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Capacitor testing

james_55@...
 

Hi all,
I have a 465 in which the minus 8v power rail had gone down to -4.89 or so volts.

It had been stored for a couple of years and was on the bench, turned on and left running with a nice square wave off of the calibrator.
After a couple of hours I noticed that there was only a 'dot' left on the screen.

Obviously after checking and fiddling around with the settings, I opened it up, half expecting to see a burnt tantalum or similar, but nothing was out of the ordinary.
After following the schematics, a few tantalums had a leg pulled and were tested but nothing came up as bad.

On a different forum someone suggested it might be that the 3000µF 35v electrolytic filter cap was leaking and that the only sure way would be to pull it and apply a DC voltage.

Anyways... It was pulled and connected up to 8v from the bench PSU, but I made a somewhat schoolbay error in that considering that it was on the 'minus' 8v rail, I connected the it with the polarity reversed. Doh!

My question is, "would that 8 volts reversed polarity effectively kill the cap and what testing can verify the situation?"

On my ESR meter it currently reads 3634µF and 0.04Ω
On my DMM it reads 3718µF

Does it need further testing?

Still feels I am missing something...

Thanks in advance


James

Chuck Harris
 

Reversing an electrolytic cap will make it get hot.
The power supply probably also shut off.

If it has good ESR, and the correct capacitance, it
is probably ok.

-Chuck Harris

james_55@... wrote:

Hi all,
I have a 465 in which the minus 8v power rail had gone down to -4.89 or so volts.

It had been stored for a couple of years and was on the bench, turned on and left running with a nice square wave off of the calibrator.
After a couple of hours I noticed that there was only a 'dot' left on the screen.

Obviously after checking and fiddling around with the settings, I opened it up, half expecting to see a burnt tantalum or similar, but nothing was out of the ordinary.
After following the schematics, a few tantalums had a leg pulled and were tested but nothing came up as bad.

On a different forum someone suggested it might be that the 3000µF 35v electrolytic filter cap was leaking and that the only sure way would be to pull it and apply a DC voltage.

Anyways... It was pulled and connected up to 8v from the bench PSU, but I made a somewhat schoolbay error in that considering that it was on the 'minus' 8v rail, I connected the it with the polarity reversed. Doh!

My question is, "would that 8 volts reversed polarity effectively kill the cap and what testing can verify the situation?"

On my ESR meter it currently reads 3634µF and 0.04Ω
On my DMM it reads 3718µF

Does it need further testing?

Still feels I am missing something...

Thanks in advance


James



 

Apply a slow charge to the cap with a resistor to allow the electrodes to reform electrochemically. Charge to about 80% of rated voltage and hold it there for a few hours. Cap. performance should be ok then.
I've been playing with supercaps this year...so that's how to get proper performance and low leakage currents.
~Ancel

james_55@...
 

Hi and thanks for the suggestion.

Have connected it up with a 100k.

Chuck Harris
 

If the case didn't bulge, and the guts didn't
escape, it is quite likely that no damage was
done.

An electrolytic capacitor reversed biased presents
itself as a pretty good short circuit. If the supply
is very strong, it can blow it, but most supplies
have significant resistance when compared to the
capacitor's reverse resistance.

In any case, aluminum electrolytic capacitors are not
as good a fuse as, say, a transistor. It takes them
a little while to heat up and start boiling their
electrolyte.

Please quote the messages you are responding to.

-Chuck Harris

james_55@... wrote:

So is it possible that no damage was done?



james_55@...
 

On Sat, Apr 4, 2020 at 03:06 PM, Chuck Harris wrote:


If the case didn't bulge, and the guts didn't
escape, it is quite likely that no damage was
done.

An electrolytic capacitor reversed biased presents
itself as a pretty good short circuit. If the supply
is very strong, it can blow it, but most supplies
have significant resistance when compared to the
capacitor's reverse resistance.

In any case, aluminum electrolytic capacitors are not
as good a fuse as, say, a transistor. It takes them
a little while to heat up and start boiling their
electrolyte.

Please quote the messages you are responding to.

-Chuck Harris

james_55@... wrote:
So is it possible that no damage was done?
Hi Chuck,
Thank you.

Was trying to find the 'quote' option...

Have now :)

Roy Morgan
 

Long and expert experience with Collins amateur equipment tells us to limit the reforming current to FIVE MILLAMPS.

Take your time. Monitor cap voltage and reforming current. Be patient.

I have a Heath variable bench supply - DC variable from about 0 to 300 volts or so, I use a 10k or 5K, 5 or 10 watt resistor in series. The current meter on the supply lets me keep reforming current low, and a separate voltmeter shows me the caps progress.

Do not follow the procedure given in the MIL-HANDBOOK on reforming electrolytics. There is real danger of disaster doing that (Parallel many caps, reform them with a light bulb series with a DC supply, limiting current to the total for all caps.)

Roy


On Apr 4, 2020, at 5:14 AM, james_55@... wrote:

Hi and thanks for the sugestion.

Any recommendation as to the resistor value?

1k 100k?

Roy Morgan
K1LKY since 1958
k1lky68@...

Randy Newman
 

Hi Roy:
I have ( in my previous work lives) reviewed various of the MIL-Cxx spec’s, and remember reading about reforming electrolytics. Sadly, I have forgotten the one containing the reforming procedure. Which particular spec are you referring to?
Btw..I have a Systron Donner 320v, 2A supply which I have been waiting to use on reforming.
The proverbial “one of these days....”

 

Hi Randy,
A Systron Donner 320V / 2A power supply must weigh at least 100 lbs and be very scary to use.
Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Randy Newman
Sent: Saturday, April 04, 2020 2:30 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Capacitor testing

Hi Roy:
I have ( in my previous work lives) reviewed various of the MIL-Cxx spec’s, and remember reading about reforming electrolytics. Sadly, I have forgotten the one containing the reforming procedure. Which particular spec are you referring to?
Btw..I have a Systron Donner 320v, 2A supply which I have been waiting to use on reforming.
The proverbial “one of these days....”





--
Dennis Tillman W7PF
TekScopes Moderator

james_55@...
 

On Sat, Apr 4, 2020 at 09:21 PM, Roy Morgan wrote:


Long and expert experience with Collins amateur equipment tells us to limit
the reforming current to FIVE MILLAMPS.

Take your time. Monitor cap voltage and reforming current. Be patient.

I have a Heath variable bench supply - DC variable from about 0 to 300 volts
or so, I use a 10k or 5K, 5 or 10 watt resistor in series. The current meter
on the supply lets me keep reforming current low, and a separate voltmeter
shows me the caps progress.

Do not follow the procedure given in the MIL-HANDBOOK on reforming
electrolytics. There is real danger of disaster doing that (Parallel many
caps, reform them with a light bulb series with a DC supply, limiting current
to the total for all caps.)

Roy


On Apr 4, 2020, at 5:14 AM, james_55@... wrote:

Hi and thanks for the sugestion.

Any recommendation as to the resistor value?

1k 100k?

Roy Morgan
K1LKY since 1958
k1lky68@...



Nice one. Thank You

Ray, W4BYG
 

I use a 1 Ma meter in series with a 47,000Ω current limiting resistor for reforming HV caps, up to 600 rated volts.

I never let the cap charge current exceed 1 ma and typically try to keep
the charge current under the rated current in microamps, for that value
and voltage of a given cap.

More current than that can easily cause a break thru in the dielectric
of a old  capacitor.  The process of increasing the voltage in small
steps to the rated voltage can take some time, but I have very good luck
with taking the time to do it that way.

I find the practice of automatically discarding older electrolytics to
be unnecessary.  (Old wax caps is another matter).  I have a large
supply of old caps as well as a number of tube equipments restored and
to restore.  I throw very few caps away using the process above.

Plus, if you turn the refurbished equipment back on at least every 5 or
6 months, you keep the caps reconditioned and working.
Ray W4BYG
--

They say a smart person learns from their mistakes. A
wise person learns from the mistakes of others.



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Roy Thistle
 

On Sat, Apr 4, 2020 at 02:29 PM, Randy Newman wrote:


Which particular [MIL] spec are you referring to?
Hi Randy:
Could it be one of these?
MIL-HDBK-1131A 14 June 2004 SUPERSEDING MIL-HDBK-1131 7 July 1999
Cheers and regards.
Roy