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Old 06-20-2021, 07:20 AM   #1
qwerki
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Question Advice for replacements for some Rubycons in a Dell Precision 670

Hello, I was hoping some of you kind souls could give me some advice.

I have a couple of old motherboards I'd like to have a go at repairing. They come from Dell Precision 670s, and the caps have gone bad.

Ultimately, I intend on getting rid of them, but it's a fun project to practise my (de)soldering on, so I want to see if I can resurrect them, even if only temporarily. I'd also like to check if there's anything on the onboard RAID array I missed backing up (I don't think there is, but you never know).

It's the VRM caps that visibly leaked, so I removed the two Rubycon MCZs next to the CPU socket that had the leakage, did a bit of research and tried replacing them with the closest equivalents I could find. The original caps were Rubycon MCZ 6.3v 1500uF, code A0443. I tried replacing them with a pair of Panasonic FM 6.3v 1500uF caps, but no dice. Obviously some of the other caps could have gone out of spec as well, or those choices could just be unsuitable.

As I found things, going by the datasheets the Rubycons have an ESR of 16 mO and a ripple current of 1960 mA, whilst the Panasonics are rated for 19 mO and 2180 mA. Going by what I read, am I correct in thinking that having a higher ripple current limit is fine, but you want a lower ESR? It was a bit of an experiment to see if 19 milliohms was close enough to 16 mO I could have gone for a bigger cap to get the ESR down - 3900uF has an ESR of 15 mO (and can handle a ripple of 3190 mA), but I wasn't sure if the extra capacitance would have been a problem. The other alternative was to get caps with a higher voltage rating, as I recall reading that can be ok as long as you don't go crazy with it. The Panasonic FM 25v 1500uF have an ESR of 15 mO and a ripple of 3190mA, so would that in theory be ok?

Ultimately, I don't really care about it working long term, even just a few hours would be fine, so if I can avoid having to recap the entire board that would be preferable. It's more the practise of the technique I'm looking to gain from this. Having it work, even if only briefly, is rather helpful to know I've done things correctly though.
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Old 06-20-2021, 08:21 AM   #2
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Default Re: Advice for replacements for some Rubycons

MCZ are the lowest ESR electrolytics ever made, you can only really replace them with polymer caps
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Old 06-20-2021, 10:34 AM   #3
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Default Re: Advice for replacements for some Rubycons

That said for a difference so small I would expect the board to work fine anyway.
Maybe only overclock a little bit less than before.
So I would say your problem now lies elsewhere, try a Port 80H post card?
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Old 06-20-2021, 11:51 AM   #4
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Default Re: Advice for replacements for some Rubycons

Thanks both. The board has pretty good diagnostic LEDs, they are still saying vrm0 failure. I don't have a port 80H, but can try and pick one up if you think it'll be a useful investment.

Before I proceeded to trying to replace a few more caps I figured I'd get some opinions on whether the first two should have been ok, so if you think they should, then I can proceed.
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Old 06-20-2021, 12:48 PM   #5
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Default Re: Advice for replacements for some Rubycons

I just now tried a post card in my Precision 470, post here: https://www.badcaps.net/forum/showthread.php?t=5974
It does display post codes (I was not 100% sure since it is a Dell) so yes it seems useful.
P.S: The 470 is the same board just with a few slots left out basically.
Yea go ahead and replace some more caps, I think it has a good rate of being successful.
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