***BANG!***

[Michael Terrell]

On Sunday, November 10, 2019 at 2:22:02 PM UTC-5, Cursitor Doom wrote:

On Sun, 10 Nov 2019 10:00:10 -0800, Michael Terrell wrote:

Those plastic capacitors were supposed to be high rel, but they didn't
expect the cases to craze, and fail. What model is your VNA?

It's the 8754A. It tops out at 1.3Ghz which was, a long time ago when I
purchased it, was a respectably high frequency! In fact it's still the
highest practicable frequency band allocated for radio hams, which was
another good reason for me to choose it.

The only manual that I have only covers operation, but it is dated 1978. The design life was typically 15 to 20 years at that time, so it lasted twice that time. It does show the IEC power connector and voltage selector on the rear panel. That is where the line filter should be. It looks like it might be a Corcom 6J4 Power Entry Module. That 6J series is available with and without a filter.

 

[Cursitor Doom]

On Sun, 10 Nov 2019 14:41:48 -0800, Michael Terrell wrote:

The only manual that I have only covers operation, but it is dated 1978.
The design life was typically 15 to 20 years at that time, so it lasted
twice that time. It does show the IEC power connector and voltage
selector on the rear panel. That is where the line filter should be. It
looks like it might be a Corcom 6J4 Power Entry Module. That 6J series
is available with and without a filter.

This one has no mains filter AFAICS; just that busted cap. I do have the
*full* service and operating manual so I'll take a look and see what that
says tomorrow as it takes a month of sundays to find anything I need in
it. But like I say, if there *is* a filter, it's going to be nano-sized
and next to useless.



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[Cursitor Doom]

On Sun, 10 Nov 2019 14:30:39 -0800, John-Del wrote:

Me:

Thanks for that, Michael. I think I've found the cap you refer to,
straddled across the mains incoming supply, looking rather jaded, bulgy
and cracked.


But did it cause a bang loud enough to be heard in a shower?

We're assuming it was this X cap as it's clearly damaged, but I admit I
would expect from the bang (which I did hear *whilst* in the shower 30'
feet away) I'd have expected to see more extreme damage to that cap than
I did.

I had a TV do that back in the 80s, and what it did was launch an
electrolytic clean off the board. I found some paper packing and
eventually the cap itself. The TV still worked and the only evidence
left on the board was two very clean leads where the cap used to sit.

Even though you found a pretty good suspect, I'd look the rest of it
over very carefully for a couple of leads and no part between them.

You're right. I need to eliminate the possibility that the x-cap damage
had maybe been there since years ago and I still haven't found the true
culprit. If that may be the case, then what other part from where could
account for it? I've already eliminated the big electrolytics from
suspicion.



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[Fox's Mercantile]

On 11/10/19 5:14 PM, Cursitor Doom wrote:

I need to eliminate the possibility that the x-cap damage
had maybe been there since years ago and I still haven't
found the true culprit.

Capacitors across the power line, when they fail, they fail
spectacularly. And usually fast enough that they don't always
take out the line fuse.

This is why they make X caps. When they fail they, in addition
to failing short, they don't explode with a vengeance or set
fire to themselves or the surrounding bits.

Y caps, on the other hand, go from the line, or neutral to
ground. They are designed to fail open.

<https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/>



--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

[Michael Terrell]

On Sunday, November 10, 2019 at 5:30:42 PM UTC-5, John-Del wrote:

On Sunday, November 10, 2019 at 10:20:06 AM UTC-5, Cursitor Doom wrote:
On Sun, 10 Nov 2019 05:59:39 -0800, Michael Terrell wrote:

The capacitors on the AC line input crack and adsorb moisture, Then the
explode, along with the smell of burning plastic. They are a common
failure in older HP test equipment. Once the short clears, the equipment
will operate, but not in a safe manor. Most people replace the entire
filter, rather than the capacitors. The ones that fail are mostly the
same brand.

Thanks for that, Michael. I think I've found the cap you refer to,
straddled across the mains incoming supply, looking rather jaded, bulgy
and cracked.


But did it cause a bang loud enough to be heard in a shower?

I had a TV do that back in the 80s, and what it did was launch an electrolytic clean off the board. I found some paper packing and eventually the cap itself. The TV still worked and the only evidence left on the board was two very clean leads where the cap used to sit.

Even though you found a pretty good suspect, I'd look the rest of it over very carefully for a couple of leads and no part between them.

I had a HP desktop computer lose an electrolytic capacitor in the power supply. The can blew out through the fan, along with flames.

I've had the large (160uF, 250V input capacitor in a v0ltage doubler explode in a TV while it was on the workbench. Confetti for about 20' radius.

I had just installed a new amplifier in a car dealership. They called the next day that t had a loud hum. Lightning had hit the early Japanese business telephone system's optional paging adapter. It blew the cover off the metal box, and it was another mess of confetti all over the old telephone equipment room. They replaced it, and that one exploded a week later. Neither design had any surge protection. Both times, their neighborhood was hit with 100W of 60Hz hum and other noise from the PA horns in their used car lot.. ;-)

 

[Cursitor Doom]

On Sun, 10 Nov 2019 17:32:51 -0600, Fox's Mercantile wrote:

Capacitors across the power line, when they fail, they fail
spectacularly. And usually fast enough that they don't always take out
the line fuse.

This is why they make X caps. When they fail they, in addition to
failing short, they don't explode with a vengeance or set fire to
themselves or the surrounding bits.

You seem to be contradicting yourself in the second paragraph. You
initially say "they fail spectacularly" then you say, "they don't explode
with a vengeance". It can't be both, so which is it?





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[Fox's Mercantile]

On 11/10/19 6:50 PM, Cursitor Doom wrote:

On Sun, 10 Nov 2019 17:32:51 -0600, Fox's Mercantile wrote:

Capacitors across the power line, when they fail, they fail
spectacularly. And usually fast enough that they don't always take out
the line fuse.

This is why they make X caps. When they fail they, in addition to
failing short, they don't explode with a vengeance or set fire to
themselves or the surrounding bits.

You seem to be contradicting yourself in the second paragraph. You
initially say "they fail spectacularly" then you say, "they don't explode
with a vengeance". It can't be both, so which is it?

Capacitors, NOT X rated.

Vs X rated.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

[Cursitor Doom]

On Sun, 10 Nov 2019 19:33:18 -0600, Fox's Mercantile wrote:

Capacitors, NOT X rated.

Vs X rated.

So you're saying the x-cap that looks like the culprit can't be, then?
Maybe you're right; we'll see in due course....



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[Michael Terrell]

On Sunday, November 10, 2019 at 7:50:14 PM UTC-5, Cursitor Doom wrote:

On Sun, 10 Nov 2019 17:32:51 -0600, Fox's Mercantile wrote:

Capacitors across the power line, when they fail, they fail
spectacularly. And usually fast enough that they don't always take out
the line fuse.

This is why they make X caps. When they fail they, in addition to
failing short, they don't explode with a vengeance or set fire to
themselves or the surrounding bits.

You seem to be contradicting yourself in the second paragraph. You
initially say "they fail spectacularly" then you say, "they don't explode
with a vengeance". It can't be both, so which is it?

They are designed to burn away the short, so there is no contradiction. The foil is very thin, so it vaporizes away from the point of failure, leaving the remaining part of the capacitor, in circuit. If you measure the capacitance of a damaged X or Y capacitor, it will be lower than marked.

 

[Fox's Mercantile]

On 11/10/19 7:44 PM, Cursitor Doom wrote:

On Sun, 10 Nov 2019 19:33:18 -0600, Fox's Mercantile wrote:

Capacitors, NOT X rated.

Vs X rated.

So you're saying the x-cap that looks like the culprit can't be, then?
Maybe you're right; we'll see in due course....

Oh for fuck's sake.
X rated capacitors are a recent development.
There weren't any when HP built the VNAs.
They used standard capacitors at the time. Like everyone else.
Which, is why, when it failed, it failed spectacularly.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

[John-Del]

On Sunday, November 10, 2019 at 6:33:20 PM UTC-5, Fox's Mercantile wrote:

On 11/10/19 5:14 PM, Cursitor Doom wrote:
I need to eliminate the possibility that the x-cap damage
had maybe been there since years ago and I still haven't
found the true culprit.

Capacitors across the power line, when they fail, they fail
spectacularly. And usually fast enough that they don't always
take out the line fuse.

Back in the mid to late 60s, RCA used a .1uf cap right across the AC interlock. They would let go on occasion and fail, well, spectacularly. They would blast bits of metal and tons of carbon under the whole chassis. Because they were wired before the breaker, they kept going until the house fuse blew or they spent their mass.

RCA recalled them and I remember putting in several hundred replacement caps through the 70s. Back then, most TVs weren't registered with the manufacturer and without internet, it was difficult getting consumers aware of the issue. But whenever we'd see one (dark brown), we'd automatically replace it with the revised cap.

 

[Cursitor Doom]

On Mon, 11 Nov 2019 04:48:42 -0800, John-Del wrote:

Back in the mid to late 60s, RCA used a .1uf cap right across the AC
interlock. They would let go on occasion and fail, well, spectacularly.
They would blast bits of metal and tons of carbon under the whole
chassis.

Hmmm. Amazing a little .1uF cap could make that much noise! I freely
admit I've never been in the same room when a cap let-go, so have only
ever heard them from some way off. I really should blow some up on
purpose so I know for myself; that's an essential part of my education
that's been missing all these years.
Is the violence you get commensurate with the capacity? Say for example I
blow up a 10,000uF electrolytic on purpose, is it a *much* bigger deal
than say a 25uF one? What does the noise approximate to - as much as a
pistol shot?



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[amdx]

On 11/10/2019 11:56 AM, Michael Terrell wrote:

On Sunday, November 10, 2019 at 11:24:08 AM UTC-5, Ralph Mowery wrote:

Michael Terrell says...

Have you seen the 'Nano VNA' handheld units? Under $50 at many places.

I bought one a while back and paid $ 57 for it. They have came down.

While not lab quality, they do work well enough for an experimenter on a
low budget.

Around 20 years or less ago , they would almost have been lab quality.
All they would need is a high stable and accurate frequency .

There is lots of stuff coming out of China that works well enough for
people like me that don't have a lot of money to buy things with but
still like to experiment and learn.

Those little component testers for around $ 20 really get to me. They
seem to work really well in most cases as to identifying components and
their values.

I bought a Nano VNA for $42.87 with some Ebay bucks that were about to expire, but I haven't tried it yet. I am currently trying to get another door and a handicap ramp installed at my home. The current exits all have too many steps to go up and down. I haven't been in my shop building for several years, because it is too much pain to get out to it, and stand at the workbench. Once the modifications are done, I will have room for a small bench in that room.

I am retired, so I no longer have access to the multiple workbenches of test equipment that I used on my last job.

I agree about those component testers being useful. I have a couple. Another one that is handy is a $10 VGA monitor test generator.

I caution others to research before you buy, some work great others
have a higher failure rate and the external appearance seems to be a
clue. The one I saw recommended has a white front with a lizard climbing
down on the right side. But,people do like them, there are several
groups running about the Nano VNA and new open source software being
written.
Mikek

 

On Sunday, 10 November 2019 16:54:24 UTC, Cursitor Doom wrote:

On Sun, 10 Nov 2019 08:14:36 -0800, Michael Terrell wrote:

What's life, without a challenge?

A challenge has to be, by definition, ultimately achievable. Removing
this cap is *not* achievable. I would have to destroy the entire VNA to
get it out. What a rotten design.

Just snip it then, it's not needed.


NT

 

[Mike Coon]

In article <qqc220$9r7$1@dont-email.me>, curd@notformail.com says...

Hmmm. Amazing a little .1uF cap could make that much noise! I freely
admit I've never been in the same room when a cap let-go, so have only
ever heard them from some way off. I really should blow some up on
purpose so I know for myself; that's an essential part of my education
that's been missing all these years.
Is the violence you get commensurate with the capacity? Say for example I
blow up a 10,000uF electrolytic on purpose, is it a *much* bigger deal
than say a 25uF one? What does the noise approximate to - as much as a
pistol shot?

It must have been in 1961 when I had a holiday job bewteen school and
college at a local PA manufacturer. I was present when the snooty
designer came to appraise the prototype of a new model. He was peering
at the build detail when an electrolytic exploded. I don't remember the
capacity or much noise, but the confetti of foil draped over him caused
a lot of bitten lips while the audience suppressed their giggles...

Mike.

 

On Sunday, 10 November 2019 15:20:06 UTC, Cursitor Doom wrote:

On Sun, 10 Nov 2019 05:59:39 -0800, Michael Terrell wrote:

The capacitors on the AC line input crack and adsorb moisture, Then the
explode, along with the smell of burning plastic. They are a common
failure in older HP test equipment. Once the short clears, the equipment
will operate, but not in a safe manor. Most people replace the entire
filter, rather than the capacitors. The ones that fail are mostly the
same brand.

Thanks for that, Michael. I think I've found the cap you refer to,
straddled across the mains incoming supply, looking rather jaded, bulgy
and cracked. I'd have thought they'd have used potted, integrated filters
like the Tek scopes of the same era did. I think IIRC I've had issues
with these caps before on other equipment. It's buried somewhat in the
guts of the device immediately behind the mains socket but I can just
about see it. 250V it says on it. Not much headroom with our 240V mains
here!
How about I just snip it out and not bother replacing it? It's a PITA to
get at.

240v is 330v peak, so no headroom at all.


NT

 

[Fox's Mercantile]

On 11/11/19 2:18 PM, tabbypurr@gmail.com wrote:

On Sunday, 10 November 2019 16:54:24 UTC, Cursitor Doom wrote:
On Sun, 10 Nov 2019 08:14:36 -0800, Michael Terrell wrote:

What's life, without a challenge?

A challenge has to be, by definition, ultimately achievable. Removing
this cap is *not* achievable. I would have to destroy the entire VNA to
get it out. What a rotten design.

Just snip it then, it's not needed.


NT

Spoken like a true hack.


--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

 

On Monday, 11 November 2019 21:10:58 UTC, Fox's Mercantile wrote:

On 11/11/19 2:18 PM, tabbypurr wrote:
On Sunday, 10 November 2019 16:54:24 UTC, Cursitor Doom wrote:
On Sun, 10 Nov 2019 08:14:36 -0800, Michael Terrell wrote:

What's life, without a challenge?

A challenge has to be, by definition, ultimately achievable. Removing
this cap is *not* achievable. I would have to destroy the entire VNA to
get it out. What a rotten design.

Just snip it then, it's not needed.


NT


Spoken like a true hack.

are you being childish again?

 

[pfjw@aol.com]

The bean counters would never let an unnecessary part into any design. Therefore, leaving a part out of a design is "hack".

That is, unless one is Mad Man Muntz.

Peter Wieck
Melrose Park, PA

 

[Clifford Heath]

On 12/11/19 3:25 am, Cursitor Doom wrote:

Is the violence you get commensurate with the capacity? Say for example I
blow up a 10,000uF electrolytic on purpose, is it a *much* bigger deal
than say a 25uF one? What does the noise approximate to - as much as a
pistol shot?

The larger cap will likely make less noise. The metal can will have
similar wall thickness, but much greater circumference, so the hoop
stress will be much bigger - it will split down one side under much less
internal pressure.

Small diameter, large C (meaning low voltage), low ESR, will make the
biggest bang.

Clifford Heath