[time-nuts] Electrolytic Capacitor Question

[ed breya]

The Ta caps in old HP gear should last virtually forever, especially 
if they have already lasted for decades. What you are referring to as 
"wet slug" Ta caps are mostly dry solid ones in hermetically sealed 
cans. There can be some actual wet slug types, but only in certain 
spots where their unique characteristics are necessary. These can 
often be identified by the end seals - they are not hermetic, but 
elastomer sealed with rubbery material, and the anode lead is a solid 
Ta wire that's butt-welded to a steel wire that's solderable. You can 
see the weld where it goes from the bluish or brownish Ta color to 
the tinned lead, except in types that put a glob of epoxy over the 
end. As I recall, a lot of the ones used by HP are marked "109D" type 
after the original Sprague (or Mallory?) product line.

The dry types have a true hermetic glass to steel seal that's 
soldered to the can and the anode lead, which is steel or copper 
alloy all the way. This is the best kind of electrolytic cap for 
lifetime and durability, in my opinion.

The wet slug ones can leak after many years, and the sulfuric acid 
electrolyte can damage things nearby. Whether they can be replaced 
readily depends on the application. Wet slug types have the lowest DC 
current leakage and highest temperature range of all electrolytic 
caps, very low ESR, and wide voltage range. If it's a low-leakage 
circuit requirement, and fairly low C (like up to a few tens of uF), 
the best bet may be to use plastic caps, even several paralleled to 
get the right amount - but the physical size will be much larger. For 
larger values, this is impractical, so the next closest thing is dry 
Ta caps, with more leakage, but may be OK for bench use at room 
temperature. One very common use in HP gear is to have a wet Ta 
switched across a YTO coil whenever the oscillator is in CW or 
narrowband mode, for extra noise filtering. Any DC leakage would 
cause errors in the tuning current.

Don't use regular Al or OSCON caps for low leakage circuits.

Ed