Great text in ur eBay listing on the 485!
I have a theory on the low Vr/Va tantalum failures, but not much data to support it. I suspect that the 485 tantalum “system” follows some form of Arrhenius behavior that is linked directly to the “live on time” of the scope. As a portable unit, some users would install the 485 on a bench and power it through the work week and maybe for many hours when the lab lights were out. Others were used only on rare occasions when a high performance portable scope was needed. After 35-40 years, these units would fall in very different places on an Arrhenius curve.
My 485 is from the latter case. A few 1000’s of hours rather than 60,000 (8hrs/day x 20years service life). I have never done a full recap. I did replace one transistor and one tantalum at some past date.
But it is also true that every time I push that on/off button it is like a one-armed bandit. Sooner or later I will get three cherries!