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Capacitor life and death

Discussion in 'Amps and Cabs [BG]' started by Jammin Johneboy, Nov 22, 2013.

  1. How old would an amp or any piece of electronic gear have to be before you started thinking about replacing its capacitors ?

    I know electrolytic capacitors diminish in value over time , do other capacitor types also diminish with age ?

    Almost everything I buy is used , thats why I am wondering . Many years ago I had a Roland digital delay I bought used . When I plugged it in and listened it was obvious that this digital delay had terrible bass response . I replaced all the electrolytic capacitors in it and the quality of sound improved tremendously.

    Other than an obvious problem like hum or poor frequency response, about at what age do you start to question the condition of capacitors and think its time to replace them as a maintenance type repair so that things continue to operate at the level that they should ? Or do you just wait for an obvious problem to show up ?

    I imagine since capacitors diminish so slowly over so long a time period , a person could even have an amp or other gear they bought new, that slowly over time has lost some of its quality of sound. But having happened so slowly over such a long period of time not realized the loss till its extreme .

    Any thoughts ?

  2. replace them when they're out of spec. Not all capacitors are created equal
  3. Capacitors, like anything else, could fail at any time, regardless of type. Obviously we'd like to be able to somehow determine a component is on the verge of failure but of course sometimes there's no advance warning.

    Electrolytic caps do have a known failure mechanism; they depend on sealing to keep the moisture that is inside the cap from drying. Seals like anything else aren't perfect. Some caps must've wound up with a pretty darn good seal and lasted forty or fifty years, but those are exceptional IMHO.

    I have a Hewlett Packard signal generator from the early 60's; at the time, HP was considered one of the best (if not THE best) test equipment company in the world and they are known to have used quality components. Their manual boasts: "The electrolytic capacitors in this instrument are high quality units which have a useful life of from five to ten years." The sig gen still worked with its original caps when I recapped it maybe 10 or so years ago. However, it should be obvious that the caps were basically living on borrowed time at that point.

    So, what's my take now? If it's more than 25 or 30 years old and I'm going to depend on it--like an instrument amp I'm going to be gigging with, or test equipment like a sig gen--it gets recapped. If it's just something like a stereo receiver in the basement, I would wait longer, I've got some stuff around here from the 70's that I am not in a rush to recap.

    Stuff older than that though, say anything from the 60's, I would say it should be recapped. Anything older than 1960, new caps go in before I even turn it on.
  4. One problem though, most capacitor test equipment measures capacitance, ESR, and leakage at only a few volts. Tube amps, particularly, may run at a B+ at 500 volts or even higher. Even solid-state amps can have rails of 85 volts. A cap that tests fine on a simple DMM could leak like a screen door at high voltage.

    Caps are cheap nowadays. IMHO replacing them is good preventive maintenance.
  5. I do tend to replace them given the choice. Some customers don't like you ripping all their original bits out of their amps though, so you have to proceed with a little more caution in those cases. Yeah testing them with a multimeter means nothing pretty much.
  6. RadioRob

    RadioRob Supporting Member

    Dec 15, 2012
    Whar are signs of a bad or failing capacitor?
  7. petrus61

    petrus61 Supporting Member

    Electrytics can also rate higher over time (.05 reading .069, etc)
  8. chucko58


    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    I would only worry about electrolytics and wax- or oil-and-paper types. Every other type of cap should be just about immortal if not abused beyond its voltage and temperature ratings. Some ceramics may drift in value with age, but a good design won't use them in applications where the value is critical.
  9. Gaolee

    Gaolee It's all about the polyester

    I have played through old solid state amps for a long time. When they start sounding a little ratty, that's when I take 'em in for work. In general, the issues tend to be caps, but they have lasted a very long time. It's probably clear from this post that I don't know diddly about the inside of the amp, but I'm pretty aware of what comes out when it's plugged in. That's how I decide when an amp needs something. Or when it doesn't do anything at all. That's happened a couple times, once from a blown capacitor and once from a blown resistor. I don't know why either happend, but when it did, it was pretty obvious there was a problem.
  10. BbbyBld


    Oct 13, 2005
    Meridian, MS
    The main killer of electrolytic capacitors is heat. The next factor, which is also related to heat, is ripple current. Ripple current basically means the rate and amount of the capacitor charging and discharging. If you have an old piece of equipment, as long as the ambient operating temperature and ripple current is low enough, electrolytics will last a very long time. Exploded or swollen electrolytics on newer equipment usually means the ripple current is too high, in which case either the wrong type of capacitor was used (standard vs Low ESR), or more capacitance is needed. A typical electrolytic is rated at 85C. What that usually means is that the life rating spec. of the cap is at that temp., so it might be 3000 hours at 85C. As the operating temperature goes down, the life rating goes up significantly.

    Some types of film capacitors are hygroscopic and will have a limited life span.
  11. PazzoBasso


    Jan 21, 2011
    Subbed... & waiting for the first "vintage caps sound better" post :)
  12. Indeed.

    I am in the computer business, and do a lot of board repairs for the above reason.
    10 years back, we had a huge infusion of CCS (cheap chinese ___) or affectionately named "craps".
    These drove ABit mother boards out of business entirely from class action suits.
    Dell GX270 and similar products had giant recalls in place for mass capacitor replacements.
    Ref: www.badcaps.net

    Old amps don't have craps installed.
    All electrolytics are "wet" by nature, and can/will eventually dry out.
    As noted above B+ voltages can be present, and cap servicing can be LETHAL for those working under cranial-rectal inversions.

    IMO, a proper recap on an old amp is every bit as warranted as new tubes + bias.
    Do it right, do it once, cry once.
  13. petrus61

    petrus61 Supporting Member

    Be careful of unscrupulous tecs that want to re-cap your vintage amp just to harvest the paper in oil/wax electrolytics for later resale at ridiculous prices. The amp may be fine only to have its astrons and blue molded's gutted and replaced with ceramics or mylar. It's not a bad thing, but ask for the old caps back.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    In amps, electrolytic capacitors in the power supply and bias circuits need to be replaced with time. If an amp is sitting unused, it is a good idea to turn it on, not in standby but in playing mode, at least every six months or so for half and hour. This will help the caps last longer. Electrolytic caps are wet inside. A chemical reaction occurs that causes the cap to deteriorate with time. Using the amp reverses this process, "reforming" them to some extent and helps the cap attain a long service life. As was mentioned a cap will dry out with time. Heat is a killer of caps. Heat makes it dry out faster. Over voltage is also harmful to caps.

    There is more information about power supply caps here. Here are a couple of application notes discussing electrolytic capacitors and how they work Note 1, Note 2.

    How long does a cap last? It varies depending on the product. Some caps are rated high temperature and long life, some aren't. In a regular gigging amp that you depend on, replace them every 5-7 years. Other amps have 15 year old caps and they are fine. It depends on how hard you push your amp, operating conditions, etc. Amps can go for years longer but changing them affords you some insurance.

    What are the signs of a bad cap? As a power supply cap ages, it slowly looses its ability to hold a charge and be recharged. The amp will loose headroom and distort sooner. Eventually you will hear a 60 or 120 Hz hum. It will get louder as the cap ages.

    When an amp is serviced, it is a good idea to change all the electrolytic caps at the same time. If you try to do it part at a time, the amp will be back in the shop sooner rather than later and it will cost more to fix in the long run with multiple visits. Also a new cap could put more stress on older caps down the line which could hasten their failure. There are certain components that are best changed with time. It costs more to check them out than it does to simply put new ones in. Power supply resistors that run hot should also be check to see if they are within spec. In a tube amp, this would include the power amp plate, screen, and cathode resistors.

    Then there is the so called "death cap" at the input of the power transformer. It is part of a line noise filter. It isn't an electrolytic cap but needs attention. When it fails, it can cause you to get a shock. If you've ever touched your lip to a mic while playing or touching your stings and got zapped, you'll know what I mean. More info on that here.
    Last edited: Oct 26, 2015
    MuthaFunk likes this.
  15. Gotta love Talk Bass , provider of great information with a little good hearted humour thrown in for good balance . lol

    I have heard the term "dried up" refering to electrolytics , but did not think they litterally dried up. I always thought it just meant , going bad . Interesting to know that they actually have moisture inside them which can eventually dry up .

    I did not know about digital multi meters (DMM) being inacurate due to the low voltage applied to the capacitor . But it makes sense that a capacitor operating at voltages higher than the multi meter puts out during its test , will not likely test correctly . New info for me - good to know .

    Also intersting to know about heat and ripple current's effect on capacitor life. I can see how that would add to the drying out of capacitors . Plus I never knew that some film capacitors are hygroscopic ( meaning they absorb moisture/water) .

    petrus61> Electolytics increasing in value is a whole new one to me , I could see this happening potentially in humid climates ?

    "A cap that tests fine on a simple DMM could leak like a screen door at high voltage." , great analogy nashvillebill .

    Everyone if I did not refer to your particular post , I want you all to know I read and appreciate every post here .

    Oh and , ........ "Do vintage caps sound better ? "
    Kidding , I was just kidding , honest !
  16. Gaolee

    Gaolee It's all about the polyester

    You are waiting for the wrong thing. Vintage transistors sound better. Wood, er, caps don't matter.
  17. beans-on-toast , Thanks for the great links , I'll read them after posting this . I know what you mean about just changing capacitors all at once . With the digital delay repair that I refered to , I figured it was easier , faster and better in the long run to just change them all instead of spending all kinds of time testing each one . They were cheap to buy.
  18. What about vintage wooden transistors ? LOL
  19. BbbyBld


    Oct 13, 2005
    Meridian, MS
    It depends on what you mean by better. They are certainly better at sounding vintage.:D
  20. Up to point they short and take out a $200 power transformer anyway? :p