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Dell GX280 motherboard capacitor replacement (Rubycon MCZ alternative)

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WhichEndToHoldTheIron?:
I've got an ancient Dell OptiPlex GX280 that runs a CNC machine via its parallel port, but unfortunately it's recently died (no response when you press the power button on the front panel).

A bit of searching indicates this model was notorious for capacitor problems on the motherboard, and the machine's symptoms appear to match those described for this problem.

Some of the caps look to be the Panasonic FL range, for which I can get replacements. However, the caps (for what I understand are the VRMs) use Rubycon MCZ. It looks as though this was an "Ultra Low ESR for PC motherboard" range, and is now discontinued in favour of polymer caps.

I found a datasheet for the Rubycon MCZ which gives the following specs for the parts I need:

6.3V 820uF: 1340mA ripple current, 21mOhm ESR
16V 1000uF: 2350mA ripple current, 12mOhm ESR

Panasonic FM equivalents have similar ripple current, but much higher ESR; and I understand that very low ESR is required for this VRM circuit?

Looking at aluminium polymer capacitors on Mouser I can see models such as the Panasonic 16SEPF1000M (16V 1000uF); which has a much higher ripple current rating of 5400mA, and an ESR of 12mOhm (matching the Rubycon MCZ).

I understand that higher ripple current is "good" (is it good in this scenario?) but that polymer caps have much higher leakage current than electrolytics. Is that a problem for this application? My understanding of ESR, ripple current and leakage current doesn't really extend much futher than their names; so I don't know how to choose an appropriate replacement capacitor.

amyk:
Those are reservoir caps for the CPU VRM. A little bit more leakage current isn't going to matter; the CPU itself draws dozens of amps.

wraper:
Do not use polymer capacitors to begin with, it may cause problems due to too low ESR. And if you still decide to do it, use 2 times lower capacitance.

WhichEndToHoldTheIron?:
> Do not use polymer capacitors to begin with, it may cause problems due to too low ESR

If the rated ESR of the polymer cap is close to the ESR of the Rubycon original, is that OK, or is the use of a polymer cap fundamentally the wrong choice?

>  And if you still decide to do it, use 2 times lower capacitance.

I've seen this advice mentioned elsewhere (in discussions of replacing an electrolytic with a polymer); can you explain (to someone with relatively little electronics knowledge - me) why a lower capacitance part should be used?

wraper:

--- Quote from: WhichEndToHoldTheIron? on January 15, 2025, 03:57:28 pm ---I've seen this advice mentioned elsewhere (in discussions of replacing an electrolytic with a polymer); can you explain (to someone with relatively little electronics knowledge - me) why a lower capacitance part should be used?

--- End quote ---
Because capacitance as such is not important in this case. Total capacitance is selected in a way that capacitors can withstand ripple current, not because there actually is a need to store this much energy. On second thought you can use polymer instead of 16V 1000uF MCZ as it has very low ESR for wet electrolyte, similar to mediocre polymer. On other hand it will be hard to find a direct substitute for 6.3V 820uF. But I personally would not overpay for polymer for no actual reliability benefit and potential problems to have.

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