Tantalum capacitors

Tantalum electrolytic capacitors consist of a pellet of porous tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounded by liquid or solid electrolyte as a cathode. They offer higher capacitance per volume, lower weight, and better high-frequency performance compared to aluminum electrolytic capacitors, but are significantly more expensive due to raw material price.

Tantalum caps in 20+ year old instruments are known often to fail short-circuited, especially those operated close to their rated voltage. Depending on the power supply current limit, such as failure can bring down the supply voltage, or sometimes cause the capacitor, resistors or board traces to burn out. Failed tantalum caps can sometimes be spotted based on decolorization.

Tantalum caps can of course be replaced with modern versions, in which case it may be prudent to select a higher voltage part (e.g. 35 V instead of 25 V) where available. Often, tantalum caps can also be replaced with modern aluminum electrolytics, using double the capacitance as a rule of thumb.

Links

• #289, 1981-08-01, p.7: Tantalum "hit list" compiled - Shows comparisons between aluminum and tantalum capacitors, and price considerations:

[...] Aluminum electrolytics are suitable replacements for tantalum caps in many of our applications.

The eye opener comes when the bill for the five caps previously mentioned is $0.50 as compared to $17 per instrument!

This modification alone will save $100K (~360,000$ in 2026 value) per year for the 7600 Series mainframes.