Transponder Problem
This is a slightly academic post as I'm shortly going to be upgrading my Narco AT150 Transponder to a Mode S model.
But the following is puzzling. I have flown over quite a large area of the UK in the last couple of years, always squawking when asked and never having a problem. But when in the vicinity of Southend I always have a transponder issue - occasionally it squawks an incorrect code, more likely it gives a negative altitude !! I always try recycling which more often than not cures the code issue but not the altitude.
My maintenance organisation has looked into it, tested the transponder and can find nothing wrong with it. Has anyone come across anything like it ?