Electrolytic capacitor reforming

[Mr Boochie]

I have done it on really old caps when I'm just trying to see if something has enough signs of life in it to rebuild without spending time and money on caps first, but it doesn't always work. I've had caps from the 60s and before come back and work perfectly, and had just as many either not even try, or work for an hour then decide they were done with life.

I fixed a Dyna ST-70 for someone, ran it on the bench a while and all was fine, hooked it up in the main rig and 5 minutes later the cap blew its guts out. Mildly annoying, but glad it happened while I still had it and could easily take care of the problem rather than involve a return trip.

Yeah. Old 'lytics can be hard to predict. I've had amps with 50-year-old OEM caps in the PSU and the darn amp doesn't hiss or hum and has plenty of reserve power. I still change them out with new caps.

One day the old puppies are fine. The next day they barf all over the chassis.

 

[Mr Boochie]

I have a reforming setup that consists of a Kepco BHK500-80MG current limited high voltage power supply combined with a microammeter(Simpson 374, since I get a feel for the progess by watching the needle bounce around, but you can use a DMM). I usually limit it to 1mA but on some larger caps I got a bit higher. I've reformed all sorts of caps when I was first learning, but these days I only bother on caps that are very expensive to replace, not easily replaceable, or I'm in a bind and want to fire something up before waiting for parts to arrive. So, mostly large screw terminal capacitors on gear from the 80's and newer. If you can even find them, they can cost upwards of $90 each at times, and some equipment was built around the very specific screw spacing on the originals - often holes in the PCB that they screw through, so it's not practical to replace them (HP test equipment, I'm talking about you) It seems like the spec on most datasheets for acceptable leakage is .03CV(capacitance in uF * Voltage) after 1 minute, or .01CV after 3 minutes, so that's what you need to check if you are successful, along with capacitance and ESR

You have some very nice equipment.

 

[egellings]

I have taken NOS caps that I was given in boxes of parts and used my bench power supply to reform them, or at least make them less leaky- electrically. The bench supply has a current readout in the mA level. I’ll start at a couple volts and watch the current. When it drops to zero I raise the voltage some more and repeat. You can watch the current drop as the cap reforms.
Testing them before and after shows measurable improvement in leakage.
I have only done this on non hifi stuff where the cap failing again is of little consequence.
If it happens in an individual part it must happen in long dormant Stereo units also.

I think that the current drop you observe as you ramp up the supply voltage is just charging current, not leakage current dropping off. The reforming process takes a long time by comparison.

 

[gadget73]

I have done it on really old caps when I'm just trying to see if something has enough signs of life in it to rebuild without spending time and money on caps first, but it doesn't always work. I've had caps from the 60s and before come back and work perfectly, and had just as many either not even try, or work for an hour then decide they were done with life.

working on a 1940 Stromberg-Carlson right now. 3/4 sections in the original filter cap seem to be working with no appreciable leakage. The lowest voltage section leaks like crazy though. Luckily thats the part that feeds the radio and I'm only fooling with the amplifier section right now.

 

[gadget73]

heh, 25 minutes at normal voltage and it started getting really hot even with the leaky section unhooked. And this is why old capacitors are not to be trusted.

ah well, a tube I don't have is dead so the amp won't run anyway. I've got caps and terminal strips, no big deal to refresh this while I'm waiting on a replacement tube.