Ok now just to make sure I understand your recommendation if I go the route of stop using tantalum NOS and only replace by Nichicon electrolytics. Say I want to replace a K100-20V tantalum (+15Vdc bus), shall I replace by Nichicon 100uF of 50V or do you need to derate by 2 its capacitance as well (i.e. 200uF and 50V) ?
To get a comparable impedance at high frequencies, an aluminum electrolytic capacitor needs to have like 4 times the capacitance of the solid tantalum capacitor it replaces. This is often reflected in old application notes which will recommend a 1 microfarad tantalum capacitor or 10 microfarad aluminum electrolytic.
Since you know both TG501 and PG506, let's see the attached pictures of 4 cases where i've added yellow arrow pointing on the failed tantalum. Ok the +5V bus rail seem to survive and I think it is because tektronix used 10V rated tantalum. However the +16.5V or the +15V are failed (shorted) where basically they put tantalum rated 20V, clearly not enough margin.
Now replacing by electrolytic with 10x capacitance cannot work, there will be no room to install them. Furthermore all these tantalum caps do have in parallel fast smaller non-polarized capacitor so these do take care of HF.
There must be a middle path of either using Al electrolytics or using modern tantalum with higher voltage rating but keeping same volume to fit the compactness of these PCBs once they're assembled together.
You know these TM500's section are very compact, I guess tektronix had no choice to save space of using these tantalum. Some voltage bus were properly rated but as I've observed so far, the problem is on the +15V or +16.5V with crazy 20V capacitor.