[Radio Wrangler]

With the passage of time, slow degradation has had more time to do its thing, so more of the transistors in the historic population have gone bad.

Parts where there is a deliberate void over the active face of the silicon and the bond wires seem to be OK, but the ones where epoxy is forced to coat the whole thing seem most at risk.

There also seems to be two mechanisms at work; contamination, and also mechanical stress on bond wires.

David