***BANG!***

C

Cursitor Doom

Guest
So I've had this HP RF VNA in mothballs for the last 10 years (and I
*know* it's exactly 10 years) which I decided to fire-up. Being as it's
been a while, I took it up very slowly with a variac, beginning Friday
evening (I checked the service manual first and it's got a linear PSU)
and by the time I got to late morning on Sunday I had it up to 230V
(which is only 10 less than the local supply) and all seemed well. It had
been fine on this for a good hour when out of nowhere there was a
**bang** and a funny smell. I was in the shower 30 feet away when it
happened but could still hear it from there so it was pretty loud.

[short time later...]

I whipped off the case and made my way to a row of large electrolytics
(the usual suspects). These are the big, blue Sprague 'Powerlytics' (TM)
That HP were very fond of using back in the day (c.1980) but they have
all tested fine for capacity and ESR. That's my prime theory out the
window, then. No visible signs inside what could have catastrophically
failed, just the unmistakable smell that *something* has. The device
still powers up fine and the screen traces are normal, so wtf else goes
*bang* and smells toxic?



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On Sunday, November 10, 2019 at 8:37:37 AM UTC-5, Cursitor Doom wrote:
So I've had this HP RF VNA in mothballs for the last 10 years (and I
*know* it's exactly 10 years) which I decided to fire-up. Being as it's
been a while, I took it up very slowly with a variac, beginning Friday
evening (I checked the service manual first and it's got a linear PSU)
and by the time I got to late morning on Sunday I had it up to 230V
(which is only 10 less than the local supply) and all seemed well. It had
been fine on this for a good hour when out of nowhere there was a
**bang** and a funny smell. I was in the shower 30 feet away when it
happened but could still hear it from there so it was pretty loud.

[short time later...]

I whipped off the case and made my way to a row of large electrolytics
(the usual suspects). These are the big, blue Sprague 'Powerlytics' (TM)
That HP were very fond of using back in the day (c.1980) but they have
all tested fine for capacity and ESR. That's my prime theory out the
window, then. No visible signs inside what could have catastrophically
failed, just the unmistakable smell that *something* has. The device
still powers up fine and the screen traces are normal, so wtf else goes
*bang* and smells toxic?

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. There are a lot of comments about these on https://groups.io/g/HP-Agilent-Keysight-equipment replacement user group for the old Yahoo groups.
 
On 11/10/19 7:59 AM, 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.

I used to repair test equipment at Hughes Aircraft.
I was surprised to find that HP used Sprague Black Beauty
capacitors in their older equipment.
Regardless of the reason it was on the bench, standard procedure
was to replace all them.



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



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On 11/10/19 9:20 AM, Cursitor Doom wrote:
How about I just snip it out and not bother replacing it?
It's a PITA to get at.

Or you could just throw it in the trash. If it's not worth
fixing, it's not worth keeping.



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

They are X or Y rated safety capacitors. I would not omit them. The 250 volt marking is AC volts, not DC. The HP manual should have the specs for the bad caps.
 
On Sunday, November 10, 2019 at 9:37:30 AM UTC-5, Fox's Mercantile wrote:
On 11/10/19 7:59 AM, 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.

I used to repair test equipment at Hughes Aircraft.
I was surprised to find that HP used Sprague Black Beauty
capacitors in their older equipment.
Regardless of the reason it was on the bench, standard procedure
was to replace all them.

Just think of all the Ebay sales you could have made to deluded musicians, with those used BB caps! come to think of it, I had a bag of NOS BBs that came from an old shop that went out of business. Maybe I'll dig them out and list them. :)
 
On Sunday, November 10, 2019 at 10:24:43 AM UTC-5, Fox's Mercantile wrote:
On 11/10/19 9:20 AM, Cursitor Doom wrote:
How about I just snip it out and not bother replacing it?
It's a PITA to get at.

Or you could just throw it in the trash. If it's not worth
fixing, it's not worth keeping.

Have you seen the 'Nano VNA' handheld units? Under $50 at many places.
 
On Sun, 10 Nov 2019 07:27:03 -0800, Michael Terrell wrote:

They are X or Y rated safety capacitors. I would not omit them. The 250
volt marking is AC volts, not DC. The HP manual should have the specs
for the bad caps.

I have a box full of Wima X and Y caps here (which are supposed to be
superior) so sourcing a replacement for this failed Rifa is not an issue.
It's getting access that's the thing.




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On Sunday, November 10, 2019 at 10:34:57 AM UTC-5, Cursitor Doom wrote:
On Sun, 10 Nov 2019 07:27:03 -0800, Michael Terrell wrote:

They are X or Y rated safety capacitors. I would not omit them. The 250
volt marking is AC volts, not DC. The HP manual should have the specs
for the bad caps.

I have a box full of Wima X and Y caps here (which are supposed to be
superior) so sourcing a replacement for this failed Rifa is not an issue.
It's getting access that's the thing.

What's life, without a challenge? :)
 
On Sun, 10 Nov 2019 07:27:03 -0800, Michael Terrell wrote:

> They are X or Y rated safety capacitors. I would not omit them.

They're "safety capacitors" in respect of the fact that they can be
connected across a (reasonably) high energy source; that's all. If they
are omitted it's NOT a safety issue. Obviously *if* one is going to
connect a capacitor across the mains supply it needs to be X rated, but
*leaving out* such a capacitor, x rated or not, is not going to kill or
injure anyone.




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In article <qq99r3$lps$1@dont-email.me>,
Cursitor Doom <curd@notformail.com> wrote:
like the Tek scopes of the same era did. I think IIRC I've had issues
with these caps before on other equipment.

Elsewhere down thread you mention "RIFA" capacitors, sounds like the
gold coloured 0.47uF/250V ones that seem to disgrace various 1980's
ASTEC branded power supplies (BBC Computer, Apple II computer, ICL
One Per Desk and others) all of which work fine for a while, and then
crackle-phut-burnt toffee flavoured smoke, but keep on working.

I guess I got lucky, they didn't fail shorted, but they definitely
cracked and bulged.


--
--------------------------------------+------------------------------------
Mike Brown: mjb[-at-]signal11.org.uk | http://www.signal11.org.uk
 
In article <0673dcfc-6859-4f94-9f08-e67a019c0895@googlegroups.com>,
terrell.michael.a@gmail.com 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 expirmenter 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 comming 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 expirment 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.
 
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.



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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.
 
On Sunday, November 10, 2019 at 11:19:15 AM UTC-5, Cursitor Doom wrote:
On Sun, 10 Nov 2019 07:27:03 -0800, Michael Terrell wrote:

They are X or Y rated safety capacitors. I would not omit them.

They're "safety capacitors" in respect of the fact that they can be
connected across a (reasonably) high energy source; that's all. If they
are omitted it's NOT a safety issue. Obviously *if* one is going to
connect a capacitor across the mains supply it needs to be X rated, but
*leaving out* such a capacitor, x rated or not, is not going to kill or
injure anyone.

X capacitors are to filter RF from the power lines. Y capacitors are for safety.
 
On Sunday, November 10, 2019 at 11:54:24 AM UTC-5, 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.

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?
 
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.



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On Sun, 10 Nov 2019 09:58:21 -0800, Michael Terrell wrote:

X capacitors are to filter RF from the power lines. Y capacitors are for
safety.

I suspect you're going to have a few contradictory views about that
sweeping statement from some of our not-so-polite regulars here! ;-)

I can *just* access enough to snip the blown cap out and discard it. I
think I'll fit a new one to the end of the power lead instead; it's my
only option realistically.



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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.
 

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