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Alex

Quadra 950 Capacitor burned (C26)

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On 1/15/2018 at 2:00 AM, Franklinstein said:

No idea what that cap actually does other than explode.

I love your reply! I hear you are an old timer on here. Great that you are back. A little humor is always amazing. No, but I read your whole answer of course and appreciate it!

 

Kind regards

—Alex

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According to the datasheet the input voltage should be between 7 to 25 volt. 16v seems plausible.

However, choosing a capacitor with a lower ripple rating than the original can cause stress in the capacitor and make it fail again. So sometimes choosing the cheapest capacitor can break things. Also, choosing a capacitor with a higher max temperature and rated longer life will usually help.

 

This video explains a lot of the capacitor differences and why it matters to choose the correct one (and why some blows ;)): https://www.youtube.com/watch?v=ZAbOHFYRFGg

Really interesting :)

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19 hours ago, LOOM said:

This video explains a lot of the capacitor differences and why it matters to choose the correct one (and why some blows ;)): https://www.youtube.com/watch?v=ZAbOHFYRFGg

Really interesting :)

Great video, I am still watching but in terms on tantalum, it appears that Tantalum Polymer is the way to go. Tantalum MnO2 (do blow up) are less and less popular, likely because Tant Polymers are used more and more in phones and tablets but not only for that reason, Tantalum Polymers don't blow up.

 

The presenter in the video does not recommend Tantalum MnO2 and he suggested the following (what he termed as his real point)

  • If you use a Tantalum MnO2 don't apply more than half the rated voltage. (get a higher rated part)
  • If you use a Tantalum Polymer reduce the rated voltage by 10% – 20%. (get a higher rated part)

So yes, a very interesting video, I learned quite a bit and some of it was over my head.

Screen Shot 2018-01-24 at 10.45.57 AM.jpg

Screen Shot 2018-01-24 at 10.47.58 AM.jpg

Edited by Alex

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So now, after this video I went to Kemet.com and am wondering which part to settle on; https://search.kemet.com/component-edge/#/browsing?search=47uF 6.3v&id=353

I did a search for 47uF 6.3v

 

I should be ordering according to a derating guideline of 20%(>10v), 10%(≤10V). The other part that confuses me is the ESR/Impedance value, I have no idea what it should be. We never discussed that part of things in this thread. Any ideas?

 

Someone help pick the right cap please :)

Edited by Alex

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Just a quick update. I believe I figured it out.

 

derating guideline of 20%(>10v), 10%(≤10V). The other part that confuses me is the ESR/Impedance value

 

I think I meant tolerance and I believe the lower the better and the same applies with ESR, the lower the better. I just wanted to put this extra bit of knowledge here.

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Also pay attention to package size.   You want a capacitor that will conveniently fit on the old pads and not be too short, nor too long.   I can't tell for sure, but from the photo it looks like you need a size D or similar.

 

ESR is sort of the internal resistance of the capacitor.   Capacitors charge and discharge at different speeds, depending on the capacitance and resistance that they see in the circuit.   A capacitor with a higher ESR is likely to respond more slowly, (charge or discharge more slowly) because the ESR adds to the R in the 1/RC time equation.    But for large capacitance bypass caps, a slower response time may actually be desirable.  In the low capacitance (1 - 100 picofarad) bypass caps meant to compensate for high frequency voltage/current variations, a low ESR is much more important.

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Did you get around to replacing it? I haven't tried reinvestigating mine since I don't have plans to use it (I have other machines to use and also don't have hard drives for this one). 

 

I am at least partially familiar with capacitors and generally don't have problems with them exploding due to improper installation. I can only surmise that a transient excessive or reverse voltage occurs to blow up the cap in this instance. This machine did come with a bad trace on the logic board that was connected to the power supply connector. I'm pretty sure I installed a jumper wire correctly but it's possible that something is/was damaged somewhere when it went bad originally. 

 

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9 hours ago, Franklinstein said:

Did you get around to replacing it? I haven't tried reinvestigating mine since I don't have plans to use it (I have other machines to use and also don't have hard drives for this one)

Hello Franklinstein, I am not sure if your message is for me but if it is, I did indeed replace the capacitor on my 950. The originals are tantalum and I replaced it with another one. To be honest now, I don't even know if I switched it on to test it, can you imagine? I would think I did though but well, there isn't much to say, whether or not I did turn it on at the time but it's one of those things, a bad cap has to get replaced and it's very cheap to do.

On a side note, the reason I question whether I turned it on after the cap replacement is that I have a pile of machines and other gear kind of sitting there. I have yet to find time to truly organize all my equipment. I have plans but I thought I should explain why I am not confident if I ran on power cycle after the fix, as I write this though I am more inclined to think I did as it's in my nature to do so.

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Yes I was mostly asking if you had fixed yours, Alex. I was curious as to whether it worked or if yours blew up the replacement too.

 

I have sort of the same tendency: get machine, fix/upgrade/play with it for a bit (or fail and get frustrated/lose interest), then put it on a shelf and move to the next project. Lately I have been on a Color Classic/NuBus Power Mac kick.

 

With my Q950, as I mentioned, it seems to mostly work without that capacitor (still haven't checked Ethernet or serial ports) but it's such a huge machine and I really don't have room to set it up so it's in the closet along with other boxes that I don't really have a use for at the moment. It's partially because this machine has the 5 drive cage and I have no drives to fill it with. It would be fun to use one as a vintage server if I acquired a stack of functional hard drives one day, especially if I got one of the WGS PDS SCSI cards to use with it.

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