Hello Arnaud,
I think there's a few things you may do to locate the source of the problem.
1 - Try operating the 3403C in DC, linear mode. This will bypass all the RMS to DC converter and the LOG circuitry, and will let you measure different DC voltages, positive or negative, and see what value is displayed. Is it always +1 ? This will be a first indication.
2 - Check the voltages which are fed to the LD110 and LD111 : power supplies and reference voltage, and check the clock input.
3 - With a known voltage at the 3403 input (for example +0.5 VDC), check the input voltage on pin 15 of the LD111.
And of course, please let me know !
Sincerely,
Jo?l Setton
