Scope keeps blowing fuses...

[Jeff Liebermann]

On Mon, 24 Oct 2022 22:17:57 +0100, Cursitor Doom <cd@notformail.com>
wrote:

I\'m no tech repairer; haven\'t got the necessary fine motor
skills, I\'m afraid.

Not a problem. Let a hand soldering robot do the precision work.
Something like this soldering robot:
<https://www.promationusa.com/9101>
More:
<https://www.promationusa.com/robotic-soldering-glance>
and even more:
<https://www.google.com/search?q=hand+soldering+robot&tbm=isch>


--
Jeff Liebermann jeffl@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Phil Allison]

legg wrote:
------------------

Cursitor Doom wrote:
---------------------------------
And if those
caps fail open circuit (which they both did despite being X2 safety
ones which are supposed to fail short)

** Really ? Where did you get that idea?

Plenty are found open or with low values after long times in service.
X2 dropping caps used in smoke alarms regularly fail open.

But those Rifa ones love to smoke and burn.

==================================================

The smoking caps were metalized paper construction. These were good
enough in their day, with a demonstrated mtbf and safe self-healing,
while seals lasted. The outer sealing body tended to shrink and crack
with age, allowing ingress of air and moisture, compromising
performance. They are no longer made.

** Rifa PME271 X2 caps are still made and widely available.
Strangely they are also the most expensive X2 caps on offer for no good reason.
Described as being of \"metalised , impregnated paper \" construction.
Rifa also claim to be the only supplier .

Other makers, dozens of them, use metalised polyester or polypropylene films as used in DC rated caps - but with one big difference.
X2 types are double wound = a technique that creates two caps in series.

Such caps are immune from *corona discharge* occurring in tiny air pockets trapped inside the cap as it it being wound.
They are still vulnerable to spike voltages of a few kV which eat away at metalisation and reduce cap value over time.

IME Rifa X2s are notorious for catching fire and filling the room with acrid smoke.
Happened twice, right in front of me, first case was a room heater and the second with a portable TV.

When used for suppression in triac dimming equipment, they invariably fail early too.
Bad news.

...... Phil

 

[Phil Hobbs]

Cursitor Doom wrote:

Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA

Yikes. When a fuse blows, you replace it *once*. If it blows again, it
ain\'t the fuse or a line transient.

Start with a DMM and check for shorted diodes in the input bridge and
then for a shorted filter cap at its output. (The schematic is more or
less completely illegible, or I\'d give the ref des.)

Next most likely is a switch FET.

Cheers

Phil Hobbs
(proud owner of a 2467 that he almost never uses)


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Cursitor Doom]

On Mon, 24 Oct 2022 20:38:01 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

Cursitor Doom wrote:
Hi all,

So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel). I opened the case and found
two RIFA caps that had obviously blown, so replaced them, but it\'s
still blowing fuses. I can\'t be 100% certain, but I\'m pretty sure
there\'s a fault with the PSU board as I get the briefest whiff of
burning from that area each time I put a new fuse in and power up. The
new fuse blows again before I get to see any smoke coming from
anywhere at all. I\'m linking to a diagram of the PSU as I\'m not
familiar with this circuit topology. If anyone can spot a component
here that could have gone low-resistance and might be a good candidate
for out-of-circuit testing, by all means let me know.
Thanks,

CD.


https://disk.yandex.com/i/UhxATIy-6Y0LIA


Yikes. When a fuse blows, you replace it *once*. If it blows again, it
ain\'t the fuse or a line transient.

Yeah, well twice in this case. Replacing the RIFAs *usually* does the
trick (I have to say, in a half-arsed defence of my appalling
conduct).

Start with a DMM and check for shorted diodes in the input bridge and
then for a shorted filter cap at its output. (The schematic is more or
less completely illegible, or I\'d give the ref des.)

Not sure what you\'re viewing it with, Phil. Others don\'t seem to have
that problem and have been able to see the part numbers okay.

Next most likely is a switch FET.

Yes, the chopper is always a prime suspect with SMPSs

Cheers

Phil Hobbs
(proud owner of a 2467 that he almost never uses)

 

[Cursitor Doom]

On Mon, 24 Oct 2022 16:52:59 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Mon, 24 Oct 2022 22:17:57 +0100, Cursitor Doom <cd@notformail.com
wrote:

I\'m no tech repairer; haven\'t got the necessary fine motor
skills, I\'m afraid.

Not a problem. Let a hand soldering robot do the precision work.
Something like this soldering robot:
https://www.promationusa.com/9101
More:
https://www.promationusa.com/robotic-soldering-glance
and even more:
https://www.google.com/search?q=hand+soldering+robot&tbm=isch

Maybe one day I can get myself one of those, Jeff! :-D

 

[legg]

On Mon, 24 Oct 2022 10:35:23 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Mon, 24 Oct 2022 09:46:18 -0400, legg <legg@nospam.magma.ca> wrote:

On Sun, 23 Oct 2022 17:13:54 -0700, Jeff Liebermann <jeffl@cruzio.com
wrote:

On Sun, 23 Oct 2022 16:46:49 -0700 (PDT), Phil Allison
pallison49@gmail.com> wrote:

Cursitor Doom wrote:
---------------------------------
And if those
caps fail open circuit (which they both did despite being X2 safety
ones which are supposed to fail short)

** Really ? Where did you get that idea?

Plenty are found open or with low values after long times in service.
X2 dropping caps used in smoke alarms regularly fail open.

But those Rifa ones love to smoke and burn.

.......Phil

RIFA caps smoke, especially the old ones. This is typical for vintage
Tektronix scopes and RIFA caps:

\"Old RIFA capacitors and a disaster story\"
https://www.eevblog.com/forum/chat/old-rifa-capacitors-and-a-disaster-story/

YouTube videos featuring smoking RIFA caps:
https://www.youtube.com/results?search_query=rifa+capacitor

I sometimes test the HV leakage in capacitors using an ancient Megger:
http://www.learnbydestroying.com/jeffl/pics/Megger/Megger-Major.jpg
https://www.vintage-radio.net/forum/showthread.php?t=73265
Unsolder the capacitor for testing. You really don\'t want a high
voltage power source connected to your oscilloscope.

The smoking caps were metalized paper construction. These were good
enough in their day, with a demonstrated mtbf and safe self-healing,
while seals lasted. The outer sealing body tended to shrink and crack
with age, allowing ingress of air and moisture, compromising
performance. They are no longer made.

When I test RIFA capacitors with the Megger, I sometimes see
capacitors the show some leakage for maybe 250 msec and then recover.
I\'ve been assuming that this demonstrates that self healing is
function. Known good capacitors don\'t do that. Since testing
required removing the capacitor from the PCB, I just replace any
capacitor that shows temporary leakage. (i.e. better safe than
sorry).

This video shows what might be two self healing RIFA capacitors in
action. Watch the current on the display:
\"40 Year Old Rifa Capacitors High Voltage Leakage Testing\"
https://www.youtube.com/watch?v=m1nDrunJ1SE> (3:09)
The effect is more visible with a panel meter than with a digital
readout (except when the meter is moving because I\'m turning the crank
on the Megger).

The video shows testing of two Y caps. These were originally
factory tested at 1500VAC/2700VDC, and have no reputation
for embarrassing field failures after 20+yrs.

X2 caps are desiged for \'impedance-limited\' applications and
are tested at around 1KVAC/1500VDC.

Metalized film parts are usually less dramatic in self-clearing
of internal faults, but in extreme cases, you still get thermal
runaway, as the faulting area expands and eats away at succesive
layers of the wrapped structure.

I haven\'t bothered to perform a post mortem on a RIFA capacitor. I
have several suitable microscopes and would expect to find holed in
the dielectric (paper or plastic) and the metalization. The RIVA caps
that failed have warped, cracked, melted, or crumbled outer plastic
cases. For meaningful microscopy, I would need an old RIFA capacitor
in an intact case.

Case degradation is a significant factor in aging of early production.
Prior to single-board assembly mfring philosophy, these parts were
shipped with long leads, loose, in bulk containers, knocking heads
all the way. They were then subjected to short and long term lead
stress in manual lead dressing and final physical attachment.

Rifa was first to use that distinctive clear lacquer. I\'m not sure
how many times that formula was \'improved\', but parts would arrive
with visible cracks and would crumble around lead wires during
lead-forming.

That being said, there were sufficient \'incidents\' in the first
20yrs of use to ensure that nobody in their right minds would
use them in new product, even in a list of alternates, in the
last 25 years.

TEK scopes that employ these parts are antiques, maintained by
enthusiasts, who will tell you all about the effects of heat,
humidity and just plain age on plastics, epoxies and other
components/materials used in them.

RL

 

[legg]

On Mon, 24 Oct 2022 17:21:00 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

legg wrote:
------------------

Cursitor Doom wrote:
---------------------------------
And if those
caps fail open circuit (which they both did despite being X2 safety
ones which are supposed to fail short)

** Really ? Where did you get that idea?

Plenty are found open or with low values after long times in service.
X2 dropping caps used in smoke alarms regularly fail open.

But those Rifa ones love to smoke and burn.

==================================================

The smoking caps were metalized paper construction. These were good
enough in their day, with a demonstrated mtbf and safe self-healing,
while seals lasted. The outer sealing body tended to shrink and crack
with age, allowing ingress of air and moisture, compromising
performance. They are no longer made.

** Rifa PME271 X2 caps are still made and widely available.
Strangely they are also the most expensive X2 caps on offer for no good reason.
Described as being of \"metalised , impregnated paper \" construction.
Rifa also claim to be the only supplier .

Phil,

You will not be able to purchase the \'exact\' replacement, as the
series number no longer has the same operating voltage markings,
safety approval listing, pert numbering or material construction.

After all the merging and take-overs are done, it\'s unlikely
to be manufactured at the same physical plant, or using
similar equipment or materials, though safety docs used to be
pretty inflexible in that regard - one of the main trade effects
of VDE/IEC safety regime.

Costs reflect a low volume mfring environment due to buyer
reluctance to bother with \'improved\' parts.

RL

Other makers, dozens of them, use metalised polyester or polypropylene films as used in DC rated caps - but with one big difference.
X2 types are double wound = a technique that creates two caps in series.

Such caps are immune from *corona discharge* occurring in tiny air pockets trapped inside the cap as it it being wound.
They are still vulnerable to spike voltages of a few kV which eat away at metalisation and reduce cap value over time.

IME Rifa X2s are notorious for catching fire and filling the room with acrid smoke.
Happened twice, right in front of me, first case was a room heater and the second with a portable TV.

When used for suppression in triac dimming equipment, they invariably fail early too.
Bad news.

..... Phil

 

[Jeff Liebermann]

On Mon, 24 Oct 2022 20:38:01 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

(The schematic is more or
less completely illegible, or I\'d give the ref des.)

https://disk.yandex.com/i/UhxATIy-6Y0LIA
The Yandex viewer is blurry, even when viewed as \"original size\".

Try downloading the PNG file and viewing it with a better viewer at
100% magnification. The details are quite readable in Irfanview
(Windoze) and Nomacs (Linux). In Irfanview, the image can be
\"Sharpened\" by hitting \"S\" on the keyboard.

--
Jeff Liebermann jeffl@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

 

[three_jeeps]

On Monday, October 24, 2022 at 2:12:12 PM UTC-4, Peter W. wrote:

OK.
What is known:

a) Scope is blowing mains fuses.
b) RIFA -branded caps blew up.
c) The power-supply board was affected.

Suggestion:
1) Remove the power-supply board.
2) Replace all, repeat: ALL the capacitors on that board of any nature, type or style with the correct (but modern) replacement type.
3) Check each other component on the board. Diodes, regulators, transistors, whatever.
4) Replace any that are either bad or the slightest bit questionable.
5) Replace board - carefully (now might be an opportunity for a DBT to \'do its thing\' - but a metered Variac would be better).

I kinda-sorta think that will do it. KISS and all that.
Peter Wieck
Melrose Park, PA

Ditto.....I was going to suggest this but some ppl want to find *the* problem. For *vintage* stuff, it makes no sense to me, Replace all the caps, check all resistors. Having a solid power supply is critical for the rest of the unit to work properly, so why screw around with replacing only one bad component? As long as you got it apart.....Do you really want to revisit this later when another cap dies?

 

[Cursitor Doom]

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

 

[Jeff Liebermann]

On Tue, 25 Oct 2022 19:33:00 +0100, Cursitor Doom <cd@notformail.com>
wrote:

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

I\'ve done worse. I don\'t like the exposed hot wires, but since you\'re
the only person likely to be working on the scope, it\'s probably an
acceptable risk.

I suggest you tack the capacitors to the PCB with a little REMOVABLE
glue, such as hot melt glue or non-acetic-acid RTV.

--
Jeff Liebermann jeffl@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

 

[JC]

Cursitor Doom wrote:

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

Probably won\'t fit back in the scope cos of the IEC filter.
Tons of info out there on this power supply and how to fix:

https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/150/

 

[Cursitor Doom]

On Tue, 25 Oct 2022 12:42:51 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Tue, 25 Oct 2022 19:33:00 +0100, Cursitor Doom <cd@notformail.com
wrote:

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

I\'ve done worse. I don\'t like the exposed hot wires, but since you\'re
the only person likely to be working on the scope, it\'s probably an
acceptable risk.

Thanks, Jeff, I thought you\'d tear me a new arsehole!

I suggest you tack the capacitors to the PCB with a little REMOVABLE
glue, such as hot melt glue or non-acetic-acid RTV.

In an ideal world and all that. Sadly, we both know that\'s not going
to happen, Jeff. :-(

 

[Cursitor Doom]

On Tue, 25 Oct 2022 15:43:14 -0400, JC <Chipbee40_SpamNo@yahoo.com>
wrote:

Cursitor Doom wrote:

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

Probably won\'t fit back in the scope cos of the IEC filter.

It does actually fit back in again - *just* about.

Tons of info out there on this power supply and how to fix:

https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/150/

Many thanks for that indeed.

 

[Cursitor Doom]

On Tue, 25 Oct 2022 09:36:35 -0400, legg <legg@nospam.magma.ca> wrote:

The video shows testing of two Y caps. These were originally
factory tested at 1500VAC/2700VDC, and have no reputation
for embarrassing field failures after 20+yrs.

X2 caps are desiged for \'impedance-limited\' applications and
are tested at around 1KVAC/1500VDC.

Do you know why these caps usually have their self-resonant frequency
marked on the side of them? That\'s always puzzled me.

 

[three_jeeps]

On Saturday, October 22, 2022 at 2:34:59 PM UTC-4, Cursitor Doom wrote:

On Sat, 22 Oct 2022 08:08:25 -0700 (PDT), \"Peter W.\"
peterw...@gmail.com> wrote:

Did the caps fail Open or Short?

Peter Wieck
Melrose Park, P
These are X2 caps and supposed to fail short. However, for whatever
reason, they both failed open.

Fail short...really? Why would you want a cap that is across the supply line to fail short? Just to make sure the fuse blows?
It is my understanding that X and Y capacitors experience line spikes, and will endure multiple small breakdowns over the years as a result of this stress. They fail open because local heating caused by a microscopic dielectric breakdown results in polypropylene melting, which seals off the \"edge\" of the breakdown.
Is this not correct??

 

[Cursitor Doom]

On Tue, 25 Oct 2022 14:19:38 -0700 (PDT), three_jeeps
<jjhudak@gmail.com> wrote:

On Saturday, October 22, 2022 at 2:34:59 PM UTC-4, Cursitor Doom wrote:
On Sat, 22 Oct 2022 08:08:25 -0700 (PDT), \"Peter W.\"
peterw...@gmail.com> wrote:

Did the caps fail Open or Short?

Peter Wieck
Melrose Park, P
These are X2 caps and supposed to fail short. However, for whatever
reason, they both failed open.

Fail short...really? Why would you want a cap that is across the supply line to fail short? Just to make sure the fuse blows?
It is my understanding that X and Y capacitors experience line spikes, and will endure multiple small breakdowns over the years as a result of this stress. They fail open because local heating caused by a microscopic dielectric breakdown results in polypropylene melting, which seals off the \"edge\" of the breakdown.
Is this not correct??

No idea if that\'s the failure mechanism or not. However, if you read
my orginal remark, I did say they are *supposed* to fail short, which
makes pretty clear that was the intention of the manufacturer even if
they were not successful in many instances.

 

[Rayner Lucas]

In article <4696384c-8b39-4bae-a924-81096d9b3da0n@googlegroups.com>,
pallison49@gmail.com says...

** Rifa PME271 X2 caps are still made and widely available.
Strangely they are also the most expensive X2 caps on offer for no
good reason.

I got a handful of brand new PME271 Y caps a few years ago, for reasons
that I\'m sure can\'t possibly have been good ones. All of them have
developed a couple of small cracks in the casing, just from sitting
unused in a parts box.

So if anyone was wondering if they\'d fixed the defects in the design at
any point in the last 40+ years, apparently not.

IME Rifa X2s are notorious for catching fire and filling the room with
acrid smoke. Happened twice, right in front of me, first case was a
room heater and the second with a portable TV.

Yep, they\'re a menace all right. These days I pre-emptively check any
mains-powered electronics from the 80s and 90s for them, which
thankfully meant I got them out of my Tek 2445A before they popped. Wish
I\'d been more alert with the 2235A; I spent ages cleaning sticky brown
crud out of the crevices of that one.

And then there was the HP spectrum analyser where I thought I\'d saved
myself a heap of trouble by extracting all eight(!) Rifa capacitors from
its power supply. Plugged it in, and half an hour later a thick cloud of
truly foul phenolic smoke poured out. Opened it up to find that it had
blown part of the case off its power inlet. Turns out that Schaffner
filtered power inlets can *also* contain Rifa (or very similar) caps,
concealed inside a metal casing and potting compound just to add extra
force and noxious smells to the eventual and inevitable explosion.

I\'m pretty sure anyone who works on older electronics eventually
develops their very own rant about these accursed objects.

Rayner

 

[legg]

On Tue, 25 Oct 2022 19:33:00 +0100, Cursitor Doom <cd@notformail.com>
wrote:

Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

You could probably have leadformed the new parts to suit.

My guess is you just prefered to solder from the comp
side.

RL

 

[Cursitor Doom]

On Wed, 26 Oct 2022 08:34:06 -0400, legg <legg@nospam.magma.ca> wrote:

On Tue, 25 Oct 2022 19:33:00 +0100, Cursitor Doom <cd@notformail.com
wrote:


Here we go, gentlemen, the finished \"repair\" on the PSU board:

Before (the faulty RIFAs are arrowed since they\'re a bit hard to spot)
https://disk.yandex.com/i/9VID9SDCNLoxQw

After (the new replacements are not arrowed, since they\'re blindingly
obvious)
https://disk.yandex.com/i/uHJW9u33lWACVQ


It\'s just awful, isn\'t it?

You could probably have leadformed the new parts to suit.

My guess is you just prefered to solder from the comp
side.

RL

It\'s really not my thing. When people say, \"Just recap anything
suspect\" and there might be fewer than half a dozen to do, the
prospect still fills me with horror. If they only knew how ironic the
\"Just\" bit strikes me in suggestions like that.