on the list somewhere, but what components
can also give rise to this issue?



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On Sun, 22 Sep 2019 10:46:57 -0000 (UTC), Cursitor Doom
wrote:

Gentlemen,

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on, what/which
is/are the most likely suspects to be considered blameworthy? I'm
guessing dry joints has to be on the list somewhere, but what components
can also give rise to this issue?

Any component can have thermal problems. I would check the ones that
feel hot after a few minutes.

Steve

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On Sun, 22 Sep 2019 12:34:22 +0100, Stephen Wolstenholme wrote:

Any component can have thermal problems. I would check the ones that
feel hot after a few minutes.

Normally I would go around with a can of freezy spray and dob various
parts of the board looking for the fault to go away, but sadly in this
case it's not possible as the board in question is one of these slot-in
types that are inaccessible to investigation when the instrument is under
power.





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In article , says...

Normally I would go around with a can of freezy spray and dob various
parts of the board looking for the fault to go away, but sadly in this
case it's not possible as the board in question is one of these slot-in
types that are inaccessible to investigation when the instrument is under
power.

Decades ago there was always a standard extension board that plugged
into the motherboard in place of the questionable one and carried only a
socket that the one under investigation could be plugged into to make it
accessible, so long as the covers were off...

Mike.

On Sun, 22 Sep 2019 15:02:22 +0100, Mike Coon wrote:

In article , says...

Normally I would go around with a can of freezy spray and dob various
parts of the board looking for the fault to go away, but sadly in this
case it's not possible as the board in question is one of these slot-in
types that are inaccessible to investigation when the instrument is
under power.

Decades ago there was always a standard extension board that plugged
into the motherboard in place of the questionable one and carried only a
socket that the one under investigation could be plugged into to make it
accessible, so long as the covers were off...

Yes, and the service engineers would no doubt have been issued with them.
The service engineers would also no doubt have been issued with duplicate
sets of identical boards to swap-out, thereby saving heaps of their very
expensive time on each job.
This particular instrument is a 10Mhz-22Ghz spectrum analyser (one of two
I have made by Hewlett-Packard) so those engineers sent out into the
field to fix them would have been very well-supported by HP.
Unfortunately, however, I'm not one of them! However, the fault appears
to be somewhere in the x-amplifier board and they used *exact* same board
for the y-amplifier so at least I can compare them. There's no point now
in swapping them over as I now *know* the fault lies somewhere in the x-
amp one.



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On 2019/09/22 4:48 a.m., Cursitor Doom wrote:
On Sun, 22 Sep 2019 12:34:22 +0100, Stephen Wolstenholme wrote:

Any component can have thermal problems. I would check the ones that
feel hot after a few minutes.

Normally I would go around with a can of freezy spray and dob various
parts of the board looking for the fault to go away, but sadly in this
case it's not possible as the board in question is one of these slot-in
types that are inaccessible to investigation when the instrument is under
power.


Lets assume you have the schematics and a bench/lab power supply and you
know what goes wrong when the board is warm. Then why not put the board
on the bench powered up by your power supply and check the parameters of
the circuit whose change can lead to the symptom you see? Check the
voltages when just turned on, then heat it up and see what changes.

You don't need an extension card, although they are very handy - I have
a dozen or so - however if the card connections are more-or-less
standard you can make your own using a plug (hacked from a dead PCB
perhaps) and socket and some wire. These will usually work, just usually
aren't very good at 100khz or higher frequencies...

It all depends on how badly you want to fix this and what your skills are...

John :-#)#

Cursitor Doom wrote:
--------------------

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on,


** If the problem appears after a time delay an warming - it ain't a ****ing intermittent.

FYI:

I was presented with a Yamaha digital reverb some years ago ( model R1000) that worked fine until it got a bit hot. Then it broke into a very loud, harsh noise - bit like pink noise.

The only thing I could find that triggered the noise was heating the micro-controller with my soldering iron - so I ordered a new one ( not cheap) and fitted it. Made no ****ing difference.

So I rang the Australian agent and described the problem to one of their staff service people - with an Asian accent (his not mine) - he went off and checked the files for relevant service bulletins.

Came back with one entitled "Big Noise".

" If the unit emits a loud noise when operating in a high ambient temp replace ICs 1,2 3, 4 ,5 ,6 ... "

The problem was a damn design fault in the first 10,000 sold !!!

The fix was to replace all the regular CMOS data selectors with high speed ( HC ) ones. Once carried out, the problem vanished.

Seems the CMOS logic used had a timing issue, exacerbated by high temps.

More BS than this little Koala can bear ...


..... Phil

On Sunday, 22 September 2019 11:46:59 UTC+1, Cursitor Doom wrote:
Gentlemen,

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on, what/which
is/are the most likely suspects to be considered blameworthy? I'm
guessing dry joints has to be on the list somewhere, but what components
can also give rise to this issue?

Anything.


NT

On 22/09/2019 11:46, Cursitor Doom wrote:
Gentlemen,

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on, what/which
is/are the most likely suspects to be considered blameworthy? I'm
guessing dry joints has to be on the list somewhere, but what components
can also give rise to this issue?


Not much to go on.

Age of board? What does it do? Digital?

Old caps can go leaky losing capacitance with temperature. If they were
decoupling something from interference ...

I'd scope out the power supply rails with a DSO, maybe you might catch
something. Can you arrange a trigger when the fault occurs?

--
Adrian C

On Sun, 22 Sep 2019 19:22:20 +0100, Adrian Caspersz wrote:

Not much to go on.

Age of board? What does it do? Digital?

30 years; CRT horizontal sweep amplifier; 100% analogue.

Old caps can go leaky losing capacitance with temperature. If they were
decoupling something from interference ...

Quite.


I'd scope out the power supply rails with a DSO, maybe you might catch
something.

Precisely what I am about to do next!

Can you arrange a trigger when the fault occurs?

It's now so recurrent that no special triggering is needed. At the pre-
driver stage (the output from the sweep gen stage) I see a nice clean saw-
tooth waveform but 3 stages later I see the same trace; same amplitude
but frizzled by severe noise and twitching like mad. So I'm closing in on
it but am hampered by the fact that the board is inaccessible when
powered up since it sits in a slot alongside other boards.





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On Sunday, 22 September 2019 21:54:10 UTC+1, Cursitor Doom wrote:
On Sun, 22 Sep 2019 19:22:20 +0100, Adrian Caspersz wrote:

Not much to go on.

Age of board? What does it do? Digital?

30 years; CRT horizontal sweep amplifier; 100% analogue.

Old caps can go leaky losing capacitance with temperature. If they were
decoupling something from interference ...

Quite.


I'd scope out the power supply rails with a DSO, maybe you might catch
something.

Precisely what I am about to do next!

Can you arrange a trigger when the fault occurs?

It's now so recurrent that no special triggering is needed. At the pre-
driver stage (the output from the sweep gen stage) I see a nice clean saw-
tooth waveform but 3 stages later I see the same trace; same amplitude
but frizzled by severe noise and twitching like mad. So I'm closing in on
it but am hampered by the fact that the board is inaccessible when
powered up since it sits in a slot alongside other boards.

Tack on a bunch of wires to points you'd like to scope. Install board, scope those points, move on. It sounds like you're very close to finding the culprit.


NT

On Mon, 23 Sep 2019 02:22:44 -0700, tabbypurr wrote:

Tack on a bunch of wires to points you'd like to scope. Install board,
scope those points, move on.

Yup, in fact that's what I had to do to find the excessive noise I
mentioned in an earlier post. I hadn't expected to find it; I wasn't
looking for noise, but I couldn't very well ignore it given it
represented about 50% of the waveform amplitude!

It sounds like you're very close to finding
the culprit.

Hopefully. But this unit has multiple issues and this is just one of
them. Fortunately for me this is just for fun. If I had to do this for a
living I wouldn't survive very long due to the time I take over these
things it simply wouldn't be economic.

Anyway, time now to get on and scope those power rails....



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As already noted, many things *can* get hot. What is in question is how heat will affect various components. In no particular order:

a) Carbon comp/film resistors will change value over time, and much more-so of heated. Some types are so awful that they will change in value simply from the heat required to install them. Given the alternatives, the only reason to use them today would be based on audiophoolery.

b) Electrolytic capacitors do not like heat at all. Sure, the come rated to 105 C. and more - but none-the-less, they continue not to like heat. In general, if an electrolytic cap in a piece of solid-state equipment gets warmer than ambient temperatures, replace it!

c) Cold-solder joints will get hot based on the current they may be carrying. And, in general, there will be some discoloration around the bad joint, or some other visible indication. Most especially if this bad joint is of long standing.

d) Broken/lifted/oxidized traces are very often heat related. And in the last stages before complete failure they may become intermittent, giving you the symptoms you hear/see.

e) If, by any chance there are small IF cans on/near this board, or within this device, silver-mica disease will create thunderstorms and breathing symptoms as the equipment heats up.

If you have access to an IR camera, try running that board until the symptoms are well-established, then pull it and photograph it. A hot component will stand out as a bright white blob. A friend of mine is a hobby photographer and has IR equipment. One day, he decided to use it to troubleshoot a piece of electronics. Within a few minutes he found the problem - that was otherwise entirely invisible under normal light.

Peter Wieck
Melrose Park, PA

On Wednesday, 25 September 2019 13:46:11 UTC+1, wrote:

As already noted, many things *can* get hot. What is in question is how heat will affect various components. In no particular order:

a) Carbon comp/film resistors will change value over time, and much more-so of heated. Some types are so awful that they will change in value simply from the heat required to install them. Given the alternatives, the only reason to use them today would be based on audiophoolery.

We use carbon film aplenty. The reasons are low cost & good availability.
Carbon comp is used on occasion for its pulse power tolerance.


NT

On Wednesday, September 25, 2019 at 6:13:15 PM UTC-4, wrote:

We use carbon film aplenty. The reasons are low cost & good availability.
Carbon comp is used on occasion for its pulse power tolerance.

In other words, kick the can down the road....

Peter Wieck
Melrose Park, PA

On Thursday, 26 September 2019 11:56:12 UTC+1, wrote:
On Wednesday, September 25, 2019 at 6:13:15 PM UTC-4, tabby wrote:

We use carbon film aplenty. The reasons are low cost & good availability.
Carbon comp is used on occasion for its pulse power tolerance.

In other words, kick the can down the road....

it's not clear what issue you're referring to there.


NT

The fact that carbon resistors have a pretty wretched service life and heat tolerance.

Peter Wieck
Melrose Park, PA

On 22/09/2019 8:46 pm, Cursitor Doom wrote:
Gentlemen,

When inspecting a board for the likely cause of an intermittent fault
believed to be induced by warming up over time from switch-on, what/which
is/are the most likely suspects to be considered blameworthy? I'm
guessing dry joints has to be on the list somewhere, but what components
can also give rise to this issue?




**All of them, but semiconductors are the most likely culprits. The
worst I've seen was a varistor used in the Marantz 2325 receiver from
the 1970s. When cold, the varistor was fine. As it warmed up, the amp
shut down, due to huge DC shifts. As it warmed further, the amp returned
to normal operation. I found that the critical temperature range was
fairly narrow. Around 5 degrees C, at around 35 degrees C. It was a
bugger to fault find, because the usual freezer spray would transition
the component from hot to cold too quickly for the fault to appear.

--
Trevor Wilson
www.rageaudio.com.au

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Whatever they all say this is the deal. First of all things that pop up when cold might be connections, if when hot they are almost always component failure.

You have to thermally cycle it several times, so go over your Grandma's place a steal an endtable. Take the case off and get a towel. Fire it up and maybe take some voltages. Cover with towel. Then when it fails take those voltages again.

Then find out where any missing voltages come from. Always remember, 70% of the time or more it only has one problem. I used to say almost all the time but that is no longer true. And things blow each other, but there is one ROOT cause of the problem.

If you can't find a case history on it you are just going to have to do it, all there is to it.

You can try Repairworld if they are still around for $11 a month. Just one month isn't going to break the bank I hope but they might not have anything on it.

Other than that, just troubleshot. Got a print ? Maybe we can look at ? I don't even know what the **** this thing is. It could be an ICBM or a toaster fr as I know.

On Sat, 28 Sep 2019 19:26:22 -0700, Jeff Urban wrote:

Other than that, just troubleshot. Got a print ? Maybe we can look at ?
I don't even know what the **** this thing is. It could be an ICBM or a
toaster fr as I know.

It's an old HP 8565A RF spectrum analyser built with good old fashioned
discrete components you can physically see.
The problem I'm having is access. I have identified the failed board: the
x-amplifier module. They've used plug-in boards which is great in one
way, except that they're so closely juxtaposed with other plug-in boards
it's impossible to carry out any traditional troubleshooting techniques
whilst under power. Believe me, I've tried. Tacking on fine wires to
various key connections and running them out for probing and whatnot but
I'd end up with a right old rat's nest of a mess if I carry on like that
much longer.
I've discovered a lot of old tower computers use the same pitch of plug
in board that HP used (3.96mm) so I've ordered a matching socket and will
make up a patch lead that will enable me to probe the board under power
out in the open where I can get at it. It's a PITA, but there's no
alternative I can see.


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On 29/09/2019 9:15 pm, Cursitor Doom wrote:
On Sat, 28 Sep 2019 19:26:22 -0700, Jeff Urban wrote:

Other than that, just troubleshot. Got a print ? Maybe we can look at ?
I don't even know what the **** this thing is. It could be an ICBM or a
toaster fr as I know.

It's an old HP 8565A RF spectrum analyser built with good old fashioned
discrete components you can physically see.
The problem I'm having is access. I have identified the failed board: the
x-amplifier module. They've used plug-in boards which is great in one
way, except that they're so closely juxtaposed with other plug-in boards
it's impossible to carry out any traditional troubleshooting techniques
whilst under power. Believe me, I've tried. Tacking on fine wires to
various key connections and running them out for probing and whatnot but
I'd end up with a right old rat's nest of a mess if I carry on like that
much longer.
I've discovered a lot of old tower computers use the same pitch of plug
in board that HP used (3.96mm) so I've ordered a matching socket and will
make up a patch lead that will enable me to probe the board under power
out in the open where I can get at it. It's a PITA, but there's no
alternative I can see.



**It's highly likely that HP made available extension adapters to suit
their test equipment back in the day. You might be able to source one.
Somewhere.

--
Trevor Wilson
www.rageaudio.com.au

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