Indigo² sound board w/blown caps

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johnnym
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Indigo² sound board w/blown caps

Unread postby johnnym » Mon Mar 20, 2017 4:20 am

Hi all Indigo² owners,

I recently got two classics for free: a R4K Indigo and a R10K Indigo². Sadly both machines are not in working order, as its guts (CPUs (both), memory (Indigo² only), chipset ASICs (Indigo), graphics ASICs (Indigo) and graphics card (Indigo²)) have been removed or destroyed (socket for chipset ASIC on IP20 board, HP2/PM2PM1 board, EISA/GIO IMPACT backplane) by some unknown person - most likely armed with a crowbar :evil: . I will later try to refit those two machines to get them back to working order. I'm not yet sure with which machine I should start, though.

But for now I'm just trying to repair the sound board of the Indigo². Here's a picture of how it looks currently:
r10k-indigo2-sound-board.jpg

I see at least one blown cap (C57 w/yellow border). From the markings (left solder pad, brown markings directly below on PCB) I assume that C54 (w/red border between C41 (w/blue border) and C57) is also missing.

I don't know what capacity and working voltage both C57 and C54 originally had. I assume both properties would be the same as for C41, where it reads "476\n16 K\n649". I make 476 Picofarad and 16 V working voltage out of this - please correct me if I'm wrong here. But I don't know for sure.

So does anyone with an (R10K) Indigo² know the properties of the missing caps?

Cheers,
johnnym
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robespierre
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Re: Indigo² sound board w/blown caps

Unread postby robespierre » Mon Mar 20, 2017 2:06 pm

I've seen this happen and heard of it happening to others.
These are tantalum MnO2 caps, which blow up if (1) Their voltage rating is exceeded; or (2) they are exposed to more than a couple volts in reverse. This should never happen, so the failure indicates that something else has gone wrong.

The three tantalum caps are 47 uF. The one on the left is 16V, and the two others are 20V. They are K tolerance (+/- 10%).
The one on the left is installed with its + mark to the right; the others have the + mark to their left.

If I was replacing these capacitors today, I would use polymer tantalums. They don't explode when they fail. I think these are 'D' case.

Another interesting bit of trivia about the audio board is that it has empty pads for a DSP56002 and its memory, as used on the Indigo. That was not delivered in the end, and Irix uses the kdsp driver instead of the hdsp driver on IP22.
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Re: Indigo² sound board w/blown caps

Unread postby robespierre » Mon Mar 20, 2017 5:41 pm

OK, I got out the calipers.
The tantalum caps are 2917 (Imperial) / 7343 (metric) case size, and 2.8 or 3.1mm height. This is called 'D' case by the manufacturers.
I don't know the original caps' ESR. Tantalums this size are made in a wide range, from 40 to 900 mOhm. But with polymers you have basically one choice, 40 or 55 mOhm.
Here are 20V polymers: http://www.mouser.com/ProductDetail/AVX ... M020R0055/
The C54, C57 are the ones that commonly fail (I have audio boards where both failed).
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Re: Indigo² sound board w/blown caps

Unread postby japes » Tue Mar 21, 2017 2:41 am

Snapped a photo for the visual folks. In yours it looked like the center pads were never populated. I just grabbed the closest Indigo2 that happened to be open for unrelated reasons.

476 is 47 x 10^6 pf or 47 uf. A lot like SMT resistors, xxn, where xx is the value and n is the number of zeros on the end of it.

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Re: Indigo² sound board w/blown caps

Unread postby johnnym » Tue Mar 21, 2017 6:56 am

japes wrote:In yours it looked like the center pads were never populated.

Yes, it looks like that, although the left pad looks like a part was soldered in there. But as I draw the red/blue border around/over it, it's hard to see. Maybe it got hot enough to desolder itself but not to blow up and fell off (only possible if the machine was standing on its feet - I didn't find any remains of it in the case though), or the previous owner(s) just removed it after it blew up.

japes wrote:476 is 47 x 10^6 pf or 47 uf. A lot like SMT resistors, xxn, where xx is the value and n is the number of zeros on the end of it.

Thanks, you're right, robespierre also already wrote that these are 47 uF types. I misunderstood how the capacity is calculated from the first line of the label. Thank god, I'm no longer in electronics, what a shame... :oops:

@robespierre, japes:
Thanks for your help. I'll try to procure some matching capacitors.

robespierre wrote:I've seen this happen and heard of it happening to others.
These are tantalum MnO2 caps, which blow up if (1) Their voltage rating is exceeded; or (2) they are exposed to more than a couple volts in reverse. This should never happen, so the failure indicates that something else has gone wrong.

Could those tantalum MnO2 caps also blow up because of old age? Because then I should perhaps also replace the intact one (C41).
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Re: Indigo² sound board w/blown caps

Unread postby robespierre » Tue Mar 21, 2017 10:49 pm

As far as I know, there is no failure mechanism that is age-related. It's usually the opposite: they fail with infant mortality when power is first applied.

The way these are made is by printing thin layers of tantalum and MnO2, then applying a forming voltage, much like an aluminum electrolytic capacitor, to create a dielectric barrier. The thickness of the barrier depends on the forming voltage, which is some multiple of the voltage it will be rated to withstand. Since the microstructure of the materials are not perfectly uniform, there will be some defects where the grown dielectric is weaker and charges can jump across more easily. The material has a limited self-healing ability: as these breakdowns happen, they rapidly heat up the MnO2, which releases its oxygen, which transfers over to the tantalum forming an insulating tantalum oxide crust. This works as long as the amount of current flowing into the defect is strictly limited. But the downside is that it is a relative of the thermite reaction: if too much heat is allowed to be released this way, the reaction runs away and burns up the whole component.

So the failure modes make a little more sense: if the capacitor sees a reverse charge, it starts to eat away at the dielectric barrier, which soon causes a failure. Or, if the forward voltage on the capacitor becomes too high (even for a short impulse), then charge starts jumping across the barrier and causing heating via the self-healing reaction. Tantalum capacitors are usually used for power bypass, so the amount of current that can flow into them is huge and the heating can be like a firecracker.

So, why would they fail after many years? If the power supply's voltage regulation is compromised, the tantalum caps might get charged to a voltage above their rating, triggering breakdown; or, if even just the ripple was higher, leading them to run hotter, and reducing the safety margin before thermal runaway. The point is that these problems are caused by other components, and will affect replacements as badly as the existing tantalums, if not more so. Tantalum capacitors are all tested to above their W.V. before they are sold, but when you solder them onto a circuit board, the heat causes defects to appear (and defects are the cause of breakdowns). So the ultimate cause of tantalum failures is soldering, and poor process control during rework could lead to even more failures than leaving them alone.
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