I just had three of these 4 channel 500MHz scopes come across my bench for repair. The symptoms were that they would power up but have no CRT display. All the front panel indicators would light and stay that way, appearing like a CPU fault. As it turns out, the on-board Lithium battery located under the vertically mounted CRT board was the culprit - I temporarily placed a 3VDC power supply across the battery leads and powered up the unit and got a display informing me that the calibration settings have been lost. I replaced the battery and sent the units out for calibration.