Reforming NOS JJ caps

[casey73]

Getting ready to build what I call a "Junk Yard Dog" or JYD amp. I've accumulated lots of parts and decided to throw together a small amp in an abandoned chassis/cabinet I have. I have three new JJ 50/50uf 500VDC caps that are at least 8 years old. Should I reform them? And if so, is there a preferred method? I've seen several methods, some including a current limiting bulb, others using a current dropping resistor while monitoring voltage. Everything I've built in the last 20 years was made out of newly purchased parts so I've not come across a situation where I have NOS caps. I could purchase new caps, but in the spirit of JYD, I'd like to use these caps if practical.

[R.G.]

I've always been fond of the series resistor method.

What you're trying to do is make the insulation resistance of the cap get big enough to withstand the voltage without over heating the cap. A big resistor in series both limits the re-forming current down to safe levels for forming, and for telling you where you are.

It's faster if you have a DC supply near the voltage you want to form them to, and then run a 100K or so resistor to the + terminal, grounding the (-) terminal. When the voltage across the resistor gets down to less than 1/10 of the total DC supply, it means that the cap's insulation resistance is nine times that big. From there, the leakage currents will usually be small enough to complete any forming on the job.

[SoulFetish]

I ask this as a recovering vintage component snob, and someone who will gladly make more work for myself to avoid spending money;
But Isn't life too short to reform electrolytics?

[casey73]

I've always been fond of the series resistor method.

What you're trying to do is make the insulation resistance of the cap get big enough to withstand the voltage without over heating the cap. A big resistor in series both limits the re-forming current down to safe levels for forming, and for telling you where you are.

It's faster if you have a DC supply near the voltage you want to form them to, and then run a 100K or so resistor to the + terminal, grounding the (-) terminal. When the voltage across the resistor gets down to less than 1/10 of the total DC supply, it means that the cap's insulation resistance is nine times that big. From there, the leakage currents will usually be small enough to complete any forming on the job.

Thanks for your input R.G.. I have the PT I'm going to use for the amp so thought I'd set up a breadboard style DC supply for the reforming process. Good to see you are still around the forums. I've been away from forum activity for a while. I worked at CE Distribution for 12 years. I left 17 months ago when I retired a second time. Don't know if you will remember this but I took a special order from you many years ago. You called and asked if you could purchase some empty FP cap cans that we used to make the multi-section Mallory style caps. It wasn't something we normally did but I told my boss about your background and he gave the okay.

[casey73]

I ask this as a recovering vintage component snob, and someone who will gladly make more work for myself to avoid spending money;
But Isn't life too short to reform electrolytics?

You have a valid point. And were I building this for someone else or was in a hurry to get it done, I'd just get some new caps. But I'm in no hurry so I'm going to give it a shot. I've accumulated a large number of parts over the years, enough to build several small amps. So it helps me pass the time and try new ideas.

[gktamps]

Along the lines of R.G.'s comment of the series resistor, I use my vintage Sprague TO-6A Telematic to routinely re-form caps that are waiting to go into amp builds if they have been sitting more than 6-9 months. It has a number of features that work well for this. I learned the hard way about watching the meter and not allowing the cap to get too warm, and to not expect much success with 30 year old multi can caps!

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