Canon A-1 meter off

Last night I found my Canon A-1 meter is off by about 3 2/3 stop. I wonder what makes it dripped that much. It was OK a couple of years ago.

I don't know if this is the root of your problem or not, but it seems that silicon photodiodes age:
https://www.sciencedirect.com/science/article/abs/pii/S0263224187800143
And if this is in fact what's happened, then ideally the fix is to find a modern part with similar performance characteristics to the original (when new!) within the visible light spectrum. Probably no point in seeking out NOS parts, as they are likely degraded.

beofre ringing hands, you might like to clean the speed and appature reading potentometers. (sensors) that tell the Microprocesor what the lens is set at. Also try it with verifing the Lens is set at "A"