I think I will replace all the tantalums, unless anyone would suggest otherwise.
While tantalums can be problematic after an extended period, I have seen them last much longer than 30 years, I have also seen them fail in much shorter times. These caps often do not show any outward signs of pending failure, they just die and unfortunately shorting out is a fairly common death for them. Many factors go into how long they are going to last, including circuit design, operating temperature and quality.
Tek usually doesn't underrate their components, I see no reason to change out the caps on that premise nor do I subscribe to some arbitrary operating voltage limit on caps, if they are within their ratings, there is no need to increase their voltage ratings, this is a fallacy running around, likely caused by people using cheap quality parts. A rule of thumb of 66% is not only arbitrary but baseless, except for cheap quality parts, there is no need for such derating.
If a tant bead has failed, replace "similarly rated" tants - especially 15V tants on a 13V line - before they take out nearby components. (On 485s the 13V "rail" isn't on the main PSU schematic!).
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If a tant bead has failed, replace "similarly rated" tants - especially 15V tants on a 13V line - before they take out nearby components. (On 485s the 13V "rail" isn't on the main PSU schematic!).
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By "tant bead" you mean the little dipped tants?
The others are operating well below their max voltage. Initially I thought to replace all tants similar to the one that failed, but now I feel that it's best just to replace the one with a better rated cap, attributing its failure to stress.
If a tant bead has failed, replace "similarly rated" tants - especially 15V tants on a 13V line - before they take out nearby components. (On 485s the 13V "rail" isn't on the main PSU schematic!).
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By "tant bead" you mean the little dipped tants? (In the case of the tek, the color coded dipped tants look like little pieces of candy all over the place). That's kinda my concern-- there's about eleven of the same tantalum on the trigger board. It's a 20V 15uF tant. One of them failed short; however, it was the only one that operated very near its maximum voltage (75%). The others are operating well below their max voltage. Initially I thought to replace all tants similar to the one that failed, but now I feel that it's best just to replace the one with a better rated cap, attributing its failure to stress.
Here is link to catalog of Tektronix part number to vendor part number cross reference 27MB .zip from Dave Miller AA4DF. Very useful for repairing 400 series. Note Tek bin ranked many parts; transistors by beta and breakdown voltage etc.
What's so special about this scope?