Classic Mac Hardware (Troubleshooting, Upgrading, & Modifying) > Mac CPU Upgrades

Why it IS important to use the right heat sink ... (Sonnet upgrade)

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robespierre:
The black soot is from a burned tantalum capacitor (see the formerly yellow boxes at C2 and C4). They do this when electrically overstressed, it has nothing to do with the heat sink at all.

FBz:
“They do this when electrically overstressed, nothing to do with the heat sink at all.”
                                                                                                               -robespierre
Thanks!

So, is it possible to simply check those tantalum capacitors (C2 and C4) using a
multimeter between ends for: (1). resistance, and/or (2). continuity
/ and thus… function? (Compared with others also present?)

AND, what about that LT 0621 “Buck Regulator”? Is that residual soot from the
tantalums overstressing or has it also suffered “overstressed” damage?

Like to see the backside too…
but might “rest” that Sonnet for a while and substitute a stock Apple 1.42 GHz
“out of an abundance of caution” /until the rare Sonnet can be checked.

robespierre:

--- Quote ---So, is it possible to simply check those tantalum capacitors (C2 and C4) using a
multimeter between ends for: (1). resistance, and/or (2). continuity
/ and thus… function? (Compared with others also present?)
--- End quote ---
Generally no, standard multimeters don't have the ability to measure capacitors in-circuit. There are specialized instruments called ESR meters that can do that (they inject a high-frequency signal, above 100 kHz, which gives a localized response of the capacitor without being affected by other components).

The dry tantalum capacitors are like little fireworks waiting to ignite: they contain a sintered mixture of tantalum and manganese dioxide, that can act as a type of thermite if it gets into "thermal runaway". The cap's normal operating temperature is up to 125°C, and ambient temperature around the heatsink is sure to be less than that. They are, however, sensitive to voltage fluctuations outside their design limit: even a brief spike beyond the safe level can cause them to ignite.

The switching regulator chip is likely fine; the black soot came from the capacitor. The actual silicon IC inside that package is very small: if it blew there would be a tiny chunk ejected from the top of the package. It would be a good idea to clean off all the soot with alcohol and a swab, including underneath as much as possible.

Bolkonskij:
aha, thanks for the clarification, robespierre! Yeah, i found the tiny chunk ejected from the top of the cap during cleaning.

So you'd attribute the problem to a failing PSU? I had mine die on me about a year ago and since replaced it. Maybe it took the caps with it?

Anyway, I'll look into getting it professionally replaced.

FBz:
Yes robespierre, greatly appreciate the very thorough and concise explanation. Thank you.

*I was partially implying that with a multimeter, that it might be possible to detect a “complete & total” failure of individual capacitors - IF no continuity could be detected whatsoever, even while still in-circuit.

My ESR meter originally touted (before purchase) the ability to measure capacitors “in-circuit”… which was later found to be partly true, except that one leg of the capacitor had to be free / removed from the circuit board in order to do so properly. Now, I just remove questionable caps completely and then test with ESR meter.

AND Bolkonskij, Knez can possibly guide / help you overclock a somewhat lesser CPU for use while the Sonnet’s “in the shop”. I’ll be looking into this too but got sidetracked yesterday with discovery of overclocking a Quicksilver’s System Bus. (Off to pick up those resistors now.) ::)

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