Stefan, no reason to doubt the AM503A schematics. The AM503 also supplied +/- 3 V and the same voltages are shown in the schematic of the A6302 itself. The P6042 used higher voltages, but I don't know if there really is a difference (improvement) in probe design from P6042 to P6302/A6302.
Still another wild guess: could the AM503B reduce the voltages when the probe slide is open?
---In TekScopes@yahoogroups.com, <stefan_trethan@...> wrote :
It's not floating and it's no different with the probe connected,
however difficult this may be to believe for us.
Perhaps the AM503A schematic is wrong and there isn't 3V there, has
anyone actually checked on a AM503A?