Forming Caps with Variac

[Reeltarded]

Yep!

[Structo]

Sure since you have some go ahead and try forming them.

Follow Larry's instructions.

If you don't have a power supply set up on a jig then use the amp power supply but be sure to disconnect the rest of the amp.

Just have the HV, into the 100K with the cap across that.

Be sure the power supply does not exceed the working voltage on the cap.

[Prairie Dawg]

Thanks Guys for all the good input! I have a better feel for the forming process now.

One reason I am contemplating re-forming caps is because it turns out I have a few NIB can caps in my junk box. I am wondering if they are worth trying to use in a build.

They are 16 uF at 600 VDC (perfect!) with date codes of 5-69 (May 1969?). They are obviously NIB.

Whaddya think? Are they worth the trouble to mess with?

I don't think so. I've reformed electrolytics in the past using a TelOhmike as well as a special purpose rig I made for testing capacitors, monitoring them for current draw and keeping leakage under 2ma all the way up to working voltage-they just never seem to work as well as fresh stock. If it's all you can get, that's fine, but it has never worked for me. Hitting them with anything resembling full voltage to start with is defeating the purpose-you have to ramp up your DC voltage slowly while monitoring leakage.

[IMG:480:640]http://img.photobucket.com/albums/v435/ ... ool1-1.jpg[/img]

[Kagliostro]

Here a circuit which purpose is to reform electrolytic capacitors

unfortunately text is in italian

http://www.leradiodisophie.it/Download/ ... ettrol.pdf

K

[rooster]

You didn't say what brand did you? If they are Sprague or Mallory then I say you will be OK. But I still think you need to warm them up before you begin the Larry process. The oven sounds scary to you perhaps, but I would still recommend it. I have done it a few times and it always turned out fine.

Getting back to another story from John Fouche, he told me how they used to get the old Fender tweed amps fired up that they had around the Fender shop when someone wanted to play one. He told me they used to use a commercial hot air gun to heat the caps before play time. Yeah, I know it sounds cavalier but he said it wasn't too different from the oven technique and it worked fine.

And since we are on this subject of heat and the electrolytic caps in the power supply, it was these very conversations with John that caused me to rethink the idea of fans in guitar amps. I decided that heat was a good thing for the dielectric in the power caps and have never looked back. I can't report a problem with this behavior for the over 20 years of messing around with guitar amps either.

[gingertube]

That Italian Circuit is pretty much what I used to use for reforming 10,000uF / 500V electrolytic capacitors from Medical Defibrillators when I was an electronics apprentice in the Biomedical Enginering Dept at a major hospital - back in the '70s.

I used a variac into a power tyransformer and a rectifier with meters for capacitor voltage and chrage current.

Start at low voltage and monitor the current into the cap which consists of charge current and leakage current. Initially the caps have large leakage current which dies away as the caps reform.

So you set a low voltage and waited for the current to drop to less than say 10mA, turn up the voltage and wait again, till you reach rated voltage.

The circuit will evetually settle to just the caps leakage current -
Use the manufacturers datasheet to determine what the leakage current should be, if you don't have any data use I = 0.006CV (I in Amps, C in Farads, V in Volts) to calculate typical leakage current value.

If the current won't settle to say no more than X2 that calculated value after 8 hours or so then chuck out the cap as being past its used by date.

Cheers,
Ian