Thanks. I opened it up (after discharging the CRT), cleaned the dust around the fan and the oscilloscope is now less noisy. Just curious, why do they need a fan in an oscilloscope? My other INSTEK 20Mhz oscilloscope does not have a fan.
Another question, if I configure the oscilloscope in dual channel (CH1 & CH2) mode with the on screen display (cursor readout) on, then I could not get a stable display using the calibration signal (1kHz, 2 Vp-p). No matter what trigger settings I use, the traces always flicker slightly. If the readout is turned off, then the trace is stable. Does this indicate any problem, or it's just the way it is?