Thermal Faults

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

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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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Re: Thermal Faults
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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.

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

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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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Re: Thermal Faults
On 2019/09/22 4:48 a.m., Cursitor Doom wrote:
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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 :-#)#

Re: Thermal Faults
On Sunday, 22 September 2019 11:46:59 UTC+1, Cursitor Doom  wrote:
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Anything.


NT

Re: Thermal Faults
On 22/09/2019 11:46, Cursitor Doom wrote:
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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

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

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

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Quite.


Precisely what I am about to do next!

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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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Re: Thermal Faults
On Sunday, 22 September 2019 21:54:10 UTC+1, Cursitor Doom  wrote:
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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

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

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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!

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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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Re: Thermal Faults
As already noted, many things *can* get hot. What is in question is how hea
t 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 f
rom the heat required to install them. Given the alternatives, the only rea
son 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 ge
neral, if an electrolytic cap in a piece of solid-state equipment gets warm
er than ambient temperatures, replace it!  

c) Cold-solder joints will get hot based on the current they may be carryin
g. 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 l
ong standing.

d) Broken/lifted/oxidized traces are very often heat related. And in the la
st 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 sy
mptoms as the equipment heats up.  

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

Peter Wieck  
Melrose Park, PA

Re: Thermal Faults
On Wednesday, 25 September 2019 13:46:11 UTC+1, snipped-for-privacy@aol.com  wrote:

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eat will affect various components. In no particular order:
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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 r
eason 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

Re: Thermal Faults
On Wednesday, September 25, 2019 at 6:13:15 PM UTC-4, snipped-for-privacy@gmail.com wrote:

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

Peter Wieck
Melrose Park, PA  

Re: Thermal Faults
On Thursday, 26 September 2019 11:56:12 UTC+1, snipped-for-privacy@aol.com  wrote:
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it's not clear what issue you're referring to there.


NT

Re: Thermal Faults
The fact that carbon resistors have a pretty wretched service life and heat tolerance.

Peter Wieck  
Melrose Park, PA  

Re: Thermal Faults
On Thursday, 26 September 2019 21:46:02 UTC+1, snipped-for-privacy@aol.com  wrote:
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you're clearly misinformed

Re: Thermal Faults
On 9/26/19 4:44 PM, snipped-for-privacy@gmail.com wrote:
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30 years of servicing vintage radios tells me you're the one
misinformed.

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Jeff-1.0
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Re: Thermal Faults
On Thursday, 26 September 2019 23:42:18 UTC+1, Fox's Mercantile  wrote:
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 heat tolerance.
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Vintage is the key word there. Ancient carbon composition are indeed prone  
to rising in value & going oc. However carbon film Rs have an excellent rel
iability record. I've repaired lots of stuff over however many years, and c
arbon film Rs are almost never a problem.  

I've no doubt your ego will not permit you to get real, so I see little poi
nt continuing this. Others who read this will make up their own minds.


NT

Re: Thermal Faults
On 9/27/19 4:08 AM, snipped-for-privacy@gmail.com wrote:
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Do you even pay attention to what you write?

"Carbon comp is used on occasion for its pulse power tolerance."

Which is the worst possible way of using a carbon composition
type of resistor.

Peter told you that. I told you that.

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Jeff-1.0
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Re: Thermal Faults
On 2019-09-27 12:42, Fox's Mercantile wrote:
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Why is that? This is IMHO not about the carbon film type but about  
modern massive carbon composition. I've used them for > 40 years in e.g.  
triac snubber networks and they never failed. All kinds of (same wattage  
rating) film types did fail. Wirewound is also OK but too expensive.

Arie

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