I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

On Friday, May 6, 2016 at 3:02:30 PM UTC-4, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

Yes, caps that read good value can still be bad. Most electrolytics that are bad will show high ESR readings (bad) before they show low capacitance. ESR meters are cheap enough that anyone who screws with electronics should have one. Despite the advice of others, I always remove the capacitor to test ESR with an "in-circuit" ESR tester.

Also, a good percentage of wonky electrolytics will show signs of venting at the top of the cap, if not an actual electrolyte leakage, then a discernible bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive. For a device to be warm up sensitive like your device it's usually a weak electrolytic , so concentrate there.

If there's only a few electrolytics, change them all with good quality (I like Panasonic) caps and they'll probably run.

In article ,
says...

I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some

Bad caps are certainly one possibility,
but there are other things to check for
as well.

There could be a cold solder joint that
makes contact when a lead expands with
heat.

A connector may have a bit of corrosion
that slips away when the metal contacts
slide past each other under heating.
Try disconnecting and reseating all
connectors.

Intermittent issues like these can be
a pain to diagnose...

--- news://freenews.netfront.net/ - complaints: ---

In article ,
says...

On Friday, May 6, 2016 at 3:02:30 PM UTC-4, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

Yes, caps that read good value can still be bad. Most electrolytics that are bad will show high ESR readings (bad) before they show low capacitance. ESR meters are cheap enough that anyone who screws with electronics should have one. Despite the advice of others, I always remove the capacitor to test ESR with an "in-circuit" ESR tester.

Also, a good percentage of wonky electrolytics will show signs of venting at the top of the cap, if not an actual electrolyte leakage, then a discernible bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive. For a device to be warm up sensitive like your device it's usually a weak electrolytic , so concentrate there.

If there's only a few electrolytics, change them all with good quality (I like Panasonic) caps and they'll probably run.

I agree with changing all the electrolytics as there are only a couple
of them. Even though the value may check out ok , there could be other
problems with them such as the ESR value.

I would guess the .068K would be a .068 UF at 250 volts. They seldom go
bag. If only a few of the 47 uf you might as well change them while you
are at it.

While you could have other problems, the heating up seems to indicate
the classic bad capacitors in the power supply.

I would first put the caps back in the circuit and see if it still behaves the same way. Your unsoldering and re-soldering may have fixed things due to a cold solder joint or other possible connection problems.

Mpfff.... Some basics on electrolytic caps:

a) an electrolytic cap, unless otherwise marked can test +100%/-20% for value and still be in spec. And unless you have the ability to test them at full operating voltage, the information from a typical DVM is incomplete.
b) an ESR meter is your friend. As noted elsewhere, testing is best done out of circuit.
c) switching power supplies are very hard on caps.

Replace them anyway, but also as noted, look for other heat sensitive failures also!

Peter Wieck
Melrose Park, PA

On Fri, 06 May 2016 12:05:35 -0700, wrote:

I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem.
(...)

Those are the classic symptoms of a defective electrolytic capacitor.
You can't test an electrolytic with just a capacitance meter. you
need an ESR (equivalent series resistance) meter. Often, I see them
with the right capacitance, but with a high ESR. It's a common
problem:
http://802.11junk.com/jeffl/pics/repair/slides/bad-caps.html
Notice how some of them are bulging at the top or bottom. As others
have mentioned, it probably best to do an pre-emptive replacement than
to try an save a few pennies seperating the good from the bad caps. In
general, if one electrolytic is bad, they're all bad.

After some provocation by Phil Allison, I ran a temperature test on
various electrolytic capacitors:
http://802.11junk.com/jeffl/Electrolytic-cap-test/
Simple setup:
http://802.11junk.com/jeffl/Electrolytic-cap-test/test-setup.jpg
Test caps:
http://802.11junk.com/jeffl/Electrolytic-cap-test/caps.jpg
Spreadsheet showing results of temp test:
http://802.11junk.com/jeffl/Electrolytic-cap-test/cap-esr-test.xls
Notice how the ESR improves (goes down) with increasing temperature.
That's why your power supply screws up at room temperature, but works
when it's warmed up.

There are a number of different ESR meters available. If you plan on
doing such repairs in the future, you really should buy one. If
you're not sure which one is suitable, I don't have any experience
with this one, but it sure is cheap:
http://www.ebay.com/itm/131645169017


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

" If you see any bulged caps, change them. "

Note that does NOT apply to the big huge ones. Like the 470/200 main raw B+ filter for example, or those 10,000 at 80 volts in an amplifier.

When it has that plastic sheath over it, the bulging top does NOT indicate it is bad.

A guy I used to work with was told that and changed a like 470/200 and that did not fix the set, of course because it was not bad and he was not a real tech, he just did what he was told. Well days later I get to work on it because nobody else could fix it. The cap was in a bag with the screws and replaced parts. The cap was still charged !

That doesn't mean it could not have high ESR, but it was definitely not the cause of the problem. There are people who like to change ALL the caps in old hifi amps, I think they're wrong. While I will agree with changing the corresponding cap in the other channel when you find a bad one, or even weak one (significant ESR compared to Xc) it does not good to just change them all arbitrarily.

It depends on where they are in the circuit, if used for bypass, what kind of current drain ? If used for coupling, what impedences ? It matters.

In article ,
says...



There are a number of different ESR meters available. If you plan on
doing such repairs in the future, you really should buy one. If
you're not sure which one is suitable, I don't have any experience
with this one, but it sure is cheap:
http://www.ebay.com/itm/131645169017


For a few dollars more you can get them already put together. I bought
a similar one a few years back. Not sure how well it does the ESR but
usually does a good job of testing many components.

Jeff Liebermann wrote:



Those are the classic symptoms of a defective electrolytic capacitor.


** Or several OR a dry solder joint or many of them.



After some provocation by Phil Allison, I ran a temperature test on
various electrolytic capacitors:
http://802.11junk.com/jeffl/Electrolytic-cap-test/
Simple setup:
http://802.11junk.com/jeffl/Electrolytic-cap-test/test-setup.jpg




** You went to lot of trouble to see the obvious.

All I did was try a few 450V electros, heat them with a hot air gun until they were darn uncomfortable to hold and note that in every case the ESR reading had plumeted by 5 to 10 times.

Then I found some graphs on-line that backed up the finding.


There are a number of different ESR meters available.


** I use the same one as in your pics - the Bob Parker design. He lives not far from me and we have conversed about that an many other electronics matters.

Have you tried your meter to test the ESR of cells ?

Easy to tell if a Lithium memory cell is good or not - also good on NiCds, NiMH and alkalines.

Also reads low value resistors, even if there is an inductor or transformer winding across one.


..... Phil

On 06/05/2016 20:05, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric


ISTR once coming across a failed zener in a smps causing this works when
warm syptom

On 06/05/2016 20:05, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric


I've seen a dot on ICs signifying PbF

On 2016-05-06 19:05:35 +0000, said:

I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

lots of esr meters to choose from : to build or to buy
http://kripton2035.free.fr/esr-repository.html

--

Jean-Yves.

On Friday, May 6, 2016 at 8:30:29 PM UTC-4, wrote:

When it has that plastic sheath over it, the bulging top does NOT indicate it is bad.

Ummmm..... Not on my watch! I do keep an ESR meter, and I also keep a cap-tester that can test at actual operating voltages and in AC or DC. ANY physical indications of ANY nature other than friction or impact will get a cap changed. And every electrolytic in an item that affects the perceived problem will get tested with that ESR meter. When in doubt, change it out.

The problem with swelling is that it may not be affecting the cap *today*. And the actual stresses on the cap may be well below its rating. But when (not if) it fails, it will be at the most awkward possible moment and take much with it. Murphy's Law. Caps, even very good ones, are much cheaper than the time it takes to replace them. Just keep that in mind.

Peter Wieck
Melrose Park, PA

On Saturday, May 7, 2016 at 8:18:08 AM UTC-4, jeanyves wrote:
On 2016-05-06 19:05:35 +0000, said:

I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

lots of esr meters to choose from : to build or to buy
http://kripton2035.free.fr/esr-repository.html

--

Jean-Yves.

I keep the "Blue" meter. Easy to use and as tested against others, quite accurate.

Peter Wieck
Melrose Park, PA

That doesn't mean it could not have high ESR, but it was definitely not the
cause of the problem. There are people who like to change ALL the caps in old
hifi amps, I think they're wrong. While I will agree with changing the
corresponding cap in the other channel when you find a bad one, or even weak
one (significant ESR compared to Xc) it does no good to just change them all
arbitrarily.

Sure it does. If the problem is with one of these caps, changing them
arbitrarily FIXES the problem.

€œPenny wise, pound foolish.€� Yes, youre right, finding the ONE cap
that is the cause is technically proficient. But just to say €œI found the
culprit!€� does not help the customer (or yourself, if its you). The unit
will be back on your bench very soon if you dont replace all electros now,
if you find one or more below spec.

Simple setup:
http://802.11junk.com/jeffl/Electrolytic-cap-test/test-setup.jpg

Bob Parker / Dick Smith / Silicon Chip meter!!!

Bob is the original designer of this meter. Last I looked Bob was still
giving help to builders of his kit.

Bob Parker / Dick Smith / Silicon Chip meter!!!

Bob is the original designer of this meter. Last I looked Bob was still
giving help to builders of his kit.

http://members.ozemail.com.au/~bobpar/esrmeter.htm

On Sat, 7 May 2016 00:02:40 -0700 (PDT), Phil Allison
wrote:

Jeff Liebermann wrote:
After some provocation by Phil Allison, I ran a temperature test on
various electrolytic capacitors:
http://802.11junk.com/jeffl/Electrolytic-cap-test/
Simple setup:
http://802.11junk.com/jeffl/Electrolytic-cap-test/test-setup.jpg

** You went to lot of trouble to see the obvious.

Cap-B was one that had failed with high ESR. There was plenty of data
and anecdotal evidence on how a normal capacitor would act. I wanted
to see how a defective capacitor acted. I also wanted to see how the
ESR meter functioned with small value and size (0.22uF 50v) caps,
which was Cap-D. The change in ESR with temp was far less radical
than the others.
http://802.11junk.com/jeffl/Electrolytic-cap-test/Cap-D.jpg
I also found as similar problem with high capacitance values (1800uF
6.3v) which was Cap-A, where the lower limit capability of the ESR
meter (about 0.03 ohms) also caused the graph to flat line. I didn't
mention this in your previous discussion on the matter, but I was
trying to determine if it made sense to use a fixed 100KHz sine wave
signal, and whether higher or lower frequencies might help test a
wider range of capacitance values. I have a pathological aversion to
accepting the obvious, and will take the time to test the obvious,
which invariably produce surprises.

All I did was try a few 450V electros, heat them with a hot air
gun until they were darn uncomfortable to hold and note that in
every case the ESR reading had plumeted by 5 to 10 times.

I wasn't interested in high voltage electrolytics. There are usually
two of those in a typical ATX power supply. They never seem to fail.
It's the low voltage electrolytics, that inhabit the output filters
and regulator circuitry that fail and were of interest to me. I
probably could have done it using your method, but I thought a hot
water bath was more interesting and accurate.

There are a number of different ESR meters available.

** I use the same one as in your pics - the Bob Parker design. He
lives not far from me and we have conversed about that an many
other electronics matters.

Yep. It's an impressive instrument, quite accurate, but a little
tricky to build (Dick Smith kit version). Methinks the later "blue"
variety might be better.
http://anatekinstruments.com/products/fully-assembled-anatek-blue-esr-meter-besr

Have you tried your meter to test the ESR of cells ?
Easy to tell if a Lithium memory cell is good or not - also good on
NiCds, NiMH and alkalines.

One of the common modifications to the original ESR meter is to add
two back to back power diodes across the input to protect the meter
from residual voltages. The problem with doing that is that the
diodes short out the battery. Since I expected to be testing far more
capacitors than batteries, this made sense.

I also have a Capacitor Wizard ESR meter that I picked up cheap at a
flea market:
http://anatekinstruments.com/products/capacitor-wizard-esr-tester-cap1b
http://anatekinstruments.com/products/capacitor-wizard-esr-tester-with-capsvr-module-cap1b-capsvr
The CapSRV model has the same diode protection circuit making it
useless for battery measurements. There's nothing particularly
wonderful about this meter except that it's very fast and convenient.
One problem is that I seem to have misplaced it and can't seem to find
it. Argh.

I also have a Peak Atlas ESR70 "purple" ESR meter.
http://www.peakelec.co.uk/acatalog/jz_esr70.html
It has a built in discharge resistor to help deal with charged
capacitors and therefore is useless for testing battery ESR.

Bottom line is that I don't use any of my ESR meters to test
batteries. I probably should build or buy something specifically for
the purpose to see what I'm missing. All of my battery testing
involve small rechargeable batteries, a discharge tester, and
computized graphs such as:
http://802.11junk.com/jeffl/LiPo/Ultrafire%2018650%20test.jpg
http://802.11junk.com/jeffl/LiPo/Ultrafire%20LiPo%203000%20ma-hr%2018650%20test.jpg
However, I'm getting into larger cells and sealed packs, where an ESR
tester is quicker and probably more useful.

Also reads low value resistors, even if there is an inductor or
transformer winding across one.

Yep. I've used that to determine the value of a charred resistor.
Usually, it's a wire wound resistor with a break somewhere along the
winding. I measure from one lead to the middle of the winding. One
end is usually open, while the other end reads half the resistor
value. However, testing electrolytics is my main use for an ESR
meter.

.... Phil

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

In article t,
Ralph Mowery wrote:

I would guess the .068K would be a .068 UF at 250 volts. They seldom go
bag.

I agree. The K probably indicates 10% tolerance.

Regardless, it is either .068 Microfarads or 68K (68,000) Picofarads,
which is the same thing!

Fred

On 06.05.2016 21:14, wrote:
On Friday, May 6, 2016 at 3:02:30 PM UTC-4, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a
minute or two the power supply starts working. I was told by
someone here or on the basics group that there is probably a bad
electrolytic capacitor causing the problem. I also have a CNC
machine that has a servo amp that acts the same. The machine must
be on for a while and when the cabinet that holds the servo amps
gets warm enough inside the amp start working and will work fine as
long as the machine is kept powered up. If the machine is turned
off for an hour or so and then powered back up the amp still works.
But if the machine is off overnight the amp acts the same as above.
I replaced the amp so there is no problem running the machine but I
would like to repair this amp so I can have a spare on the shelf.
Anyway, it has only 3 physically large electrolytic caps. I
unsoldered one lead from each cap and measured the capacitance with
my Extech multimeter. The two 10 uf caps measured 11. something uf
and the 33 uf cap measured 37. something. Could these caps be bad
anyway? There are also three blue colored dipped caps that have
printed on them: .068K 250. I don't what the dot in the .068K means
but I don't think it's a decimal point. Maybe it means lead free.
There are also a couple 47 uf wound polymer caps. Could either of
these types of caps need to be warm before they work properly? I
have no schematic for this amp and have not been able to get one.
Furthermore, the company that makes this servo amp has ground the
markings off of any device with more than three leads. Thanks,
Eric

Yes, caps that read good value can still be bad. Most electrolytics
that are bad will show high ESR readings (bad) before they show low
capacitance. ESR meters are cheap enough that anyone who screws
with electronics should have one. Despite the advice of others, I
always remove the capacitor to test ESR with an "in-circuit" ESR
tester.

Also, a good percentage of wonky electrolytics will show signs of
venting at the top of the cap, if not an actual electrolyte leakage,
then a discernible bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive.
For a device to be warm up sensitive like your device it's usually a
weak electrolytic , so concentrate there.

If there's only a few electrolytics, change them all with good
quality (I like Panasonic) caps and they'll probably run.

Good advice. But there is one more small-ish pitfall for the unwary. If
a power supply refuses to start when cold, it's usually an electrolytic
cap. But in many cases it's not one of the big caps but the small one
that is located in the start-up / auxiliary section.

Mains power supplies usually have an auxiliary section that is used to
power the internal needs of the primary side switching circuitry. This
section is powered by a small auxiliary winding on the transformer in
steady-state. But because it needs to start up first, and because at
first the main transformer is not yet operating, it is precharged with a
high value resistor (or sometimes a string of 2 or 3 resistors in
series) directly from the rectified mains. There is a small (47 uF or
similar, 25 to 35 V typically) electrolytic. At first it gets precharged
through the resistor up to the point where the controller chip can start
up. Once the controller starts, it would rapidly drain the small cap, so
the main switching circuitry has to come on line fast in order to top up
the the auxiliary supply cap and keep it from discharging.

Now this small cap only provides little reserve. It's usually sized very
spartanically and it lasts for just one try and only if the start-up
timing of the switching circuitry is not delayed for any reason. If it
drains before the switcher can fully start, the switcher will stop and
wait for another retry at a later time.

Often this small cap is considered "non-critical" and carelessly placed
by the power supply designer into whatever corner had whatever little
free space. This may happen to literally be a "corner" of a heatsink!

When this cap dries out either from overheating of just being selected
from the cheapest manufacturer, it will rise in ESR and thus no longer
provide the peak current that the controller needs in order to start.
This either delays the start to the point where the cap is drained or
causes it to outright abort prematurely because the auxiliary supply
rail tanks right at the first few switching cycles.

When warmer it has lower resistance, thus it can provide higher peak
currents and have better chances of starting the supply successfully.
Also if the supply is working continuously for many years and never
switched off, the condition of this capacitor is of no consequence
during operation. Even if somebody just cut it out of a working power
supply, there are usually some small ceramics on the rail and the
auxiliary transformer winding will keep them topped up, so that the
supply would quite likely keep on working. But if the supply ever gets
switched off, even for a very short time, it will never start up again.

If you see this type of behavior, find and replace the cap in the
auxiliary section. You can normally identify it as being physically
small, a low voltage type (from 25 to 50 V) and being connected to a
sizeable precharge resistor that gets supplied from the high voltage
DC rail.

If you don't have an exact replacement, it's normally OK to use the
next higher capacitance as long as the voltage rating fits. If you
find that yours sits in the corner of a heatsink, use a low ESR high
temperature rated replacement (no matter what the original has been)
and some reasonable reserve capacitance-wise won't hurt either.

Dimitrij

Jeff Liebermann wrote:
Phil Allison


** You went to lot of trouble to see the obvious.

Cap-B was one that had failed with high ESR. There was plenty of data
and anecdotal evidence on how a normal capacitor would act. I wanted
to see how a defective capacitor acted.


** Well, that depends how *defective* it actually is !!!

An electro that has lost nearly all of its fluid and reads off-scale on Bob Parker's meter is not likely to come good with a bit of heat.


I also wanted to see how the
ESR meter functioned with small value and size (0.22uF 50v) caps,
which was Cap-D. The change in ESR with temp was far less radical
than the others.

** Even a 0.22uF film cap reads 7 ohms on the Bob parker meter - IOW the impedance of the thing at 100kHz. Bob's meter does not read actual ESR values, rather it reads impedance vales at 100kHz.

This *fact* is pointed out in the instructions and the lowest value electro you can reliably test is about 1uF - as shown on the front panel table.

A man has gotta know the limitations of his ESR meter.
------------------------------------------------------


All I did was try a few 450V electros, heat them with a hot air
gun until they were darn uncomfortable to hold and note that in
every case the ESR reading had plumeted by 5 to 10 times.

I wasn't interested in high voltage electrolytics.


** They have higher ESR values than low voltage caps, so there is plenty of room for ESR readings to come down without approaching the low reading limit of Bob's meter.

I used a value of 47uF, so its reactance at 100kHz was negligible.

Pays to think ahead, you know.


..... Phil

In article -
september.org, says...

That doesn't mean it could not have high ESR, but it was definitely not the
cause of the problem. There are people who like to change ALL the caps in old
hifi amps, I think they're wrong. While I will agree with changing the
corresponding cap in the other channel when you find a bad one, or even weak
one (significant ESR compared to Xc) it does no good to just change them all
arbitrarily.

Sure it does. If the problem is with one of these caps, changing them
arbitrarily FIXES the problem.

?Penny wise, pound foolish.? Yes, you?re right, finding the ONE cap
that is the cause is technically proficient. But just to say ?I found the
culprit!? does not help the customer (or yourself, if it?s you). The unit
will be back on your bench very soon if you don?t replace all electros now,
if you find one or more below spec.


Unless the capacitors are very expensive, it is best to just replace
them all even if only one or two is showing up to be the problem.

When working a factory repair man came in to repair a 300 HP electric
motor varitable speed drive. This was a 3 phase AC input. He determined
2 out of 3 large diodes were bad. I asked him to replace the 3 rd one
and he said those were about $ 100 each. I said so what, the equipment
beind down was costing several thousand dollars an hour and another $
20,000 to start it up and shut it down. At that ratio of dollars to
production I thought it would be a good idea to replace it even if it
was good.
I am sure it would have started, and may have ran from then on, but why
take a chance it would crap out a week or even a year later.

On Sat, 7 May 2016 18:18:16 -0700 (PDT), Phil Allison
wrote:

Jeff Liebermann wrote:
Phil Allison


** You went to lot of trouble to see the obvious.

Cap-B was one that had failed with high ESR. There was plenty of data
and anecdotal evidence on how a normal capacitor would act. I wanted
to see how a defective capacitor acted.


** Well, that depends how *defective* it actually is !!!

Well, cap-B was labeled 2200uF 10v and was bulging at the top.
Room temp ESR was measured at about 2.3 ohms:
http://802.11junk.com/jeffl/Electrolytic-cap-test/Cap-B.jpg
and should have been about 0.1 ohms according to the latest chart for
my ESR meter:
http://802.11junk.com/jeffl/crud/ESR.txt
It's the 2nd from the left in:
http://802.11junk.com/jeffl/Electrolytic-cap-test/caps.jpg
and shows no signs of leakage except for the bulging top.

An electro that has lost nearly all of its fluid and reads
off-scale on Bob Parker's meter is not likely to come good
with a bit of heat.

True. However at 100C, the ESR dropped to 0.25ohms which is not
perfect but would probably be adequate for most purposes.
http://802.11junk.com/jeffl/Electrolytic-cap-test/Cap-B.jpg

I also wanted to see how the
ESR meter functioned with small value and size (0.22uF 50v) caps,
which was Cap-D. The change in ESR with temp was far less radical
than the others.

** Even a 0.22uF film cap reads 7 ohms on the Bob parker meter -
IOW the impedance of the thing at 100kHz. Bob's meter does not
read actual ESR values, rather it reads impedance vales at 100kHz.

Right. The impedance is the vector sum of the resistive (ESR)
component and the capacitive reactance of the capacitor. At 100KHz:
Xc = 1 / (2*Pi*MHz*uF) = 1 / 2*Pi * 0.1 * 0.22 = 7.23 ohms
If the ESR were the typical less than 1 ohm, the bulk of what the
meter is reading is from the capacitive reactance.

The meter was reading exactly 7.00 ohms, I can get a rough idea of the
resistive ESR from:
R = sqrt(Xc^2 - Z^2) = sqrt(7.2^2 - 7.0^2)
= sqrt(52.3-49) = 1.8 ohms

However, let's pretend that this capacitor was defective and it
exhibited a higher ESR, such as 6 ohms. What would the ESR meter
read? Same formula:
Z = sqrt(R^2 + Xc^2) = sqrt(6.0^2) + 7.2^2)
= sqrt(36+52.3) = 9.4 ohms
You would be able to see the difference between a good 7.0 ohm reading
and a bad 9.4 ohm reading, but only if you did the calculations in
advance. I once played with an HP/Agilent LRC meter that did all this
automatically. it would be nice if the next generation of ESR meters
could also do that.

Raising the frequency to 1MHz would reduce the capacitive reactance to
0.723 ohms, which would produce something closer to the real ESR.
However, the ESR changes with frequency, so accuracy will suffer.

This *fact* is pointed out in the instructions and the lowest
value electro you can reliably test is about 1uF - as shown on
the front panel table.

A man has gotta know the limitations of his ESR meter.

I just hate reading the instructions. However, the 1uF lower limit is
printed on the front of almost every ESR meter. My Capacitor Wizard
has it boldly displayed on the front panel.
http://www.howardelectronics.com/images/thumbnails/1138/1138/detailed/1/CAP-WIZARD.jpg?t=1440164290

All I did was try a few 450V electros, heat them with a hot air
gun until they were darn uncomfortable to hold and note that in
every case the ESR reading had plumeted by 5 to 10 times.

I wasn't interested in high voltage electrolytics.

** They have higher ESR values than low voltage caps, so there is
plenty of room for ESR readings to come down without approaching
the low reading limit of Bob's meter.

I used a value of 47uF, so its reactance at 100kHz was negligible.

Pays to think ahead, you know.

Yeah, but I think you missed my point. I don't do tube amplifiers and
therefore do not run into too many high voltage capacitors. The one's
I see most of are low voltage and high capacitance electrolytics as
found on PC motherboards, ATX power supplies, LCD monitors, wall
warts, and various consumer electronic devices.

Incidentally, speaking of tubes (valves), you might be amused by this:
https://www.kickstarter.com/projects/1815752970/hybrid-tube-amp-for-the-raspberry-pi

.... Phil

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:


I also wanted to see how the
ESR meter functioned with small value and size (0.22uF 50v) caps,
which was Cap-D. The change in ESR with temp was far less radical
than the others.

** Even a 0.22uF film cap reads 7 ohms on the Bob parker meter -
IOW the impedance of the thing at 100kHz. Bob's meter does not
read actual ESR values, rather it reads impedance vales at 100kHz.

Right.

** Then you should not have posted what you just did. The actual change in ESR of a 0.22uF 50V electro was not readable on a Bob Parker meter.

What is really happening is the electrolyte becomes more conductive at higher temps, many times more in the range from 20C to 100C. It is the resistance of this fluid that dominates the ESR value.


A man has gotta know the limitations of his ESR meter.

I just hate reading the instructions.


** Right - you take zero notice of anything others tell you.




I wasn't interested in high voltage electrolytics.

** They have higher ESR values than low voltage caps, so there is
plenty of room for ESR readings to come down without approaching
the low reading limit of Bob's meter.

I used a value of 47uF, so its reactance at 100kHz was negligible.

Pays to think ahead, you know.

Yeah, but I think you missed my point.


** No I didn't.

You missed mine, as usual.


..... Phil

On Saturday, May 7, 2016 at 8:18:08 AM UTC-4, jeanyves wrote:
On 2016-05-06 19:05:35 +0000, said:

I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

lots of esr meters to choose from : to build or to buy
http://kripton2035.free.fr/esr-repository.html

--

Jean-Yves.

(Nice link, thanks.)
George H.