Hi,

I have this synthesizer Studio electronics ATC-1 form 1996. It works well but there is some disturbing kind of sine (i guess) sound at around 8 Khz. For now I filter it out with an eq and can perfectly use the synthesizer. But I wonder what this could be. Any idea on what to verify (I only have a multimeter) or what to look at or possibly replace to understand where this disturbing sound is coming from an how to maybe fix it? The photos added are the synth inside and two print screens (synth switch on and synth switched off) of a software spectrum analyzer showing the two loudest...