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What causes caps to become resistive or otherwise unsuitable?


garyrc

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When caps become resistive, or otherwise unsuitable, is that due to age alone, use alone, or both together? Do caps that have current run through them everyday last a longer or shorter length of time than caps in storage?

Everyone seems to talk about replacing caps in speaker crossover networks, but how about caps in amplifiers and other common pieces of equipment?

If one buys a used amplifier, that is 5, 10, 20 years old, would the caps need replacing? What is the usual life of a piece of electronic equipment (solid state) if it is used every day?

Thanks

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garyrc,

not being a tech person, i can only share what BEC shared with me on the question of caps drifting off spec and becoming resistive with age. 20 or 25 years in a crossover will often be a candidate for replacement. However, he also notes that caps that live in the sealed cabs of Heresy or Cornwall seem to have longer lives than the caps that reside in the open backs of LaScalas or Khorns.

that suggests it's not just age as a factor

I have been astonished at the improvement of sound in my old Heritage sweethearts that simple recapping has brought. This IMO is as close to factory spec and sound as one can reasonably expect for a minimal investment. And the sound is very good to my ears.

i'm also curious about how that issue translates into the performance of vintage SS gear. Would love to hear more from some of the resident experts.

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Capacitor life is related to many factors including internal core temperature, operating voltage as a percentage of maximum voltage rating, ripple current, surges, and so on. For example, an electrolytic cap may be rated for 2000 hrs service at its maximum voltage/ripple current/core temperature. But, if used far below those max. ratings, the life could be 50,000 hours or more. So, it's hard to make a blanket statement. And, electrolytic caps can actually go bad if not used enough. As far as a particular age at which they need replacement, there's really no answer. I've worked on communications receivers from the late 40s and found the electrolytic caps were still fine. Many of the coupling and bypass caps which were used in the equipment of the 40s, 50s and 60s were crap (Sprague, in particular, made a cap which worked great when new- the only problem is that they aged rapidly and were very prone to failure.) On the other hand, many still perform like new. So, replacing caps is definitely a judgment call based on all the potential issues which may arise. Caps in 5 year old equipment which was left on 24/7 may be in worse shape than those in something 25 years old.

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Many of the coupling and bypass caps which were used in the equipment of the 40s, 50s and 60s were crap (Sprague, in particular, made a cap which worked great when new- the only problem is that they aged rapidly and were very prone to failure.)

When I was a teenager I took a bad cap apart and discovered that it was made of wax paper and aluminum foil wound into a cylindrical shape, mounted into a cardboard tube sealed with wax. The foil was simply wound around the wires with no soldering or bonding at all. No wonder there were so many leaky caps and cap failures back then. I believe that the better manufacturing methods for modern caps is a major reason for the improved performance of amplifiers these days.

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When I was a teenager I took a bad cap apart and discovered that it was made of wax paper and aluminum foil wound into a cylindrical shape, mounted into a cardboard tube sealed with wax. The foil was simply wound around the wires with no soldering or bonding at all. No wonder there were so many leaky caps and cap failures back then. I believe that the better manufacturing methods for modern caps is a major reason for the improved performance of amplifiers these days.

Thankfully, the paper and foil variety wasn't extensively used in the better audio equipment. Radios and TVs were another matter! It is very common to find the ends of such caps literally blown out like tootsie rolls (especially notable when used for power line bypass.) What's amazing is that some equipment continues to function in spite of it. Many times someone brings me a radio to restore which they plugged in and found that it still plays. When looking under the chassis I can't figure out how that can possibly be the case. And, you are absolutely correct about modern capacitor construction. Better in every respect, including long term reliability. In addition, the size reduction in comparison with the caps used back then is wonderful as well making for a much neater appearance post-restoration.

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Thanks, guys.

In this day when some writers want you to burn in everything from your speaker wire to your coffee table, does burning in caps do them any good, or does it just contribute to their wear?

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Thanks, guys.

In this day when some writers want you to burn in everything from your speaker wire to your coffee table, does burning in caps do them any good, or does it just contribute to their wear?

You will get many different opinions on that question! Personally, I consider it totally unnecessary with one exception: when I order electrolytic caps to keep in stock for use when needed, I "form" them (using a device which charges the cap up to it's rated voltage very slowly over a long period of time, and discharging slowly as well) before putting them away. Doing so helps them to maintain their properties when not in use. In fact, I have some caps which are 5-7 years old which are every bit as fresh and functional today as they were when first purchased. Generally, though, I prefer to form them annually for good measure. There are some good online articles which explain the forming process in detail. You need to be careful about where the electrolytic caps are purchased, and shouldn't get them from a supplier which doesn't rotate its stock constantly. Buying an unformed cap which has been sitting in the bin for 10 years isn't a good idea.

Maynard

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Heat is the great killer of signal caps. Power supply caps usually fail when the rectifier(s) die and the caps are hit with pure mains AC. The big electrolytics in power supplies start out somewhat over labelled value and then the value declines slowly over time.

If you are buying an older tube amp, the caps all need replacing (heat aging). If you are buying an older SS amp, you need to see what the signal paths caps are. Not a few phono preamps had small electrolytics at the inputs that should be replaced with MKP or better caps, Any signal path caps need to be at least MKP, but physical size may be an issue getting them installed. I recapped a few 70s receivers and had to get creative installing caps that were alrger than the stock units.

For crossovers, I'd say that any speaker over 20 yrs old, or any that came with "Mylar" or electrolytic caps, need recapping with MKP units. I recently recapped my Corn IIs, and had to mount the big honking 68 uF MKP on a bracket on the crossover board, since it was about 10X larger than the electrolytic it replaced.

PS I have had exactly one (1) film cap go bad in a crossover, to the point where it was making a midrange driver audibly lower than its mate in the other system. This cap was about 35 years old at the time.

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