The front panel switches control the voltage range indicated on the display
correctly, but don't seem to affect the magnitude of the noise on the trace.
I suspect the problem may be on the A to D converter card.

Is it possible to clean the front switch?

Greg wrote:
I have a Philips PM3323 digital oscilloscope which has not been used for
some time. It has developed a problem where both A and B channels have
become very noisy such that the noise on the display is now larger than
the wanted signal - making the scope unusable. Timebase and y-axis
functions all appear to be still working OK.
Can anybody tell me whether this fault is likely to be economically
repairable?