Thanks for your suggestions!
Tried to attach a load 100
resistor - did't help, same problem. So I decided to look what's inside. At first glance noting wrong, nothing looks like burned... visually. But definitely something wrong.. So, here is how this FG looks from inside
What I have however noticed that on couple of BJTs inside on some legs are signals like that
(this is for square signal, same s# happens for other functions)
Is that OK at all?
Because to me it looks like there is some high frequency parasitic resonance in circuit.. The frequency selected on front panel was 150kHz