Bipolar capacitors repair - Reforming by DC Volts

[Kiriakos GR]

Internet has become a huge place and this has impact when you seek for information’s too.
By percentage you will fall first over outdated ones or over information which an individual will exaggerate regarding steps to be taken for getting some results in your application.
That is a bit of information with allot of ketchup sauce.

In my case I am restoring my old but expensive pair of speakers and crossovers network are now easy to be tested with my LCR at hand.

I am writing those lines almost shocked by the results after restoring my first of the four capacitors which show signs of changing capacity to a higher number.
For example one 3.3uF was measuring as 4.7uF, after feeding it with 30V AC for 2-3 minutes it capacity restored to 3.9uF which is better but not good enough.

My last crazy thought which paid out was to combine two external DC power supply so to get 80V DC and feed with that the capacitor.
Their named specifications’ are 100V DC / 35V AC and so if those capacitors are healthy they should withstand that voltage and keep going with out problems.

By feeding my capacitor an 3.3uF (4.7uf) with 80V it did start to require an amount of 200mA which it started falling but very slowly, the capacitor become hot as 50 Celsius in just a minute and then by starting to recover it temperature started falling to room temperature.
By monitoring the DC current from the starting point it dropped too as low as 140uA but it needed to be connected at those high volts for 30 minutes. 
After seeing that DC current were stabilised at that low value, I removed the capacitor from my test bed and after a good discharge I measured it again with the LCR and now it is measuring 3.218 uF ( 1 KHz) as it should or close to it true specifications.

Before I start screaming Eureka !!  Now I am using the same method to an 8.3uF capacitor which has the same symptoms’ by measuring 10.2uF.
Currently the capacitor is connected to DC for 10 minutes and it temperature has fallen to 27.5 which is the room temperature, therefore the recovering signs are visible.   

To be continued…

[Kiriakos GR]

Part 2…

This story filled me with allot of excitement because accidentally discovered more things which the mind cannot predict, or better said undocumented.
 
My second 3.3uF also fully recovered by simply staying connected to 80V DC, capacity restored and is now almost 3.2uF which still is close to specifications’ as this capacitors tolerance is at 10 or 15%.

The 8.2uF capacitors responded very differently in this reforming process, by getting connected to DCV their temperature got as high as 80 Celsius for two minutes and started dropping among with their consumption in mA which in about 30 minutes it stabilize down to 150uA.

After removing them from my test bed the first measurement in my LCR shown that the capacitor A was at 12.5uF and the capacitor B was at 9 uF.
For a moment I forgot the one capacitor connected on the LCR (1 kHz) and I got busy with another task, by returning I noticed that D factor and capacity was lowering slowly but stably and this made me think that some how this constant 1 kHz signal it was helping them to truly recover.

To confirm that I connected both capacitors in parallel and up to my function generator as this has stronger output than my LCR.
By monitoring this connection with my Oscilloscope I did noticed the volt output to rising slowly, as the ESR was starting to further improve.

The capacitor connected to the function generator it is seen as load and it internal resistance it influence the max voltage of the output.
When internal resistance decreases, this is shown by the voltage which increases, and in my case after having both capacitors connected for 30 minutes the results are unbelievable.
The cap A now measures as D0036 (impedance) and 9.515 uF capacity.
The cap B ………………… D 0052 ………………. 7.987 uF

After this finding I got wild and thought to try a frequency sweep as test signal so to see if there will be further improvement, mostly for the cap A which capacity seems still high even after all this exercise. 
I did set my Philips generator to sweep mode starting from 800Hz and end point the 2 kHz and connected both caps together in parallel for one complete hour so to have two comparison points in my test.
 
The result was no further improvement in capacity and a bit lower ESR but nothing dramatic which worth mentioned as improvement. 

In summary by my experiments I did succeed restoring three capacitors from the four in their factory specifications.
Before starting my experiments I had all ready order new NOS replacements of ROE Gold series, and therefore I was not worrying if my testing will deliver any good results.

I got lazy taking any pictures of this adventure of my, I am just sharing this story so to motivate any inquiring minds out there to investigate this possibility.
Some one may ask …  Why to perform this 1kHz workout over the workbench?  Just use the crossover network with the speaker for an hour and it would be restored by it self.
Well I wanted to be 100% confident that capacity will be 100% equal in both capacitors even after this workout, and you can not confirm that with out taking both speakers apart for a second time.

[Kiriakos GR]

Just received from Belgium eight ROE EBT Gold capacitors NOS old/new stock (product of 2004).
I got them as possible replacement of my own 3.3uF L-X5 TONFREQUENZ bipolar before performing any reforming to them.

Today I have mixed feelings after testing those 1.5uF 100V ROE EBT, mostly because their ESR is tremendously high at the range of 6 Ohm in comparison with my old L-X5  at 1.5 Ohm ESR.

No I did not fall over bad stock, or better said completely, two of the six has more ESR than the specified in their datasheet and now I hope for a small refund from the seller.

Either way what looks as important for making this follow up, is that I did tested my reforming process to 
these capacitors too.
The interesting part was that temperature did no raise at all when 60DC was applied for 30 minutes.
And after connecting them at 1kHz 2.5V sine-wave their inner condition did not change significantly.

Below are few pictures that I took,  with black ink are the values after reforming.
 


 

[Kiriakos GR]

Just received from China my last order regarding capacitors, these are ROE - EGW series (Gold capacitors).
They are truly remarkable Low-ESR capacitors even when compared with my old ones.

While the capacitors seemed great by the first LCR measurement, I wanted to try reforming on these too at 100 DCV this time that is followed by the additional capacitance fine-recovery process at 1 kHz (2V AC) for an hour.

The gain from the capacitor reforming process was just 1% correction, which translates that those capacitors are that fresh at a point to not need any reforming.
Even so I did confirm even with this set of capacitors that the connection with a frequency generator it does help them so to come closer to their original capacity, and now I am very happy for my discovery.


And now I will mention few additional details of what I am actually dealing with.
The specific capacitors are not common grade electrolytic.
They are AUDIO grade capacitors and even their specifications’ datasheet is different compared to common electrolytic.
ROEDERSTEIN 8.2uF EGW series datasheet gives ESR information’s with test frequency at 100Hz and also at 1 kHz and also at 10 kHz.
Also this datasheet is that long in length which specifies delta D at all frequencies’ range starting for:
50 Hz
100Hz
200Hz
500Hz
1k
2k
3k
5k
6k
7k
8k
9k
10k Hz

Therefore this old class of AUDIO grade capacitors comes with advanced engineering which makes me wonder of why there are no such modern replacements available today?

Possibly the special engineering of these capacitors is the reason of why they respond that way after feeding them with 1 kHz signal for some time.
I think that regular electrolytic or low quality bipolar capacitors will not gain much or anything at all from been connected to 1 kHz source.

Either way my specific contribution serves just one goal, which is to pinch your interest about exploring this theory by your selves.