All electronic components have a limited life. An aluminium electrolytic cap that lasts for ten years is doing very well.
The claim that the capacitor should be non-polarised is nonsense. In that circuit, a polarised capacitor (which can go in either way if there's no voltage across it) or non-polarised will function equally well. If some have failed, it's just old age, not because they should have been non-polarised types.
In all other respects, they are good articles. In particular, the reverse-biased electros on the 3640 are a real worry. Although they are unlikely to burst after all this time, if they dry out through overheating and go open circuit, they aren't going to do their job.
Probably the excessive heat inside the case of an A4000D is the worst cause of component failures.
tony
