Re: What replacement for 22uF tantalum beads?


Brian Bloom
 

I don't have any specific gear for measuring ESR, so I used a Wavetek 145 & a scope. I set the FG to 300mV @ 100Khz square wave for in-circuit measurement. I first set the scope for properly viewing the full square wave, then changed vertical to 5mV/div.
(I should just buy an ESR meter, but I keep finding other gear to buy when I've got the money to spare.... )

I compared them against new 199D & T351.

I still have the old caps stashed in a box. I tend to keep all the parts I replaced - bagged for each unit.

Your hypothesis makes sense to me considering the rate of failure of the 22uf 15V caps in that specific scope. The two I have of the B189000 SN don't suffer the same 22uf tantalum failure. I've yet to check the B144000 SN unit.

How did you measure their ESR?
>So far all of the solid tantalum capacitors I have tested using my ESI
>250DA impedance bridge had a dissipation (D = ESR/Xc = ESR * 2piFC)
>right in line with their specifications whether they were new, NOS
>(new old stock), or working parts removed from decades old
>instruments.

>My hypothesis for the failures of old solid tantalum capacitors is
>contamination in the raw materials causing defects in the dielectric
>which grow over time. Eventually a large enough defect forms that the
>solid tantalum self healing mechanism where heating converts MnO2 to
>Mn2O3 fails because of thermal runaway.

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