Nineteen years ago I retired a spectrum analyser from field work because it was getting a bit long in the tooth, but mainly because it needed a new battery. The analyser was installed on the bench. Every time the bench power was on an external PSU fed it, whether it was in use or not. It was always the case until recently that the fan ran indefinitely, and the charge light was on. In the last six months or so the fan and charge light have stayed on for less and less time, until it is now about ten minutes.
Today the analyser wouldn’t turn off. It just ignored the on/off button. It was going through some sort of initialisation routine, over and over again. I turned off the mains supply to the PSU, assuming that the battery, which had become useless 19 years ago, would soon run down. It didn’t; it powered the analyser for two hours. This analyser has a CRT and two hours is about as good as it would do when the battery was new.
I’m just puzzled.