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
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
On Saturday, October 22, 2022 at 10:26:11 AM UTC-4, 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
Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.
Did the caps fail Open or Short?
Peter Wieck
Melrose Park, P
I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.
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
On 10/22/2022 10:26 AM, 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
Thia might help:
https://www.tek.com/en/manual/2465a-and-2467-service-manual
Ed
On Sat, 22 Oct 2022 08:57:06 -0700 (PDT), "ohg...@gmail.com" <ohg...@gmail.com> wrote:
On Saturday, October 22, 2022 at 10:26:11 AM UTC-4, 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
Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.Yes, well, my options are limited due the to the *dumb* design
implementation here. Plus I can't probe the test points or look for
voltage drops when the power supply is constantly interrupted within miliseconds of switch-on. Looks like I'll have to break out my dim
bulb tester (if I can find it).
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,
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,
** Did both fail at the same time?
Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.
.... Phil
I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.
A dim-bulb tester is a reasonable tool for this purpose as a rubber spoon makes a reasonable carving knife. Just not fine enough for any kind of accurate diagnosis.
Peter Wieck
Melrose Park, PA
** Did both fail at the same time?
Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.
.... Phil
Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.
It is already almost a certainty that the issue is in the power-supply. All that rubber spoon will tell you is that the issue is in the power-supply. At which point, you will need to troubleshoot at the component level.
Each time you apply power to what amounts to a dead-short, you may be damaging other components downline. When the second fuse blows, *STOP THERE*. Or the first, if there is any sort of associated symptoms such as heat, smoke, noise or a vaporized fuse.
a) Do you have a VOM? And, if so, how good is it?
b) Not that testing capacitors in-situ generally accurate, but you can usually tell the difference between an open cap and a dead-shorted cap using the Ohms scale.
c) Similarly, transistors.
d) And it is possible that the RIFAs were symptoms, not the cause.
Which would be a start.
Peter Wieck
Melrose Park, PA
On Sun, 23 Oct 2022 04:20:47 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:
st...@swingnn.com wrote:
Phil Allison
-----------------
** Did both fail at the same time?
Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.
.... Phil
Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.
** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?
It sure as hell ain't smoothing...
....Phil
I was pointing out a common misunderstanding with multiple capacitors.
Rifa are alone, in parallel or in series and can all fail at the same
time as can smoothing capacitors in parallel.
st...@swingnn.com wrote:
Phil Allison
-----------------
** Did both fail at the same time?
Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.
.... Phil
Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.
** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?
It sure as hell ain't smoothing...
....Phil
If the alternative is landfill, then heroic measures are justified.
If the alternative is 'sending it out', unless you wish to did deeper - and all indications are that you do not - then go ahead and send it out.
There is no magic bullet.
Hi all,
So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel).
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.
On Saturday, October 22, 2022 at 7:26:11 AM UTC-7, 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.
First thing to check is the rectifier; that diode bridge can fail, and
apply AC to the filter capacitors, and... that'll take out the fuse.
Not sure why the RIFA capa goes, though; maybe just a power surge some
time last week?
There seem to be some surge-suppressors (gas discharge?) in the diagram, they might
have failed short in a surge..
On Sat, 22 Oct 2022 15:26:04 +0100, Cursitor Doom <cd@notformail.com>
wrote:
Hi all,
Bah Humbug.
So my main analogue scope, a Tek 2465A, has gone faulty. Keeps blowing
main fuses (the 2A one on the rear panel).
"Oscilloscope Restoration Project - Repair Tektronix 2465B 400MHz" ><https://cdn.hackaday.io/files/1638907011869760/Tektronix-2465A-B-Oscilloscope-Restoration-Repair.pdf>
"Symptoms: This is my lab unit, which suddenly started blowing its
main AC fuse."
"Amazing Tektronix 2465 Repair Information" ><https://jestineyong.com/amazing-tektronix-2465-repair-information/>
Good luck
** Did both fail at the same time?
Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.
.... Phil
Two capacitors in parallel can cause a problem when they both fail. I
had a computer power supply that had four smoothing capacitors in
parallel. It worked with just one so all four had to fail before the
fault was detected.
** So you have no clue what a piar of Rifa 68nF supreso caps do in an off line SMPS ?
It sure as hell ain't smoothing...
I was pointing out a common misunderstanding with multiple capacitors.
Rifa are alone, in parallel or in series and can all fail at the same
time as can smoothing capacitors in parallel.
And if those
caps fail open circuit (which they both did despite being X2 safety
ones which are supposed to fail short)
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
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.
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
On Sun, 23 Oct 2022 16:46:49 -0700 (PDT), Phil Allison
<palli...@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.
.......PhilRIFA 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.
--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558
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.
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.
Film parts will generally fail at the end schoopage joint, where
metalization and film stresses accumulate in mfring.
RL
On Sunday, October 23, 2022 at 8:14:05 PM UTC-4, Jeff Liebermann wrote:
On Sun, 23 Oct 2022 16:46:49 -0700 (PDT), Phil Allison
<palli...@gmail.com> wrote:
Cursitor Doom wrote:RIFA caps smoke, especially the old ones. This is typical for vintage
---------------------------------
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
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.
I know about the RIFA caps exploding - thanks for the interesting video tho. >What is the recommended replacement type? (assuming they can physically fit into the same space on the board)
I have a few vintage Tek scopes that I should do some preventative maintenance work on....
J
On Mon, 24 Oct 2022 10:29:52 -0700 (PDT), three_jeeps
<jjhudak@gmail.com> wrote:
On Sunday, October 23, 2022 at 8:14:05 PM UTC-4, Jeff Liebermann wrote:
On Sun, 23 Oct 2022 16:46:49 -0700 (PDT), Phil Allison
<palli...@gmail.com> wrote:
Cursitor Doom wrote:RIFA caps smoke, especially the old ones. This is typical for vintage
---------------------------------
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
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.
I know about the RIFA caps exploding - thanks for the interesting video tho. >>What is the recommended replacement type? (assuming they can physically fit into the same space on the board)
I have a few vintage Tek scopes that I should do some preventative maintenance work on....
J
"RIFA - Replacement Choices" ><https://www.eevblog.com/forum/beginners/rifa-replacement-choices/>
Kemet bought RIFA. My older Tek scopes survive 30 to 40 years and
usually blow electroltyics before RIFA caps. Rather than experiment,
I simply bought the same value/voltage X2 "safety" caps from Kemet
(via Mouser or Digikey): ><https://www.kemet.com/en/us/capacitors/film/safety.html?40=133>
I wasn't very careful when I ordered replacement caps many years ago.
I think I ordered "metalized paper", which in retrospect, was not the
best choice. Going down the list of dielectrics, only "metalized
paper" and "metalized polypropylene" are available for "safety" caps.
Kemet is recommending their R53 X2 caps with "metalized polypropylene
film" dielectric.
<https://www.mouser.com/new/kemet-electronics/kemet-miniature-emi-suppression-capacitors/>
<https://www.digikey.com/en/product-highlight/k/kemet/r53-film-capacitors>
X2 is for "line to line" filtering while Y2 is for "line to ground" >filtering.
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.
I'm no tech repairer; haven't got the necessary fine motor
skills, I'm afraid.
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
==================================================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.
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>
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)
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).
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.
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
(The schematic is more or
less completely illegible, or I'd give the ref des.)
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
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.
On Sat, 22 Oct 2022 08:08:25 -0700 (PDT), "Peter W."
<peterw...@gmail.com> wrote:
Did the caps fail Open or Short?
Peter WieckThese are X2 caps and supposed to fail short. However, for whatever
Melrose Park, P
reason, they both failed open.
On Saturday, October 22, 2022 at 2:34:59 PM UTC-4, Cursitor Doom wrote:polypropylene melting, which seals off the "edge" of the breakdown.
On Sat, 22 Oct 2022 08:08:25 -0700 (PDT), "Peter W."
<peterw...@gmail.com> wrote:
Did the caps fail Open or Short?These are X2 caps and supposed to fail short. However, for whatever
Peter Wieck
Melrose Park, P
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
Is this not correct??
** 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.
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.
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