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I have a Fender solid-state amplifier nearing its 40th birthday. I've been going through it, replacing some failed logic chips and updating some of the op-amps (gaining about 10dB in S/N ratio!).

The low-voltage (±18V) DC supply has typical 220µF 25V electrolytics. I haven't tried to test them but they seem to work OK. But they're almost 40 years old! The amp has been used only sporadically over its lifetime—I wouldn't hazard a guess as to total hours but it's not many.

Should I replace them? I'm thinking yes, but if total hours are low, is it necessary?

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  • \$\begingroup\$ Insight into how long electrolytic capacitors can last \$\endgroup\$
    – Andy aka
    Apr 16 '20 at 15:40
  • \$\begingroup\$ I have fitted new bulbs that blow after 30 seconds, others that last ages... This is like asking how long is a piece of string? ( exact answer : twice the distance from the middle to one end..) But the point is they may continue happily or the new ones blow in short order.. you takes your choice. \$\endgroup\$
    – Solar Mike
    Apr 16 '20 at 15:40
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    \$\begingroup\$ anecdotally, many of the life-limited caps we hear about are from the late 80s and mid 90s, when they found a cheap way to make them that ended up sucking long-term. Older and newer caps have better life expectancy. \$\endgroup\$
    – dandavis
    Apr 16 '20 at 18:36
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If you're already in there messing with it all over the place, I'd replace all the big ones for sure. All of them if you like. With replacements from a top-tier manufacturer.

Otherwise, no need to break something that isn't broken (broken would be excessive hum or possibly intermittent oscillation).

Lifetime is related to time and temperature mostly, and temperature goes up when in use, so the lifetime is reduced. 40 years is not uncommon for good capacitors kept cool, but it's also possible some have excessive ESR. It's usually not a catastrophic failure, just an increase in ESR (equivalent series resistance) as the electrolyte dries out. At some point the circuit doesn't work well enough. In the case of your power supply that would be excessive ripple and probably some resulting hum. If there is regulation in the power supply the onset of hum might be more sudden than gradual.

As a rule of thumb life is halved for every 10°C temperature rise, so it's usually good to buy 105°C-rated capacitors rather than 85°C, all other things being equal. The lifetime ratings at full temperature are very short (thousands of hours only). Higher voltage rating than the original is also better. Since capacitors have gotten smaller over time for the same ratings you can definitely substitute a really good part that fits in the same space as the original.

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    \$\begingroup\$ One thing in its favor is that the amp, being solid-state, doesn't have fire bottles in there cooking all the other components. It runs quite cool. \$\endgroup\$
    – Rob Lewis
    Apr 16 '20 at 15:48
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    \$\begingroup\$ Regarding your comment about oscillation: at one point during this project I was getting some "motorboating" of the amp. I never found the source—it could have been a loose connector—but it seems to have gone away. I did find that one of the original 4558 op amps had no supply bypass caps so when I replaced it with a 5532 I added them. Probably helped. \$\endgroup\$
    – Rob Lewis
    Apr 16 '20 at 15:53
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    \$\begingroup\$ motorboating can be a symptom of a high impedance power supply. \$\endgroup\$
    – Neil_UK
    Apr 16 '20 at 20:23
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It can actually be worse if they are not used for a long time. Being used helps keep the dielectric in good shape.

What ages them is a chemical reaction that often goes faster if they are not charged.

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    \$\begingroup\$ I know electrolytics can be "re-formed". Does this undo any of the degradation? \$\endgroup\$
    – Rob Lewis
    Apr 16 '20 at 15:43
  • \$\begingroup\$ Yes, this can work quite well. I wouldn't consider doing it here, however. Replacing is cheap and easy since you are already into it. There can be other types of degradation that re-forming the dielectric won't fix too (seals being a major one). \$\endgroup\$ Apr 16 '20 at 15:45

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