Leaky Electolytics in Philips 'scope

[Cursitor Doom]

Hi all,

I've just got around to troubleshooting this old Philips scope that was
given to me by some chap who was moving abroad. It's the PM3264 model and
appears to be a well-constructed four channel job. It's blowing the 2A
fuses that are contained in the fuse-holder at the rear of the
instrument, so something's gone low-res. I can't see any obvious signs of
anything having burned-out on any of the boards, but a lot of the
electroytics - most of them in fact, are testing as leaky in-circuit
according to my ESR meter and before I go any further I was wondering if
that fact alone could account for the blowing fuses?

Thanks,
cd.

 

[Cursitor Doom]

On Sun, 22 Mar 2015 14:19:14 +0000, Cursitor Doom wrote:

I should have said the caps I refer to are the small, low voltage ones on
the signal boards, NOT the large ones in the PSU section.

 

[Cursitor Doom]

On Sun, 22 Mar 2015 14:46:03 +0000, N_Cook wrote:

On 22/03/2015 14:30, Cursitor Doom wrote:
On Sun, 22 Mar 2015 14:19:14 +0000, Cursitor Doom wrote:

I should have said the caps I refer to are the small, low voltage ones
on the signal boards, NOT the large ones in the PSU section.


Unlikely caps problem. Hard (ie black stained)quickly blown fuses or
slowish soft blown?
How do the fuses fare if you try running off a variac at say 60 percent
mains?

T-rated fuses. In fact I was running off a Variac when the replacement
fuse blew at only about 80V!
I've got one of the PSU covers off now and have identified a suspect
component. It's some sort of capacitor with the following markings:

250V, 0.22uF@X fo=3,00Mhz

and it has a fine web of cracks over its encapsulation. I had one of
these go short-circuit once before in a microscope power supply and that
was the cause of it blowing fuses too.
I don't understand the markings on the component, though. IIRC, "fo"
refers to the self-resonant frequency and I don't see what purpose there
would be in that at 50hz mains where it's located in that part of the
device pre-PSU!

 

[Cursitor Doom]

On Sun, 22 Mar 2015 16:36:05 +0000, Gareth Magennis wrote:

"Cursitor Doom" wrote in message news:memj12$tl2$1@dont-email.me...

Hi all,

I've just got around to troubleshooting this old Philips scope that was
given to me by some chap who was moving abroad. It's the PM3264 model
and appears to be a well-constructed four channel job. It's blowing the
2A fuses that are contained in the fuse-holder at the rear of the
instrument, so something's gone low-res. I can't see any obvious signs
of anything having burned-out on any of the boards, but a lot of the
electroytics - most of them in fact, are testing as leaky in-circuit
according to my ESR meter and before I go any further I was wondering if
that fact alone could account for the blowing fuses?

Thanks,
cd.



Doesn't "Leaky - in circuit" mean that either the capacitor is leaky, OR
the circuitry around it is discharging it whilst the meter is trying to
measure it, and thus it can't be measured whilst in circuit, and it
might be perfectly OK?

I suspect you are correct. This is a brand new meter I'm entirely
unfamiliar with, first time I've used it, and the readout messages aren't
terribly grammatical. But I've moved on now and narrowed the fault down
to the pre-PSU area.
There's one of those aluminium cased mains filters next inline down from
the 2A fuse compartment and it's measuring only 1ohm across it! However,
measuring the resistance looking into the scope circuitry after
disconnecting this results in an open circuit reading, so replacing the
filter alone will not fix the problem. Probably the filter just blew and
took out something further down the line. More investigation required....

 

[N_Cook]

On 22/03/2015 14:30, Cursitor Doom wrote:

On Sun, 22 Mar 2015 14:19:14 +0000, Cursitor Doom wrote:

I should have said the caps I refer to are the small, low voltage ones on
the signal boards, NOT the large ones in the PSU section.

Unlikely caps problem. Hard (ie black stained)quickly blown fuses or
slowish soft blown?
How do the fuses fare if you try running off a variac at say 60 percent
mains?

 

[Gareth Magennis]

"Cursitor Doom" wrote in message news:memj12$tl2$1@dont-email.me...

Hi all,

I've just got around to troubleshooting this old Philips scope that was
given to me by some chap who was moving abroad. It's the PM3264 model and
appears to be a well-constructed four channel job. It's blowing the 2A
fuses that are contained in the fuse-holder at the rear of the
instrument, so something's gone low-res. I can't see any obvious signs of
anything having burned-out on any of the boards, but a lot of the
electroytics - most of them in fact, are testing as leaky in-circuit
according to my ESR meter and before I go any further I was wondering if
that fact alone could account for the blowing fuses?

Thanks,
cd.



Doesn't "Leaky - in circuit" mean that either the capacitor is leaky, OR the
circuitry around it is discharging it whilst the meter is trying to measure
it, and thus it can't be measured whilst in circuit, and it might be
perfectly OK?



Gareth.

 

[Reinhard Zwirner]

Cursitor Doom schrieb:

[...]

250V, 0.22uF@X fo=3,00Mhz

and it has a fine web of cracks over its encapsulation. I had one of
these go short-circuit once before in a microscope power supply and that
was the cause of it blowing fuses too.
I don't understand the markings on the component, though. IIRC, "fo"
refers to the self-resonant frequency and I don't see what purpose there
would be in that at 50hz mains where it's located in that part of the
device pre-PSU!

Search for "X capacitor", e. g.

here:
<http://www.okaya.com/products/noise-surge/capacitors/capacitors-faq/>

HTH

Reinhard

 

[Cursitor Doom]

On Sun, 22 Mar 2015 18:17:28 +0100, Reinhard Zwirner wrote:

Cursitor Doom schrieb:

[...]
250V, 0.22uF@X fo=3,00Mhz

and it has a fine web of cracks over its encapsulation. I had one of
these go short-circuit once before in a microscope power supply and
that was the cause of it blowing fuses too.
I don't understand the markings on the component, though. IIRC, "fo"
refers to the self-resonant frequency and I don't see what purpose
there would be in that at 50hz mains where it's located in that part of
the device pre-PSU!

Search for "X capacitor", e. g.

here:
http://www.okaya.com/products/noise-surge/capacitors/capacitors-faq/

HTH

Reinhard

Thanks for that. Interesting, but it still doesn't answer what the
relevance of the "fo=3Mhz" parameter is. Neither does the Wiki page on
the subject!

 

[mike]

On 3/22/2015 8:01 AM, Cursitor Doom wrote:

On Sun, 22 Mar 2015 14:46:03 +0000, N_Cook wrote:

On 22/03/2015 14:30, Cursitor Doom wrote:
On Sun, 22 Mar 2015 14:19:14 +0000, Cursitor Doom wrote:

I should have said the caps I refer to are the small, low voltage ones
on the signal boards, NOT the large ones in the PSU section.


Unlikely caps problem. Hard (ie black stained)quickly blown fuses or
slowish soft blown?
How do the fuses fare if you try running off a variac at say 60 percent
mains?

T-rated fuses. In fact I was running off a Variac when the replacement
fuse blew at only about 80V!

Simple experiment.
80V x 2A = 160W.
run it at 75V and see what melts.

I've got one of the PSU covers off now and have identified a suspect
component. It's some sort of capacitor with the following markings:

250V, 0.22uF@X fo=3,00Mhz

and it has a fine web of cracks over its encapsulation. I had one of
these go short-circuit once before in a microscope power supply and that
was the cause of it blowing fuses too.

Depending on the circuit, remove it and see if the fuse still blows.

I don't understand the markings on the component, though. IIRC, "fo"
refers to the self-resonant frequency and I don't see what purpose there
would be in that at 50hz mains where it's located in that part of the
device pre-PSU!

 

[Cursitor Doom]

On Sun, 22 Mar 2015 16:43:12 -0700, mike wrote:

Had a laptop I couldn't fix because the power supply wouldn't stay up.
had to put a DC supply downstream of the inductors and stuffed an amp
into the board. Took a millivoltmeter to trace the short to a SMT cap
under a heat sink. Was really shorted, didn't get warm at all.

Interesting point there I'd not considered before: if something's
*completely* shorted-out, then it won't get warm!

 

[Phil Allison]

Cursitor Doom wrote:

Interesting, but it still doesn't answer what the
relevance of the "fo=3Mhz" parameter is.

** fo is the *series* resonant frequency of the cap.

Such caps across the AC supply act as an EMI filter so their high frequency characteristics are important.

Above 3MHz, the impedance of that cap will begin rising and its effectiveness lost. A lower value cap will have a higher resonant frequency.

Allow about 15nH inductance and you will easily see how he game works.


.... Phil

 

[Ralph Mowery]

"mike" <ham789@netzero.net> wrote in message
news:men52i$925$1@dont-email.me...

T-rated fuses. In fact I was running off a Variac when the replacement
fuse blew at only about 80V!

Simple experiment.
80V x 2A = 160W.
run it at 75V and see what melts.

When fuses are blowing, that is not a good idea. You often melt the
transformer or something else in adition to what blows.

Fuses are often rated at somewhat above the normal current draw. With a
very bad overload , they will blow quickly enough, but at just a slight
overload, they will not blow and often the transformer or other components
will also overheat and go bad.


For simple circuits like lighting circuits or where there is lots of big
heavy duty components, I have used a light bulb in place of a fuse. Just
sort of use a bulb that has a low wattage compaired to the normal current
draw.

 

[mike]

On 3/22/2015 3:13 PM, Ralph Mowery wrote:

"mike" <ham789@netzero.net> wrote in message
news:men52i$925$1@dont-email.me...
T-rated fuses. In fact I was running off a Variac when the replacement
fuse blew at only about 80V!

Simple experiment.
80V x 2A = 160W.
run it at 75V and see what melts.

When fuses are blowing, that is not a good idea. You often melt the
transformer or something else in adition to what blows.

Fuses are often rated at somewhat above the normal current draw. With a
very bad overload , they will blow quickly enough, but at just a slight
overload, they will not blow and often the transformer or other components
will also overheat and go bad.


For simple circuits like lighting circuits or where there is lots of big
heavy duty components, I have used a light bulb in place of a fuse. Just
sort of use a bulb that has a low wattage compaired to the normal current
draw.

Well, if that makes you nervous, run it at much lower current.
Only takes a couple of watts to make something warm enough to detect
by touch.

Had a laptop I couldn't fix because the power supply wouldn't stay up.
had to put a DC supply downstream of the inductors and stuffed
an amp into the board. Took a millivoltmeter to trace the short to a
SMT cap under a heat sink. Was really shorted, didn't get warm at all.

 

[mike]

On 3/22/2015 5:18 PM, Cursitor Doom wrote:

On Sun, 22 Mar 2015 16:43:12 -0700, mike wrote:

Had a laptop I couldn't fix because the power supply wouldn't stay up.
had to put a DC supply downstream of the inductors and stuffed an amp
into the board. Took a millivoltmeter to trace the short to a SMT cap
under a heat sink. Was really shorted, didn't get warm at all.

Interesting point there I'd not considered before: if something's
*completely* shorted-out, then it won't get warm!

Yep, in this case, I had to move the injection point twice
because the isolation inductors in the path were getting
too hot for comfort.

That's one of the problems fixing the old Tektronix scopes.
Those tantalum caps are REALLY shorted. Of course, the one
that fails most often requires removing the backplane just to get
at something to test.

 

[Arfa Daily]

"Cursitor Doom" <curd@notformail.com> wrote in message
news:memrl1$tl2$4@dont-email.me...

On Sun, 22 Mar 2015 16:36:05 +0000, Gareth Magennis wrote:

"Cursitor Doom" wrote in message news:memj12$tl2$1@dont-email.me...

Hi all,

I've just got around to troubleshooting this old Philips scope that was
given to me by some chap who was moving abroad. It's the PM3264 model
and appears to be a well-constructed four channel job. It's blowing the
2A fuses that are contained in the fuse-holder at the rear of the
instrument, so something's gone low-res. I can't see any obvious signs
of anything having burned-out on any of the boards, but a lot of the
electroytics - most of them in fact, are testing as leaky in-circuit
according to my ESR meter and before I go any further I was wondering if
that fact alone could account for the blowing fuses?

Thanks,
cd.



Doesn't "Leaky - in circuit" mean that either the capacitor is leaky, OR
the circuitry around it is discharging it whilst the meter is trying to
measure it, and thus it can't be measured whilst in circuit, and it
might be perfectly OK?

I suspect you are correct. This is a brand new meter I'm entirely
unfamiliar with, first time I've used it, and the readout messages aren't
terribly grammatical. But I've moved on now and narrowed the fault down
to the pre-PSU area.
There's one of those aluminium cased mains filters next inline down from
the 2A fuse compartment and it's measuring only 1ohm across it! However,
measuring the resistance looking into the scope circuitry after
disconnecting this results in an open circuit reading, so replacing the
filter alone will not fix the problem. Probably the filter just blew and
took out something further down the line. More investigation required....

Do you actually have the schematics for this unit ? If so, you will see that
there is a cap directly between line and neutral within the filter section,
and it is almost certainly this that has failed if you read one ohm across
those two legs. The power supply itself is a switch mode type, so downstream
of the filter - "looking into the scope circuitry" as you say - an open
circuit or thereabouts is exactly what you should expect to see ...

I would suggest that you do as someone else said, and apply some power, via
a variac, directly to the power input after the filter, and with the filter
disconnected.

Arfa

 

[John-Del]

On Sunday, March 22, 2015 at 12:47:18 PM UTC-4, Cursitor Doom wrote:

There's one of those aluminium cased mains filters next inline down from
the 2A fuse compartment and it's measuring only 1ohm across it! However,
measuring the resistance looking into the scope circuitry after
disconnecting this results in an open circuit reading, so replacing the
filter alone will not fix the problem. Probably the filter just blew and
took out something further down the line. More investigation required....

Taking a resistance measurement into a circuit can lead to abnormally low or high resistance readings depending on the dmm used, and sometimes you can recheck them and get totally different readings. I would try feeding fused AC into it past the disconnected filter and see what happens.

 

[Cursitor Doom]

On Mon, 23 Mar 2015 01:14:32 +0000, Arfa Daily wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:memrl1$tl2$4@dont-email.me...
On Sun, 22 Mar 2015 16:36:05 +0000, Gareth Magennis wrote:

"Cursitor Doom" wrote in message news:memj12$tl2$1@dont-email.me...

Hi all,

I've just got around to troubleshooting this old Philips scope that
was given to me by some chap who was moving abroad. It's the PM3264
model and appears to be a well-constructed four channel job. It's
blowing the 2A fuses that are contained in the fuse-holder at the rear
of the instrument, so something's gone low-res. I can't see any
obvious signs of anything having burned-out on any of the boards, but
a lot of the electroytics - most of them in fact, are testing as leaky
in-circuit according to my ESR meter and before I go any further I was
wondering if that fact alone could account for the blowing fuses?

Thanks,
cd.



Doesn't "Leaky - in circuit" mean that either the capacitor is leaky,
OR the circuitry around it is discharging it whilst the meter is
trying to measure it, and thus it can't be measured whilst in circuit,
and it might be perfectly OK?

I suspect you are correct. This is a brand new meter I'm entirely
unfamiliar with, first time I've used it, and the readout messages
aren't terribly grammatical. But I've moved on now and narrowed the
fault down to the pre-PSU area.
There's one of those aluminium cased mains filters next inline down
from the 2A fuse compartment and it's measuring only 1ohm across it!
However, measuring the resistance looking into the scope circuitry
after disconnecting this results in an open circuit reading, so
replacing the filter alone will not fix the problem. Probably the
filter just blew and took out something further down the line. More
investigation required....



Do you actually have the schematics for this unit ? If so, you will see
that there is a cap directly between line and neutral within the filter
section, and it is almost certainly this that has failed if you read one
ohm across those two legs. The power supply itself is a switch mode
type, so downstream of the filter - "looking into the scope circuitry"
as you say - an open circuit or thereabouts is exactly what you should
expect to see ...

Do'h! You're right of course; a massive oversight on my part. I don't
have any schematics as yet; I've tried searching and the only hit I got
was some French forum where you have to join to get anything. I'm
considering it...

I would suggest that you do as someone else said, and apply some power,
via a variac, directly to the power input after the filter, and with the
filter disconnected.

Excellent suggestion - but with a fuse in line too, naturally. I'll give
it a go and report back later.

 

[Cursitor Doom]

On Sun, 22 Mar 2015 17:55:27 -0700, Phil Allison wrote:

Cursitor Doom wrote:



Interesting, but it still doesn't answer what the relevance of the
"fo=3Mhz" parameter is.


** fo is the *series* resonant frequency of the cap.

Such caps across the AC supply act as an EMI filter so their high
frequency characteristics are important.

Above 3MHz, the impedance of that cap will begin rising and its
effectiveness lost. A lower value cap will have a higher resonant
frequency.

Allow about 15nH inductance and you will easily see how he game works.


... Phil

Thanks for that. I'll try simulating it later. And this is for what
purpose exactly? To prevent noise on the mains from other appliances
getting into the scope, or to prevent noise from the scope getting into
the mains?

 

[Cursitor Doom]

On Mon, 23 Mar 2015 01:14:32 +0000, Arfa Daily wrote:

"Cursitor Doom" <curd@notformail.com> wrote in message
news:memrl1$tl2$4@dont-email.me...
On Sun, 22 Mar 2015 16:36:05 +0000, Gareth Magennis wrote:

"Cursitor Doom" wrote in message news:memj12$tl2$1@dont-email.me...

Hi all,

I've just got around to troubleshooting this old Philips scope that
was given to me by some chap who was moving abroad. It's the PM3264
model and appears to be a well-constructed four channel job. It's
blowing the 2A fuses that are contained in the fuse-holder at the rear
of the instrument, so something's gone low-res. I can't see any
obvious signs of anything having burned-out on any of the boards, but
a lot of the electroytics - most of them in fact, are testing as leaky
in-circuit according to my ESR meter and before I go any further I was
wondering if that fact alone could account for the blowing fuses?

Thanks,
cd.



Doesn't "Leaky - in circuit" mean that either the capacitor is leaky,
OR the circuitry around it is discharging it whilst the meter is
trying to measure it, and thus it can't be measured whilst in circuit,
and it might be perfectly OK?

I suspect you are correct. This is a brand new meter I'm entirely
unfamiliar with, first time I've used it, and the readout messages
aren't terribly grammatical. But I've moved on now and narrowed the
fault down to the pre-PSU area.
There's one of those aluminium cased mains filters next inline down
from the 2A fuse compartment and it's measuring only 1ohm across it!
However, measuring the resistance looking into the scope circuitry
after disconnecting this results in an open circuit reading, so
replacing the filter alone will not fix the problem. Probably the
filter just blew and took out something further down the line. More
investigation required....



Do you actually have the schematics for this unit ? If so, you will see
that there is a cap directly between line and neutral within the filter
section, and it is almost certainly this that has failed if you read one
ohm across those two legs. The power supply itself is a switch mode
type, so downstream of the filter - "looking into the scope circuitry"
as you say - an open circuit or thereabouts is exactly what you should
expect to see ...

Turns out to be 1M ohm or thereabouts now I've remeasured it with a
decent DVM.

I would suggest that you do as someone else said, and apply some power,
via a variac, directly to the power input after the filter, and with the
filter disconnected.

Have now done this and it isn't blowing fuses any more. However, I'm up
to 100V on the variac and no sign of life from the scope yet (230V main
here). Unfortunately the PSU section is sandwiched in the middle of other
boards, so probing for voltages within it is not possible without removal
(which looks a right PITA). I'll have to have a good think about how to
proceed further.

 

[Cursitor Doom]

On Mon, 23 Mar 2015 10:41:27 -0700, DaveC wrote:

How is 115/230 volts mains selected? Is it in the line filter/fuse/IEC
connector housing? Or...?

Switch on the rear panel (yes, it's in the right pos.)

Of course when bypassing the line filter you're inserting your mains
voltage to the proper transformer taps...?

It's a SMPS!