Boards are obviously originally made w/ crummy Al electrolytic caps for cost reasons. But when recapping the labor is going to be more than the entire initial board cost.
So, when recapping, why not use tantalum caps? Those aluminum electrolytics aren't even allowed in avionics. You generally won't be able to use equally high values but you won't need them either due to the lower ESR, better RF performance, and the vastly better lifetime values of the Ta caps.
The only problem I can see is that a couple of the caps may affect the control loops of low voltage regulators or just in general might not be bypass caps, and therefore a big change in value might cause problems. This could be tested by identifying the supply rails and checking resistance, or perhaps by just guessing that the widely repeated value is the bypass cap value.
A hybrid approach would be possible, too. Minimize exposure to the various Al electrolytic failure modes by using a combination of extra-big Al electrolytic recaps plus some (most likely smaller) tantalum replacements.
Many people have objected to originally mfr'ing these boards with life-limited caps. Even the very best ones are still woefully inferior to solid caps. I must say I've never recapped a board but I've designed many...
So, when recapping, why not use tantalum caps? Those aluminum electrolytics aren't even allowed in avionics. You generally won't be able to use equally high values but you won't need them either due to the lower ESR, better RF performance, and the vastly better lifetime values of the Ta caps.
The only problem I can see is that a couple of the caps may affect the control loops of low voltage regulators or just in general might not be bypass caps, and therefore a big change in value might cause problems. This could be tested by identifying the supply rails and checking resistance, or perhaps by just guessing that the widely repeated value is the bypass cap value.
A hybrid approach would be possible, too. Minimize exposure to the various Al electrolytic failure modes by using a combination of extra-big Al electrolytic recaps plus some (most likely smaller) tantalum replacements.
Many people have objected to originally mfr'ing these boards with life-limited caps. Even the very best ones are still woefully inferior to solid caps. I must say I've never recapped a board but I've designed many...
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