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Old 16th January 2013, 07:02 AM   #1
atnaat is offline atnaat  Wales
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Default Can capacitors, how do they fail?

Just got a leak stereo 20 so I can have something decentish to listen to music through my computer, it's been recapped many years ago and then not used for decades. Everything's in the post to refurb it but I'm pretty impatient, brought it up slowly with a variac, been on for about an hour now but I bottled it and turned it off. The smoothing caps are in what appears to be a sealed aluminium cylinder with no pressure release, if these go will I get any warning?
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Old 16th January 2013, 07:06 AM   #2
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Quote:
Originally Posted by atnaat View Post
Just got a leak stereo 20 so I can have something decentish to listen to music through my computer, it's been recapped many years ago and then not used for decades. Everything's in the post to refurb it but I'm pretty impatient, brought it up slowly with a variac, been on for about an hour now but I bottled it and turned it off. The smoothing caps are in what appears to be a sealed aluminium cylinder with no pressure release, if these go will I get any warning?
Pressure release will be at the base of the cap. You may, or may not get any warning.
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Old 16th January 2013, 09:22 AM   #3
tomchr is online now tomchr  United States
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You say it's been recapped. So it has relatively modern caps? Those will have a pressure relief of some sort. Usually, the metal top of the can is scored so it breaks open in a predictable way. Or on chassis mount caps, there's a pressure relief valve by the pins (this is usually fairly obvious - a circle 3-4 mm in diameter).

I've seen some electrolytic cans where the electrolyte has leaked out. This looks like battery corrosion by the pin end of the cap (or anode/plus end for an axial cap). But a lot of the time, the electrolyte has just dried out and the cap has lost its capacitance. This leads to hum but usually doesn't let the cap blow up.

But if you can bring the amp up on a variac and run it for an hour, I'd say use it. You may try measuring the temperature of the caps with an IR thermometer or something. If they got hot (say > 50 deg C), replace them. But if the amp can run for an hour without anything blowing up, I'd trust it.

If the amp has electrolytic caps in it that do not have a pressure relief of some sort, I'd say replace them. Even if they're not bad now, who knows when they decide to blow. Caps without a pressure relief will explode like a hand grenade.

~Tom
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Last edited by tomchr; 16th January 2013 at 09:24 AM.
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Old 16th January 2013, 10:29 AM   #4
DF96 is offline DF96  England
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The way to check electrolytics is to monitor their leakage current. Either use a separate HT supply, or insert a resistor (10K-47K?) in series with the rectifier cathode. (Or HT secondary CT ground link). Pull out other valves. Monitor the capacitor voltage, so seeing how much voltage gets dropped across the resistor. If it rises quickly the cap is fine. If it rises slowly then the cap is reforming itself. If it get stuck then the cap is not reforming so switch off and replace the cap.

A variac does not give you any information. The cap either works or it goes bang.
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Old 16th January 2013, 01:20 PM   #5
atnaat is offline atnaat  Wales
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Nevermind my new caps have arrived! It sounds so much nicer than the transistor amp it replaced, even if the circuit is a bit flawed. Can't wait to modify this thing.
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