PDP 11 gear finally moved

Well, at least 2 possibilities... Firstly, the tolerance of the capacitance of an electrolytic capacitor is very wide -- -20% to +80% is not uncommon. So it's quite possible they started off at the top end of that range, have deteriorated over the years, and are still within spec. Of course nobody can prove that (unless there are records of the values meaured 50 years ago) and nobody really knows how they will continue to change (if indeed they do). Secondly, I have no idea what was measured. The capacitance value is not the whole story by any means. In fact the most important thing most of the time is ESR (Effective Series Resistance) which increases as the electrolyte dries up. The ESR of these components could well still be OK after 50 years, but again nobody knows what it was when they were new. That said, I keep on with the comment that the important thing is 'does the circuit behave as required', If so, then the capacitor is almost by definition OK in that circuit. -tony I'm very confused... Noel