I just checked by touching the connectors where I have soldered the capacitors for the mod from the post 1 of this thread and when touching with small screwdriver the speakers are producing the crackling sound, meaning that there is some signal coming from that point, also when touching the opamps board, same sound is coming out.
Could something be wrong with input stage of the 7520? Static?
p.s. I also used can spray for blowing air in order to clean all possible shortcuts (e.g. under the DAC 1716 chip).