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Hello, everyone!
I am new here, on this forum and I need some help. Here is the story.
I have come across a number of old aluminum can capacitors rated between 20-170 uF at 200-500V. Some of them were salvaged from old equipment, some of them are NOS. I have checked them for leakage using a Heathkit IT-11, which does not function very well and is not very reliable. I trashed some of the caps, but I also kept about 20 of them that checked OK. I am now trying to reform them using one of those simple circuits found on the web with a few mods. I added a milliampermeter in series with the capacitor and a voltmeter across it. Therefore, when I charge the capacitors I can measure the current through them at different voltage values. I am getting currents between 40-500 uA, depending of the capacitor value and the test voltage.
Here comes the BIG QUESTION!!!
Could any of you, please, give me an idea what the allowable current through these old reformed capacitors might be? By “allowable current” I mean something like how far can I go before the caps don’t filter well and I get hum in my tube circuits. Of course, I could do it by trial and error, but it would be nice to know ahead of time what I should expect.
Thank you in advance for you input.
Follow Ups:
Here's a link to a cap reforming unit I built years ago, from an article in Glass Audio. Pages 4 and 5 have information on what data you should be looking for when reforming a cap.
I hope you're using a variac. I keep the current draw to no more than 10 to 15 mA while reforming.
Less than 1 mA should be OK - it represents less than a half-watt dissipation at 450V. I WOULD be concerned if you see any jumps in current after it has stabilized. Still no guarantee that they won't short out tomorrow... be sure any circuit where you use these is fused (if slow-blow, 1.2 to 1.5 times normal current)
Filtering is dependent on capacitance value and ESR (equivalent series resistance). High ESR can result in heating even if the DC leakage is OK. You can use an ESR meter or measure it with a 'scope and audio generator at 100 KHz or so.
Dump them. You can purchase new can type caps that keep the factory stock appearance.
Forget about reforming if they are aluminum electrolytics and replace them. You're gambling with death! Ray
"The gift of imagination is a gift of the Gods imparted to a few who receive innumerable kicks in the a$$ their entire life." Le Corbusier (Charles-Édouard Jeanneret)
Page 22 of this pdf will tell you what Sencore thinks are allowable current leakage.
Tre'
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