ESR METERS

[Phil Allison]

Cursitor Doom wrote:

Be aware that the ESR meters intended for service work are only good
for electrolytic caps ( regular aluminium and tantalum ) and
interpreting the readings correctly requires significant operator
knowledge and/or a similar electro to compare with.

...Or a table of expected values vs. voltage rating vs. capacitance -
which comes with the Peak meter.


** IME, such tables are far too vague and out of date for modern electros.

The goal in servicing & refurbishing work is to find electro caps that
are *on the way out* as well as obviously dead ones.

If in doubt, comparison with a the closest new electro available is a
good move.

Well maybe. But it'll slow the process down massively as it's not just
the capacitance that causes ESR values to change. As you well know.

** See page 9 of the Peak ESR meter user's guide.

http://www.peakelec.co.uk/resources/esr60_userguide_en.pdf

The ESR values shown in the table up to 470uF are several times higher than normally found with new caps of the same ratings.

When servicing, the task is to identify electros whose ESR has risen significantly, indicating end of useful life. Cos most of the electrolyte that was there when new has escaped and evaporated.



..... Phil

 

[Ralph Mowery]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:mnqp40$9qv$1@dont-email.me...

The cheapo fleabay one, I could not find the pk-pk voltage in ESR mode, in
the specs. Do you know what it is? I consider that to be a vital
requirement as most of my testing is in-circuit , unlike the shown
examples of isoltated componenent testing. Must be no more than .2V and
ideally .1V, so as not to turn on anything active, in circuit

The fleabay one uses the same basic circuit design as the Peak meter. Peak
is screwing the people as they make one for non simiconductor devices and
another one for simiconductors. Whe ebay one does both and the origional
software for them does both using the same circuits.

I don't know the specs for it, but I did put it on a analog scope and from
what I can tell when I hook a capacitor across it there is a large voltag
pulse around 4 votls or so and then a low voltage pulse of about .2 volts.
Both going +and -.

 

[Cursitor Doom]

On Sat, 11 Jul 2015 22:06:30 -0700, Phil Allison wrote:

Cursitor Doom wrote:


Be aware that the ESR meters intended for service work are only
good for electrolytic caps ( regular aluminium and tantalum ) and
interpreting the readings correctly requires significant operator
knowledge and/or a similar electro to compare with.

...Or a table of expected values vs. voltage rating vs. capacitance
-
which comes with the Peak meter.


** IME, such tables are far too vague and out of date for modern
electros.

The goal in servicing & refurbishing work is to find electro caps
that are *on the way out* as well as obviously dead ones.

If in doubt, comparison with a the closest new electro available is a
good move.

Well maybe. But it'll slow the process down massively as it's not just
the capacitance that causes ESR values to change. As you well know.


** See page 9 of the Peak ESR meter user's guide.

http://www.peakelec.co.uk/resources/esr60_userguide_en.pdf

This bit makes no sense to me:

"To allow the self-protection mechanism to function, always ensure that
the Atlas ESR has completed any analysis before connecting the test
probes to a component." :-/

 

[N_Cook]

On 12/07/2015 03:57, Ralph Mowery wrote:

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:mnqp40$9qv$1@dont-email.me...

The cheapo fleabay one, I could not find the pk-pk voltage in ESR mode, in
the specs. Do you know what it is? I consider that to be a vital
requirement as most of my testing is in-circuit , unlike the shown
examples of isoltated componenent testing. Must be no more than .2V and
ideally .1V, so as not to turn on anything active, in circuit

The fleabay one uses the same basic circuit design as the Peak meter. Peak
is screwing the people as they make one for non simiconductor devices and
another one for simiconductors. Whe ebay one does both and the origional
software for them does both using the same circuits.

I don't know the specs for it, but I did put it on a analog scope and from
what I can tell when I hook a capacitor across it there is a large voltag
pulse around 4 votls or so and then a low voltage pulse of about .2 volts.
Both going +and -.

Thanks for that, I suppose at that price its worth getting as a
spare/second opinioner. My old one is 15KHz sine , or was before
clamping with reversed germanium diodes across the probes, to limit the
applied voltage, but it is good enough for go/no-go of >5uF or so electros.

 

[Mark Justice]

On Fri, 10 Jul 2015 23:18:40 -0400, Ralph Mowery wrote:

"Trevor Wilson" <trevor@SPAMBLOCKrageaudio.com.au> wrote in message
news:d0bahrF11cbU2@mid.individual.net...
**Up until last year, I was using the Bob Parker ESR meter. Excellent
device. Foolproof, cheap and very handy. OTOH, I bought one of these
last year:

http://www.peakelec.co.uk/acatalog/jz_esr70.html

Very, VERY nice instrument. I use it every day. If you budget can
stretch,
it is highly recommended.



If that is too much, you can get about the same thing without a caes
from China for $ 16 including shipping.
There are several on ebay similar to this one. They check almost any
small signal component.

Ebay number

271611840945

www.aliexpress.com
search term: Mega328
$9.99

On your recommendation I'm ordering one.
Allow 3 - 4 weeks delivery.
I've ordered about 30 times from them and all good so far.

 

[M Philbrook]

In article <mntii7$noo$3@dont-email.me>, curd@notformail.com says...

On Sat, 11 Jul 2015 22:06:30 -0700, Phil Allison wrote:

Cursitor Doom wrote:


Be aware that the ESR meters intended for service work are only
good for electrolytic caps ( regular aluminium and tantalum ) and
interpreting the readings correctly requires significant operator
knowledge and/or a similar electro to compare with.

...Or a table of expected values vs. voltage rating vs. capacitance
-
which comes with the Peak meter.


** IME, such tables are far too vague and out of date for modern
electros.

The goal in servicing & refurbishing work is to find electro caps
that are *on the way out* as well as obviously dead ones.

If in doubt, comparison with a the closest new electro available is a
good move.

Well maybe. But it'll slow the process down massively as it's not just
the capacitance that causes ESR values to change. As you well know.


** See page 9 of the Peak ESR meter user's guide.

http://www.peakelec.co.uk/resources/esr60_userguide_en.pdf

This bit makes no sense to me:

"To allow the self-protection mechanism to function, always ensure that
the Atlas ESR has completed any analysis before connecting the test
probes to a component." :-/

Why? At least it's providing some sort of internal protection.

Just wait for it to fully boot or put some unknown charged cap on
the leads before switching it on. At least you can help them
to stay in business.

Jamie

 

"To allow the self-protection mechanism to function, always ensure that
the Atlas ESR has completed any analysis before connecting the test
probes to a component." :-/

Why? At least it's providing some sort of internal protection.

Just wait for it to fully boot or put some unknown charged cap on
the leads before switching it on. At least you can help them
to stay in business.

Jamie "

To say the least, this is not enticing me to buy one. One kilohertzz square wave at 400 mV amplitude positive going, with a scope freewheeled into sync and DC coupled. The most effective lytic checker in the world once you lern to read it.

Once you learn to read it. I invented it, I used it to great profit and productivitly and I taught others how to use it, but only where I worked.

Now that I am older and ready to die Imight be tempted to reveal all the details. the bottom line is that tis thing read directly RIGHT NOW, no waiting for some stupid program to run. Like the one they had at opjne job, the son of a bitch sat there and said "DISCHARGING". The fuck you cre if it is charged ? Just read the AC resistance.

Well my scope doodad does exactly that and if the cap is charged then the scope will still read correctly on AC and not iondicate the cap is charged. BTW, does it occur to anyone that IF the cap is charged that it holds a charge and therefore is probably good ? Or are you testing for 0.05 ohms of ESP in the filter caps of an audio amp and giving name to the term "audiophool" ?

It is amazing they do not teach this. you put a signal to a device - the DUT - and then measure what is does. What is not simple about that ? God damn, I figured out this cap checker witht eh scope thing when I was about 16. ON MY OWN. Fuck Dick Smith. Well not really but you know what I mean. In fact I made a trade with Bob Parker. Some knobs for a Ysamaha receiver for an elcheapo boomerang. Hey, it came back a few times which proved it was not just a stick, but then the kids got ahold of it and it was then a stick. Know what I mean ?

 

[Phil Allison]

Ralph Mowery wrote:

That should be done with any piece of equipment. Practice with it and learn
the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the limitations of his test gear ... "

Where I worked there were many wires in a conduit carring 120 volt control
voltage. With a digital meter there was so much induced voltage you could
not tell if a wire was really active or not. You had to put some kind of a
load on them. YOu could take an analog meter and start with a high voltage
range and then switch to a lower range. If the meter stayed in the same or
near the same physical position, it was just showing the induced voltage.
Even with a wire disconnected at each end,it would still shock the crap out
of you due to that voltage.

** Electricians once regularly used 40W bulbs in a protected, hand held fittings to test if circuits were live - see pic.

https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg

The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...


.... Phil

 

[Ralph Mowery]

"Michael Black" <et472@ncf.ca> wrote in message
news:alpine.LNX.2.02.1507110952010.9567@darkstar.example.org...

Surely if one buys an ESR meter, they should be practicing with it.
Check
every electrolytic and tantalum, get a feel for the variation. Don't just
use the meter to "check some electrolytics that look bad" and be done with
it, until the next time you need to use the meter.

That should be done with any piece of equipment. Practice with it and learn
the odd things about them.

That $ 15 component tester will sometimes show odd results of the older
transistors and if you change the leads around it will show up differant.
This is also in some of the documentation of the tester.

Where I worked there were many wires in a conduit carring 120 volt control
voltage. With a digital meter there was so much induced voltage you could
not tell if a wire was really active or not. You had to put some kind of a
load on them. YOu could take an analog meter and start with a high voltage
range and then switch to a lower range. If the meter stayed in the same or
near the same physical position, it was just showing the induced voltage.
Even with a wire disconnected at each end,it would still shock the crap out
of you due to that voltage.

 

[Rheilly Phoull]

"Phil Allison" wrote in message
news:c22de235-2de2-4439-a467-e540492785c6@googlegroups.com...

Ralph Mowery wrote:

That should be done with any piece of equipment. Practice with it and
learn
the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the limitations
of his test gear ... "

Where I worked there were many wires in a conduit carring 120 volt control
voltage. With a digital meter there was so much induced voltage you could
not tell if a wire was really active or not. You had to put some kind of a
load on them. YOu could take an analog meter and start with a high
voltage
range and then switch to a lower range. If the meter stayed in the same
or
near the same physical position, it was just showing the induced voltage.
Even with a wire disconnected at each end,it would still shock the crap
out
of you due to that voltage.

** Electricians once regularly used 40W bulbs in a protected, hand held
fittings to test if circuits were live - see pic.

https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg

The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...


.... Phil

Sure was, then there was the 2 15w pilot lamp bulbs wired in series for the
3 phase stuff (Hi - tech)!!
Along came the "Wiggy" which was more robust,
http://www.ebay.com/bhp/wiggy-voltage-tester
The important part was that the tester was putting a load on the circuit
where neon testers and digital
testers don't.

 

[John G]

Phil Allison has brought this to us :

Ralph Mowery wrote:


That should be done with any piece of equipment. Practice with it and learn
the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the limitations
of his test gear ... "

Where I worked there were many wires in a conduit carring 120 volt control
voltage. With a digital meter there was so much induced voltage you could
not tell if a wire was really active or not. You had to put some kind of a
load on them. YOu could take an analog meter and start with a high voltage
range and then switch to a lower range. If the meter stayed in the same or
near the same physical position, it was just showing the induced voltage.
Even with a wire disconnected at each end,it would still shock the crap out
of you due to that voltage.

** Electricians once regularly used 40W bulbs in a protected, hand held
fittings to test if circuits were live - see pic.

https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg

The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...

... Phil

Some used 2 bulbs in paper bakelite tubes wired in series then they
could test line to line or line to neutral with the same device without
worrying about the voltage or about spurious couplings of mythical
voltages like you can get with meters especially Digital meters.

Once in college Elec Eng Lab, a cotractor wanted to test 3 wires,
rubbed 2 together and closed down whole large building. when asked our
instructor agreed that was not the accepted method. :-?

--
John G Sydney.

 

[N_Cook]

On 13/07/2015 07:22, John G wrote:

Phil Allison has brought this to us :
Ralph Mowery wrote:


That should be done with any piece of equipment. Practice with it
and learn the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the
limitations of his test gear ... "


Where I worked there were many wires in a conduit carring 120 volt
control voltage. With a digital meter there was so much induced
voltage you could not tell if a wire was really active or not. You
had to put some kind of a load on them. YOu could take an analog
meter and start with a high voltage range and then switch to a lower
range. If the meter stayed in the same or near the same physical
position, it was just showing the induced voltage.
Even with a wire disconnected at each end,it would still shock the
crap out of you due to that voltage.


** Electricians once regularly used 40W bulbs in a protected, hand
held fittings to test if circuits were live - see pic.

https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg

The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...


... Phil


Some used 2 bulbs in paper bakelite tubes wired in series then they
could test line to line or line to neutral with the same device without
worrying about the voltage or about spurious couplings of mythical
voltages like you can get with meters especially Digital meters.

Once in college Elec Eng Lab, a cotractor wanted to test 3 wires, rubbed
2 together and closed down whole large building. when asked our
instructor agreed that was not the accepted method. :-?

Is it apochriphal? . I was told that civil engineering subcontractors,
laying new utilities water/sewage/electricity etc in trenches, would
confirm that any power cables they came across that should be dead ,
were dead or not.
Using a cartridge powered cutting blade , that was explosively shot
across the cable, severing it.

 

[jeanyves]

On 2015-07-10 14:42:06 +0000, KenO said:

Hi,

Am new to ESR Meters so have been searching the Internet and Forums for info.

Appreciate comments from others that have been using ESR Meters as to
what features would be included in an "Ideal ESR Meter"

Thanks

Ken

you should see all available on this repository, and then choose to buy
or build one
http://kripton2035.free.fr/esr-repository.html

best regards,
--

Jean-Yves.

 

[Cursitor Doom]

On Sun, 12 Jul 2015 18:41:57 -0700, jurb6006 wrote:

To say the least, this is not enticing me to buy one. One kilohertzz
square wave at 400 mV amplitude positive going, with a scope freewheeled
into sync and DC coupled. The most effective lytic checker in the world
once you lern to read it.

Hmmm. Your device is still going to switch on low barrier height diodes
like Schottkys and whatnot at that voltage level.

 

[Phil Allison]

Cursitor Doom wrote:

One kilohertzz
square wave at 400 mV amplitude positive going,
with a scope freewheeled into sync and DC coupled.
The most effective lytic checker in the world
once you lern to read it.

Hmmm. Your device is still going to switch on low barrier height diodes
like Schottkys and whatnot at that voltage level.

** So you think having an electro connected across the square generator will have NO effect on the output voltage ??

Got any idea of the impedance of a 100uF cap at 1kHz ??




.... Phil

 

[Baron]

Phil Allison prodded the keyboard with:

Ralph Mowery wrote:


That should be done with any piece of equipment. Practice with it
and learn the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the
limitations of his test gear ... "


Where I worked there were many wires in a conduit carring 120 volt
control
voltage. With a digital meter there was so much induced voltage
you could not tell if a wire was really active or not. You had to
put some kind of a
load on them. YOu could take an analog meter and start with a high
voltage
range and then switch to a lower range. If the meter stayed in the
same or near the same physical position, it was just showing the
induced voltage. Even with a wire disconnected at each end,it would
still shock the crap out of you due to that voltage.


** Electricians once regularly used 40W bulbs in a protected, hand
held fittings to test if circuits were live - see pic.


https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg

The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...


... Phil

I have two of those test bulbs ! I forget who made them. But if
there is really voltage there they don't lie.

--
Best Regards:
Baron.

 

[Dimitrij Klingbeil]

On 13.07.2015 08:58, N_Cook wrote:

On 13/07/2015 07:22, John G wrote:
Phil Allison has brought this to us :
Ralph Mowery wrote:


That should be done with any piece of equipment. Practice
with it and learn the odd things about them.

** As Clint Eastwood might have said: " a man's gootta know the
limitations of his test gear ... "


Where I worked there were many wires in a conduit carring 120
volt control voltage. With a digital meter there was so much
induced voltage you could not tell if a wire was really active
or not. You had to put some kind of a load on them. YOu could
take an analog meter and start with a high voltage range and
then switch to a lower range. If the meter stayed in the same
or near the same physical position, it was just showing the
induced voltage. Even with a wire disconnected at each end,it
would still shock the crap out of you due to that voltage.


** Electricians once regularly used 40W bulbs in a protected,
hand held fittings to test if circuits were live - see pic.

https://img1.etsystatic.com/068/0/6320982/il_fullxfull.780718127_nbxf.jpg




The AC plug was replaced with two probes in the examples I saw.

I guess it was important to test the bulb before each use ...


... Phil


Some used 2 bulbs in paper bakelite tubes wired in series then they
could test line to line or line to neutral with the same device
without worrying about the voltage or about spurious couplings of
mythical voltages like you can get with meters especially Digital
meters.

Once in college Elec Eng Lab, a cotractor wanted to test 3 wires,
rubbed 2 together and closed down whole large building. when asked
our instructor agreed that was not the accepted method. :-?


Is it apochriphal? . I was told that civil engineering
subcontractors, laying new utilities water/sewage/electricity etc in
trenches, would confirm that any power cables they came across that
should be dead , were dead or not. Using a cartridge powered cutting
blade , that was explosively shot across the cable, severing it.

Yep, that instrument used to be called a "Kabelbeschußgerät" here in
Germany. Nowadays they fall under the firearms regulations, which made
them a royal pain in the back end to own and operate, so they're
actively being replaced by remote-operated pneumatic equivalents now.

Dimitrij

 

[M Philbrook]

In article <82f2ea2f-dc27-4e51-92c9-6f2c717d2f02@googlegroups.com>,
jurb6006@gmail.com says...

Just wait for it to fully boot or put some unknown charged cap on
the leads before switching it on. At least you can help them
to stay in business.

Jamie "

To say the least, this is not enticing me to buy one. One kilohertzz square wave at 400 mV amplitude positive going, with a scope freewheeled into sync and DC coupled. The most effective lytic checker in the world once you lern to read it.

Once you learn to read it. I invented it, I used it to great profit and productivitly and I taught others how to use it, but only where I worked.

I doubt many much that you invented the process.

That trick dates back to the days of early electronics, very early..

Now that I am older and ready to die Imight be tempted to reveal all the details. the bottom line is that tis thing read directly RIGHT NOW, no waiting for some stupid program to run. Like the one they had at opjne job, the son of a bitch sat there and said "DISCHARGING". The fuck you cre if it is charged ? Just read the AC resistance.

Well my scope doodad does exactly that and if the cap is charged then the scope will still read correctly on AC and not iondicate the cap is charged. BTW, does it occur to anyone that IF the cap is charged that it holds a charge and therefore is probably good ? Or are you testing for 0.05 ohms of ESP in the filter caps of an audio amp and giving name to the term "audiophool" ?

It is amazing they do not teach this. you put a signal to a device - the DUT - and then measure what is does. What is not simple about that ? God damn, I figured out this cap checker witht eh scope thing when I was about 16. ON MY OWN. Fuck Dick Smith. Well not really but you know what I mean. In fact I made a trade with Bob Parker. Some knobs for a Ysamaha receiver for an elcheapo boomerang. Hey, it came back a few times which proved it was not just a stick, but then
the kids got ahold of it and it was then a stick. Know what I mean ?

Back then it may have worked with out worry, most likely due to the
fact that many square wave generators were not able to generate the
raise time like they do today. There ware other things to consider now.

Jamie

 

>"** So you think having an electro connected across the square generator will have NO effect on the output voltage ?? "

I wrote that wrong.

OK, the 1 KHz square wave I generally get from the probe calibrator but not all of them are compatible. They have to be more than 400 mV and less than 360 ohms source impedance, then I modify it to be 400 mV and 360 ohms souirce impedance.

This is wired directly to the vertical input and set for the top and bottom of the square wave to be equidistant from the center of the screen. The DUT shunts it.

You can tell the differenc between low and high value caps. A 100 uF generally results in a flat line if good. If it has ESR you will see some square wave still. Lower value caps are not as easy to read of course because they give you a quarter sine wave. However, remember you are ooking for the vertical parts of the trace. The fast rise and fall time indicates the ESR.

Try it, and get a bunch of caps and try it with resistors in series with the caps. When there is resistance there is a part of the trace with fast rise/fall times. THAT is excatly what you are looking for.

Another thing to remem,ber is that sme caps are more critical than others. I see where people simply change ALL of them in vintage audio equipment and that is very rarely ever needed. they say it sounds better and I do not disagree, but in most cases they would have gotten the same results by changing maybe a half dozen, rather than fifty of them.

I also got to the point where I didn't pay much attention in TVs anymore. My sister's PC monitor went down with someting like a 1,000 uF at 35 volts. I didn't have ANYTHING at the house at the time aand wanted to just see it work. I put a 100 uF at 160 volts in and it worked. I know that is ridiculous and I told her it is liable to fail soon. But I would get the right cap and put it in. So now I have some stock of caps, or electros as you like to call them and here it is like a YEAR later and that SOB still works.

TV, PC mobos and all that, I determine which caps are all in a bank. Alot of them use four, five, all in parallel. You only need ONE. So I put ONE in for test. Id on't even cut the leads, I reuse the same test caps over and over. If it works and I find out the pamel and the Tcon are good, then I proceed with a repair and change them. but I don''t wate them by changing them all and then have a bunch of caps in the drawer with the leads trimmed, like alot of assholes out there. That costs MONEY, and almost all businesses that are still in buisiness here are wo rkjing on a very small margin. You can't afford to waste parts even if they are only a dollar or two each. Hell, I am to the point of ratyioning solderwick. Seriously, I keep it in a puill bottle so it doesn't dry out and it only comes out when there is money involved.

>"Got any idea of the impedance of a 100uF cap at 1kHz ?? "

Ummm, damn I forget so much.

1 / 2 pi F C and then the vector sum of the ESR and ESL I think. Somnething like that. Don't you have the impedance nomograph ?

http://cjh.polyplex.org/electronics/nomograph/

It was gifted by someone on one of the sci.electronics newgroups. I, for one, appreciate it. It will save to your HD but I couldn't get it to run in Firefox right so it is saved in MHT format, which means my copy will only open in IE. You might have different results, or as they say YMMV.

I also started to design a small cap checker based on reading the fast rise and fall times of a square wave. Funny, I recently ran across it, on paper of course. If I design, I do it on paper. I am trying to learn Spice but it is aggravating at times. But seriously, if I ever buoold it it will do what I want. It will read caps both large and small and do it IMMEDIATELY with easily readable display on a simple (multiple for different frequencies) LED bar graph.

However I am not really impelled to finish it. The work I am doing now simply does not require any more advanced solutions than I alreay have. And what's more, that apply the waveform to the DUT thing has even more uses. I was trying to figure out one for inductors but haven't gone that far with that.

Bottom line, it just ain't worth it. I can tell with a scope all I nneed to know.

And BTW, where is Jerry G ? He croak on us or what ? We are in a fifteen year old thread here. Hell, back then we still had Jim Yanik. He is probably pushing up daisys as well. Lenny is still hanging in there. Ho many others are just gone ?

These Googlers who resurrect old threads might be a PITA, but really, now I am reminded of how this group used to be.

Tripped out.

 

>"Hmmm. Your device is still going to switch on low barrier height diodes >like Schottkys and whatnot at that voltage level. "

Yes, but I am aware of that so it is alright. I did try to lower the applied voltage but then the scope picks up too much noise because I use regular voltmeter type probe for ease ot use.

The usual scope probe is simply inappropriate for checking caps like that. Plus the ground doesn't usually go to ground, like when checking coupling, rather than decoupling caps.