Attention Gurus- New Unused Caps- Shelf Life and Should I Charge Them

[silverfox]

Que the Wives Tales, Fables and Carmudgeon Disputes now. Seriously though, I need learnded opinions from experienced peoples and appreciate advice as I am half full cup student.

I purchased many electrolytic caps at about 2 years ago. I don't anticipate using the majority of them any time soon.

Should I jumper them into a B+ circuit to form them or will they go bad soon?? The shelf life is what? 5 years or 2 years I think. After they have had B+ put to them for say 24 hours, will they be okay for a couple years??



Silverfox- Half a Loaf of Kung Fu.

[olddawg]

I can't give you anything but anecdotal experience. But I have used electrolytic caps out if stock that are 20+ years old with no problems. I would not deliberately purchase old caps, but have never had a problem with old stock off of my own shelf.

[g1]

I would think the shelf life would be longer if no voltage is ever applied until actual installation for use.

[Enzo]

I have been soldering and using caps for almost 60 years now - 1954 was my first electronics kit - and in that entire time, I have never found the need to "form" a cap.

[Joe L]

Realizing that I'm on thin ice disagreeing with Enzo, I just had to contend with unformed caps on my '74 Marshall Superlead last Saturday.

I turned on the amp at my bench and when I switched it off standby, it promptly blew the 1 amp HV fuse. I suspected the LCR caps that I replaced about ten years ago so I put a 100K resistor across the standby switch and put in a 1/2 amp fuse. I pulled the tubes except for the PI so that I wouldn't stress the 400v rated film caps and when I flipped off standby, the fuse held. I monitored the preamp voltage until it stabilized at around 275v on the screen resistors after 1/2 hour. At that point, I swapped out the 100k for a 22K and powered back up. After about ten more minutes, the voltages were high enough to trust the caps so I removed the resistor. I put everything normal except for the 1/2 amp fuse and successfully powered back on. I let it cook for a few minutes but it started popping loudly so I shut everything down and did some drinking - it was Saturday after all

The next day I fired it up with some sacrificial output tubes and determined the problem was not PA related and subsided after about ten minutes.

I'm fortunate to have a Sencore Z-meter so I put the amp to the side for now but I am going to fully analyze the LCRs when I feel like getting back on it.

Also, this link should shed some unbiased light on cap reforming.

[Chuck H]

I don't want to be a turd in the punch bowl, but I disagree with all of the above

I've had many electrolytics go bad in the drawer/on the shelf. Sometimes the performance improves and they actually do cook in and perform fine. More often they get worse.

I use to use Atom caps from Mouser. I was having a lot of trouble with new caps that weren't doing their job. Admittedly never tested, but the symptoms were those of bad electrolytics and replacing them fixed the problem. More than one person here told me that it could have been a shelf life problem. fter looking into the product I decided to stop using them for other reasons anyway. But one thing I found along the way was that once upon a time, yes there were expiration dates on caps. Most were five years.

Caps dry up and become useless faster when NOT used than when they are (thus, your inquiry about periodic charging). There was a long time when mid to late 90's era Fender amps seemed to be suffering disproportionate electrolytic failures after only six to ten years. I think these were IC brand caps. Imagine one of those caps after five unused years. Well...

I got to see what those IC caps do after twenty years anyway. Two brand new amps. Never used and still in their original cardboard boxes. NOS amps. Both required a full recap. One roasted, popped, hummed and smoked in about three minutes. The other was brought up slowly with a current limiter and left on the limiter for about eight hours. That amp worked, but hummed. We did replace the caps soon thereafter.

Problems with old caps are real. Ignore the possibility at your own risk. I recently tossed some old caps that were about ten years old. Found them in a box in the closet. I probably wouldn't put a cap over two years old (from my inventory anyway) into a customers amp. Only because of my personal experiences. I wouldn't feel right about it.

Not accepting flames on this one guys. These are my own personal experiences on the matter and I don't like it any more than you do

[Joe L]

Chuck, I completely agree if the amp isn't mine. And if I still gigged, my stage amp would have fresh caps. But I have so many amps around the house that I would go broke keeping new caps in them.

Yeah, I know. Sounds like a high quality problem

[Enzo]

Joe, disagree with me all you like, that's part of me learning.

[g1]

To get back to the question about new unused/never installed caps. I'll assume any new gear is fired up at least once before it goes in the box, so I won't consider those caps unused.
Re-forming caps in used gear that has sat may have merits, sure won't hurt anything.
But the caps that you have never put a charge to and are sitting on the shelf, will they last longer on the shelf if you put a charge to them, and periodically re-form them? My opinion is the failure rate would probably be about the same, or maybe the "never charged" would have a longer shelf life as the electrolyte has never been disturbed/activated.

[silverfox]

Wow Enzo that's a long time in the trade. Thanks for the thoughts everyone.

Are the newer caps as susceptibel to leakage as they were 4 years ago? It seems to me that leakage is the killer if the caps are formed when put in a circuit prior to full power use.

Silverfox.

[Tooboob]

Talk of B+ cap failure made me nervous. I haven't had a problem with it, and I've got quite a few amps, but people keep replacing them just to be safe. I told myself that the talk is driven by the vintage amps, with ancient paper/oil caps. Change them, and you risk losing the magic and degrading the amp's value (perhaps unfairly), and they do die. I've been using large caps in my designs at work, and I worry much more about reliability there, so I did some research. Big tantalum caps still stink. Manufacturers talk about 2000 hour product life. Aluminum electrolytics can have million hour life, but their ESR is significantly higher. I don't know about OSCONs. It's all dependent on ripple current, voltage, and temperature.

Since it is an issue, manufacturer tend to provide clear reliability guidelines. The answer you want depends very much on the capacitor technology and manufacturer. I didn't see anything in my short search on "re-forming", and it may indeed be a solution to a problem only vintage capacitors and their late model clones have. I suggest that you research your particular capacitors at the manufacturer site, and even pose the question with them (reps don't have much to do these days...).

[R.G.]

Let me suggest a middle ground - test them. Hook a new/unused cap up to the rectifiers/etc. it will eventually power, but (1) temporarily open the loads by removing tubes or opening bleeder networks, and (2) temporarily hook a 100K resistor in series with the + terminal of the cap. Now measure the voltage across that 100K resistor. If it's less than 10% of the total voltage, you're probably good to go, no issues with the cap. If it's more than 10%, be suspicious. A cap that leaves more than 10% of the incoming DC across a 100K resistor has a leakage resistance lower than 1M, so there may be leakage issues.

By the way, leaving a leaky cap in this setup until the voltage across the 100K resistor drops to less than about 5% of the incoming re-forms it. If it won't slowly go to less than about 5%, it's permanently damaged. "Slowly" may mean " a day or so". Or you can simply ditch it as too leaky and/or too risky.

[Joe L]

I have an interesting case. I bought some Mallory FP240 50/50 450V caps in '88. I have two of them that are still nos except for reforming them every few years. On my cap tester, I just got the following readings:

62 uF
Dialectric Absorbtion 2% (good)
ESR .16 ohms
Leakage at 450v - 126 uA

Knowing this, would any of you throw this cap away? Would you hesitate to use it?

[nickb]

According to Vishay's paper on electrolytics, they say they may be stored for up to three years under suitable conditions and then full rated voltage may still be applied. After that time no specified recommendation is given, but my reading is that the full rated voltage should not be applied.

[R.G.]

Knowing this, would any of you throw this cap away? Would you hesitate to use it?

Been re-formed occasionally and still tests good? I'd use them.