Here are postings I made on this topic on this group on August 1, 2001
and April 22, 1999. I'm sure you can find a lot of other relevant
postings with a google groups search.

--------

You apply a variable DC voltage through a current limiting resistor.
Adjust the voltage so that the capacitor draws a few mA of current.
(What you want to avoid is excessive heating of the capacitor.) As the
dielectric layer thickens, the current will slowly drop. When it does,
increase the voltage again to get the current back up to a few mA.
Continue this until you've applied the full rated voltage and the
current at that voltage has dropped to a very low level. Or you can use
a fixed voltage source equal to the rated capacitor voltage, and vary
the resistance to maintain the desired current.

--------

One point that hasn't been mentioned here is that the voltage-handling
ability of electrolytic capacitors declines if they're stored for long
periods without use. So a capacitor might be bad after long storage,
but can be brought back to life.

The insulator is a coating which is plated onto one of the electrodes
by an electroplating process. This process naturally occurs due to the
applied voltage when the capacitor is in use. When stored for long
periods, the coating deteriorates and must be re-formed to restore the
capacitor to its original voltage rating. (An associated problem is
that when in use, the dielectric coating will, over a long period,
naturally degrade to match the applied voltage. So if changes are made
in the circuit which raise the capacitor voltage, say by replacing an
old rectifier tube, the capacitor can leak or explode due to the
suddenly increased leakage current.)

An electrolytic capacitor is re-formed by applying a voltage through a
current-limiting resistor. The voltage is slowly increased until the
leakage current reaches some specified value, then left at that level
until the leakage drops to a low value as the dielectric layer forms.
Then the voltage is increased until the leakage current again
increases to the limit and left at that value for forming, and so
forth, until the leakage is adequately low at the rated voltage.

There are various rules of thumb for how large a leakage current can
be tolerated during re-forming. One reference I have for modern
capacitors gives 5 mA for small PC-board-mount capacitors, and 10 mA
for large computer grade capacitors. It seems to me the important
thing is to limit the temperature rise, so the tolerable current would
really be a function of capacitor size and the voltage across it. A
quick look at the _ARRL Handbook_ index didn't reveal anything about
this topic, but I know it was in the Handbook many years ago. The
folks on the .boatanchors newsgroup should be able to provide a lot
more practical information on the topic for those who are interested.

Roy Lewallen, W7EL

Steve Nosko wrote:

"Per-Ake Andersson" wrote in message
news
I have a powersupply which have been stored for 15 years.
Now I want to use it. But I think the high volt(500 V) capacitors
need to be refreshed (right word?). (To slowly increase the voltage.)

Questions are for ex.: how fast, how long time at each level.
Cant find any info on the net or in the handbook.
Is there any info on the net ? Or here ?

/Per-Ake



Wish I could remember where, but there was something within the last 6 mo
where it talked about monitoring the CURRENT. I would think that HOT caps
are a BAD thing and should be avoided. My guess is that t15 years isn't
long, but better safe...

Bill Turner wrote:
On Tue, 13 Apr 2004 09:55:33 +0000 (UTC), (Per-Ake
Andersson) wrote:


I have a powersupply which have been stored for 15 years.
Now I want to use it. But I think the high volt(500 V) capacitors
need to be refreshed (right word?). (To slowly increase the voltage.)


__________________________________________________ _______

This isn't quite what you asked, but I'd recommend just replacing them.
Electrolytics are notorious for drying out and 15 years is really
pushing it. I'd just replace them with some new ones.

Wow, I have lots of 15 y.o.+ electrolytics that are still going strong.


I didn't have a variac, so here's what I did to rejuvenate a SB-200 I
restored a while back:

I made a fixture of a standard light bulb socket.

This socket was wired in series with the amplifier power cord.

Starting out with a 10 watt light bulb, I plugged it in, and turned it
on. I left it running in this setup for an hour.

Next I replaced the 10 watt bulb with a 25 watt bulb. Repeat the burn
in process.

I worked my way up the wattage scale until I reached 100 watts. At that
point I put the tubes back in, turned it on and it worked just fine.


- Mike KB3EIA -

Bill Turner wrote:
On Tue, 13 Apr 2004 09:55:33 +0000 (UTC), (Per-Ake
Andersson) wrote:


I have a powersupply which have been stored for 15 years.
Now I want to use it. But I think the high volt(500 V) capacitors
need to be refreshed (right word?). (To slowly increase the voltage.)


__________________________________________________ _______

This isn't quite what you asked, but I'd recommend just replacing them.
Electrolytics are notorious for drying out and 15 years is really
pushing it. I'd just replace them with some new ones.

Wow, I have lots of 15 y.o.+ electrolytics that are still going strong.


I didn't have a variac, so here's what I did to rejuvenate a SB-200 I
restored a while back:

I made a fixture of a standard light bulb socket.

This socket was wired in series with the amplifier power cord.

Starting out with a 10 watt light bulb, I plugged it in, and turned it
on. I left it running in this setup for an hour.

Next I replaced the 10 watt bulb with a 25 watt bulb. Repeat the burn
in process.

I worked my way up the wattage scale until I reached 100 watts. At that
point I put the tubes back in, turned it on and it worked just fine.


- Mike KB3EIA -

I worked at a broadcast radio station that had been in service about twenty
years and we were starting to get random failures in the capacitors in
timing circuits. These capacitors had been in service for the full time
that the station was on the air and the station was a 24/7 operation.

I would vote on replacing the capacitors and still doing the gradual in-
crease of the input voltage. Normally it doesn't really help to do that
but there have been rare cases where a new capacitor surprises the new owner
with a loud bang.

I worked at a broadcast radio station that had been in service about twenty
years and we were starting to get random failures in the capacitors in
timing circuits. These capacitors had been in service for the full time
that the station was on the air and the station was a 24/7 operation.

I would vote on replacing the capacitors and still doing the gradual in-
crease of the input voltage. Normally it doesn't really help to do that
but there have been rare cases where a new capacitor surprises the new owner
with a loud bang.

In article , Bill Turner
writes:

On Wed, 14 Apr 2004 23:09:40 GMT, Allan Butler
wrote:

there have been rare cases where a new capacitor surprises the new owner
with a loud bang.

_________________________________________________ ________

This is so rare as to be a non-issue. If a new electrolytic goes
"bang", you have almost certainly done one of two things wrong:

1. Installed a new capacitor with too low a voltage rating.

-or-

2. Installed the new one backwards.

Or both. :-)

Otherwise, don't worry about it.

Bill, I have to agree with you. This is a NON-ISSUE.

Having had some experience in electron-pushing, mostly hands-on,
since 1947 (which includes those "B+" high-working-voltage
electrolytics lots are so "afraid of" nowadays), I've been through
the same two boo-boos you mentioned...but only a few times and
that was long ago.

I've got a few electrolytics that have been around since the year
dot in the workshop and they still measure good enough, with
or without this "re-forming." Those were obtained some time
between 1958 and 1964, putting them 46 to 40 years ago since
first manufactured. :-)

No explosions, no going "poooof," just sitting there doing their
thing, tack-soldered into a quasi-breadboard for a hybrid "B+"
supply that is regulated and I'm interested in trading off how
much capacity needed for achieving a reasonable regulator.
Not building a production prototype, not worried about a job if
10,000 ordered parts will be scrapped if the wrong thing is
ordered or worried about future QC shouting and hollering about
"failure rates." It works or it don't.

This is a HOMEBREWER newsgroup. We aren't building space-
craft that must operate unattended on Mars. We should have
- as hobbyists on the bench - some kind of power supply handy.
It's a bloody simple task to just get some clip leads and a handy
series resistor and roll up some voltage from whatever supply is
there, then see if the electrolytics can hack it. Takes at most
a half hour to set up, do it, make notes, take down...about the
time needed for some to sit and agonize over problems they
don't have yet on the Internet. Geez, at worst, a cap will get
fried and a new one has to be bought. Maybe. One #$%^!!!
better than "replacing _all_ 'old' ones" as another suggested.

Len Anderson
retired (from regular hours) electronic engineer person

In article , Bill Turner
writes:

On Wed, 14 Apr 2004 23:09:40 GMT, Allan Butler
wrote:

there have been rare cases where a new capacitor surprises the new owner
with a loud bang.

_________________________________________________ ________

This is so rare as to be a non-issue. If a new electrolytic goes
"bang", you have almost certainly done one of two things wrong:

1. Installed a new capacitor with too low a voltage rating.

-or-

2. Installed the new one backwards.

Or both. :-)

Otherwise, don't worry about it.

Bill, I have to agree with you. This is a NON-ISSUE.

Having had some experience in electron-pushing, mostly hands-on,
since 1947 (which includes those "B+" high-working-voltage
electrolytics lots are so "afraid of" nowadays), I've been through
the same two boo-boos you mentioned...but only a few times and
that was long ago.

I've got a few electrolytics that have been around since the year
dot in the workshop and they still measure good enough, with
or without this "re-forming." Those were obtained some time
between 1958 and 1964, putting them 46 to 40 years ago since
first manufactured. :-)

No explosions, no going "poooof," just sitting there doing their
thing, tack-soldered into a quasi-breadboard for a hybrid "B+"
supply that is regulated and I'm interested in trading off how
much capacity needed for achieving a reasonable regulator.
Not building a production prototype, not worried about a job if
10,000 ordered parts will be scrapped if the wrong thing is
ordered or worried about future QC shouting and hollering about
"failure rates." It works or it don't.

This is a HOMEBREWER newsgroup. We aren't building space-
craft that must operate unattended on Mars. We should have
- as hobbyists on the bench - some kind of power supply handy.
It's a bloody simple task to just get some clip leads and a handy
series resistor and roll up some voltage from whatever supply is
there, then see if the electrolytics can hack it. Takes at most
a half hour to set up, do it, make notes, take down...about the
time needed for some to sit and agonize over problems they
don't have yet on the Internet. Geez, at worst, a cap will get
fried and a new one has to be bought. Maybe. One #$%^!!!
better than "replacing _all_ 'old' ones" as another suggested.

Len Anderson
retired (from regular hours) electronic engineer person

Bill Turner wrote:

On Wed, 14 Apr 2004 23:09:40 GMT, Allan Butler
wrote:


there have been rare cases where a new capacitor surprises the new owner
with a loud bang.


__________________________________________________ _______

This is so rare as to be a non-issue. If a new electrolytic goes
"bang", you have almost certainly done one of two things wrong:

1. Installed a new capacitor with too low a voltage rating.

-or-

2. Installed the new one backwards.

Or both. :-)


Which of course is a possibility when replacing them. So the restorer
could be causing a problem where there was noeIf you want to carte
blance replace them, that's fine, but I'm fine with taking a few hours
to reform them. Generally if there is anything really wrong with the
caps, it'll show up quickly.

- Mike KB3EIA -

Bill Turner wrote:

On Wed, 14 Apr 2004 23:09:40 GMT, Allan Butler
wrote:


there have been rare cases where a new capacitor surprises the new owner
with a loud bang.


__________________________________________________ _______

This is so rare as to be a non-issue. If a new electrolytic goes
"bang", you have almost certainly done one of two things wrong:

1. Installed a new capacitor with too low a voltage rating.

-or-

2. Installed the new one backwards.

Or both. :-)


Which of course is a possibility when replacing them. So the restorer
could be causing a problem where there was noeIf you want to carte
blance replace them, that's fine, but I'm fine with taking a few hours
to reform them. Generally if there is anything really wrong with the
caps, it'll show up quickly.

- Mike KB3EIA -

Thanks all information.
Now I know how to do the re-forming.

/Per-Ake

--
Remove "extra" in my e-mail adress !!