tek PG506 calibrator generator voltage error

[gifsim]

good morning to the whole group

I ask for help because I'm not understanding anything anymore ....
I have a pg506a that makes a strange defect

I turn on the pg506a and connect an oscilloscope (I have tried with various oscilloscopes) to the amplitude output in standard mode
the output voltages are not correct (they are higher) in any position of the amplitude switch
if I extract and rotate the pot variable out nothing changes and the display remains fixed on a value

if I connect a multimeter or a battery powered oscilloscope (ths730) to the amplitude output, everything works correctly
the voltages are correct and the pot variable out works as expected

It seems to be a ground fault

I connected the pg506a to the extender from the tm504 to measure the supply voltages and they are all correct (I followed the service manual)

I hope I have explained myself as well as possible
ask if anyone have any suggestions to be able to understand the problem

thanks
Simon