Siglent SDS1202X-E scope noise

[howard123]

Good morning

I have had this scope for 4 years and it quite suddenly developed a 308kHz sine wave noise signal superimposed on top of the regular signal. It is difficult to quantify the signal size as it remains a more or less constant size when the probe scale (*1, *10,*100) is changed, but does change size when the amplitude (volts/ cm) is changed. Everything else seems ok. 

Anyone had a similar experience perhaps. I have thought to change the PSU caps. A PSU circuit would be very helpful.

thanks